Sunday, May 31, 2015

Orbital Evolution of Planetesimals in Gaseous Protoplanetary Disks

Orbital evolution of planetesimals in gaseous disks

Author:

Kobayashi

Abstract:

Planets are formed from collisional growth of small bodies in a protoplanetary disk. Bodies much larger than approximately 1\,m are mainly controlled by the gravity of the host star and experience weak gas drag; their orbits are mainly expressed by orbital elements: semimajor axes a, eccentricities e, and inclinations i, which are modulated by gas drag. In a previous study, a˙, e˙, and i˙ were analytically derived for e≪1 and i≪H/a, where H is the scale height of the disk. Their formulae are valid in the early stage of planet formation. However, once massive planets are formed, e and i increase greatly. Indeed, some small bodies in the solar system have very large e and i. Therefore, in this paper, I analytically derive formulae for a˙, e˙, and i˙ for 1−e2≪1 and i≪H/a and for i≫H/a. The formulae combined from these limited equations will represent the results of orbital integration unless e≥1 or i>π−H/a. Since the derived formulae are applicable for bodies not only in a protoplanetary disk but also in a circumplanetary disk, I discuss the possibility of the capture of satellites in a circumplanetary disk using the formulae.

The Science Case for the Thirty Meter Telescope

Thirty Meter Telescope Detailed Science Case: 2015

Authors:

Skidmore et al

Abstract:

The TMT Detailed Science Case describes the transformational science that the Thirty Meter Telescope will enable. Planned to begin science operations in 2024, TMT will open up opportunities for revolutionary discoveries in essentially every field of astronomy, astrophysics and cosmology, seeing much fainter objects much more clearly than existing telescopes. Per this capability, TMT's science agenda fills all of space and time, from nearby comets and asteroids, to exoplanets, to the most distant galaxies, and all the way back to the very first sources of light in the Universe.

More than 150 astronomers from within the TMT partnership and beyond offered input in compiling the new 2015 Detailed Science Case. The contributing astronomers represent the entire TMT partnership, including the California Institute of Technology (Caltech), the Indian Institute of Astrophysics (IIA), the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC), the National Astronomical Observatory of Japan (NAOJ), the University of California, the Association of Canadian Universities for Research in Astronomy (ACURA) and US associate partner, the Association of Universities for Research in Astronomy (AURA).

Modeling Protoplanetary Disks With Adaptive Meshes

Modeling gravitational instabilities in self-gravitating protoplanetary disks with adaptive mesh refinement techniques

Authors:

Lichtenberg et al

Abstract:

The astonishing diversity in the observed planetary population requires theoretical efforts and advances in planet formation theories. Numerical approaches provide a method to tackle the weaknesses of current planet formation models and are an important tool to close gaps in poorly constrained areas. We present a global disk setup to model the first stages of giant planet formation via gravitational instabilities (GI) in 3D with the block-structured adaptive mesh refinement (AMR) hydrodynamics code ENZO. With this setup, we explore the impact of AMR techniques on the fragmentation and clumping due to large-scale instabilities using different AMR configurations. Additionally, we seek to derive general resolution criteria for global simulations of self-gravitating disks of variable extent. We run a grid of simulations with varying AMR settings, including runs with a static grid for comparison, and study the effects of varying the disk radius. Adopting a marginally stable disk profile (Q_init=1), we validate the numerical robustness of our model for different spatial extensions, from compact to larger, extended disks (R_disk = 10, 100 and 300 AU, M_disk ~ 0.05 M_Sun, M_star = 0.646 M_Sun). By combining our findings from the resolution and parameter studies we find a lower limit of the resolution to be able to resolve GI induced fragmentation features and distinct, turbulence inducing clumps. Irrespective of the physical extension of the disk, topologically disconnected clump features are only resolved if the fragmentation-active zone of the disk is resolved with at least 100 cells, which holds as a minimum requirement for all global disk setups. Our simulations illustrate the capabilities of AMR-based modeling techniques for planet formation simulations and underline the importance of balanced refinement settings to reproduce fragmenting structures.

Saturday, May 30, 2015

Chemistry in Protoplanetary Disks

Chemistry in Protoplanetary Disks: the gas-phase CO/H2 ratio and the Carbon reservoir

Authors:

Reboussin et al

Abstract:

The gas mass of protoplanetary disks, and the gas-to-dust ratio, are two key elements driving the evolution of these disks and the formation of planetary system. We explore here to what extent CO (or its isotopologues) can be used as a tracer of gas mass. We use a detailed gas-grain chemical model and study the evolution of the disk composition, starting from a dense pre-stellar core composition. We explore a range of disk temperature profiles, cosmic rays ionization rates, and disk ages for a disk model representative of T Tauri stars. At the high densities that prevail in disks, we find that, due to fast reactions on grain surfaces, CO can be converted to less volatile forms (principally s-CO2, and to a lesser extent s-CH4) instead of being evaporated over a wide range of temperature. The canonical gas-phase abundance of 10−4 is only reached above about 30-35 K. The dominant Carbon bearing entity depends on the temperature structure and age of the disk. The chemical evolution of CO is also sensitive to the cosmic rays ionization rate. Larger gas phase CO abundances are found in younger disks. Initial conditions, such as parent cloud age and density, have a limited impact. This study reveals that CO gas-phase abundance is heavily dependent on grain surface processes, which remain very incompletely understood so far. The strong dependence on dust temperature profile makes CO a poor tracer of the gas-phase content of disks.

Fast Pebble Growth Near Condensation Fronts in the HL Tau Protoplanetary Disk

Evidence of fast pebble growth near condensation fronts in the HL Tau protoplanetary disk

Authors:

Zhang et al

Abstract:

Water and simple organic molecular ices dominate the mass of solid materials available for planetesimal and planet formation beyond the water snow line. Here we analyze ALMA long baseline 2.9, 1.3 and 0.87 mm continuum images of the young star HL Tau, and suggest that the emission dips observed are due to rapid pebble growth around the condensation fronts of abundant volatile species. Specifically, we show that the prominent innermost dip at 13 AU is spatially resolved in the 0.87 mm image, and its center radius is coincident with the expected mid-plane condensation front of water ice. In addition, two other prominent dips, at distances of 32 and 63 AU, cover the mid-plane condensation fronts of pure ammonia or ammonia hydrates and clathrate hydrates (especially with CO and N2) formed from amorphous water ice. The spectral index map of HL Tau between 1.3 and 0.87 mm shows that the flux ratios inside the dips are statistically larger than those of nearby regions in the disk. This variation can be explained by a model with two dust populations, where most of solid mass resides in a component that has grown into decimeter size scales inside the dips. Such growth is in accord with recent numerical simulations of volatile condensation, dust coagulation and settling.

The Evolution of Vicosity Over Time in Viscous Disks

EVOLUTION OF FINITE VISCOUS DISKS WITH TIME-INDEPENDENT VISCOSITY

Author:

Lipunova

Abstract:

We find Green functions for the accretion disk with fixed outer radius and time-independent viscosity. With the Green functions, a viscous evolution of the disk with any initial conditions can be described. Two types of inner boundary conditions are considered: the zero stress tensor and the zero accretion rate. The variable mass inflow at the outer radius can also be included. The well-known exponential decline of the accretion rate is a part of the solution with the inner zero stress tensor. The solution with the zero central accretion rate is applicable to disks around stars whose magnetosphere's boundary exceeds the corotation radius. Using the solution, the viscous evolution of disks in some binary systems can be studied. We apply the solution with zero inner stress tensor to outbursts of short-period X-ray transients during the time around the peak. It is found that for the Kramers' regime of opacity and the initial surface density proportional to the radius, the rise time to the peak is ${{t}_{{\rm rise}}}\approx 0.15\;r_{{\rm out}}^{2}/{{\nu }_{{\rm out}}}$ and the e-folding time of the decay is ${{t}_{{\rm exp} }}\approx 0.45\;r_{{\rm out}}^{2}/{{\nu }_{{\rm out}}}$. Comparison to non-stationary α-disks shows that both models with the same value of viscosity at the outer radius produce similar behavior on the viscous time-scale. For six bursts in X-ray novae, which exhibit fast-rise–exponential-decay and are fitted by the model, we find a way to restrict the turbulent parameter α.

Friday, May 29, 2015

The Frequency of Brown Dwarf Binaries in the Pleiades

On the Binary Frequency of the Lowest Mass Members of the Pleiades with Hubble Space Telescope Wide Field Camera 3

Authors:

Garcia et al

Abstract:

We present the results of a Hubble Space Telescope Wide Field Camera 3 imaging survey of 11 of the lowest mass brown dwarfs in the Pleiades known (25-40 Mjup). These objects represent the predecessors to T dwarfs in the field. Using a semi-empirical binary PSF-fitting technique, we are able to probe to 0.03" (0.75 pixel), better than 2x the WFC3/UVIS diffraction limit. We did not find any companions to our targets. From extensive testing of our PSF-fitting method on simulated binaries, we compute detection limits which rule out companions to our targets with mass ratios of q≳0.7 and separations a≳4 AU. Thus, our survey is the first to attain the high angular resolution needed to resolve brown dwarf binaries in the Pleiades at separations that are most common in the field population. We constrain the binary frequency over this range of separation and mass ratio of 24-40 Mjup Pleiades brown dwarfs to be less than 11% for 1σ (less than 26% at 2σ). This binary frequency is consistent with both younger and older brown dwarfs in this mass range.

New Brown Dwarfs Found While Age-Dating the Young Moving Group

Planets Around Low-Mass Stars (PALMS). V. Age-Dating Low-Mass Companions to Members and Interlopers of Young Moving Groups

Authors:

Bowler et al

Abstract:

We present optical and near-infrared adaptive optics (AO) imaging and spectroscopy of 13 ultracool (greater than M6) companions to late-type stars (K7-M4.5), most of which have recently been identified as candidate members of nearby young moving groups (YMGs; 8-120 Myr) in the literature. The inferred masses of the companions (~10-100 Mjup) are highly sensitive to the ages of the primary stars so we critically examine the kinematic and spectroscopic properties of each system to distinguish bona fide YMG members from old field interlopers. 2MASS J02155892-0929121 C is a new M7 substellar companion (40-60 Mjup) with clear spectroscopic signs of low gravity and hence youth. The primary, possibly a member of the ~40 Myr Tuc-Hor moving group, is visually resolved into three components, making it a young low-mass quadruple system in a compact (less than 100 AU) configuration. In addition, Li 1 λ6708 absorption in the intermediate-gravity M7.5 companion 2MASS J15594729+4403595 B provides unambiguous evidence that it is young (less than 200 Myr) and resides below the hydrogen burning limit. Three new close-separation (less than 1") companions (2MASS J06475229-2523304 B, PYC J11519+0731 B, and GJ 4378 Ab) orbit stars previously reported as candidate YMG members, but instead are likely old (greater than 1 Gyr) tidally-locked spectroscopic binaries without convincing kinematic associations with any known moving group. The high rate of false positives in the form of old active stars with YMG-like kinematics underscores the importance of radial velocity and parallax measurements to validate candidate young stars identified via proper motion and activity selection alone. Finally, we spectroscopically confirm the cool temperature and substellar nature of HD 23514 B, a recently discovered M8 benchmark brown dwarf orbiting the dustiest-known member of the Pleiades.

J0219-3925B: a new L Class "Nemesis" Brown Dwarf

BANYAN. VI. Discovery of a companion at the brown dwarf/planet-mass limit to a Tucana-Horologium M dwarf

Authors:

Artigau et al

Abstract:

We report the discovery of a substellar companion to 2MASS J02192210-3925225, a young M6 γ candidate member of the Tucana-Horologium association (30 - 40 Myr). This L4 γ companion has been discovered with seeing-limited direct imaging observations; at a 4" separation (160AU) and a modest contrast ratio, it joins the very short list of young low-mass companions amenable to study without the aid of adaptive optics, enabling its characterization with a much wider suite of instruments than is possible for companions uncovered by high-contrast imaging surveys. With a model-dependent mass of 12-15MJup, it straddles the boundary between the planet and brown dwarf mass regimes. We present near-infrared spectroscopy of this companion and compare it to various similar objects uncovered in the last few years. The J0219-3925 system falls in a sparsely populated part of the host mass versus mass ratio diagram for binaries; the dearth of known similar companions may be due to observational biases in previous low-mass companion searches.

Thursday, May 28, 2015

Stable Conic-Helical Orbits of Circumbinary Exoplanets

STABLE CONIC-HELICAL ORBITS OF PLANETS AROUND BINARY STARS: ANALYTICAL RESULTS

Author:

Oks

Abstract:

Studies of planets in binary star systems are especially important because it was estimated that about half of binary stars are capable of supporting habitable terrestrial planets within stable orbital ranges. One-planet binary star systems (OBSS) have a limited analogy to objects studied in atomic/molecular physics: one-electron Rydberg quasimolecules (ORQ). Specifically, ORQ, consisting of two fully stripped ions of the nuclear charges Z and Z' plus one highly excited electron, are encountered in various plasmas containing more than one kind of ion. Classical analytical studies of ORQ resulted in the discovery of classical stable electronic orbits with the shape of a helix on the surface of a cone. In the present paper we show that despite several important distinctions between OBSS and ORQ, it is possible for OBSS to have stable planetary orbits in the shape of a helix on a conical surface, whose axis of symmetry coincides with the interstellar axis; the stability is not affected by the rotation of the stars. Further, we demonstrate that the eccentricity of the stars' orbits does not affect the stability of the helical planetary motion if the center of symmetry of the helix is relatively close to the star of the larger mass. We also show that if the center of symmetry of the conic-helical planetary orbit is relatively close to the star of the smaller mass, a sufficiently large eccentricity of stars' orbits can switch the planetary motion to the unstable mode and the planet would escape the system. We demonstrate that such planets are transitable for the overwhelming majority of inclinations of plane of the stars' orbits (i.e., the projections of the planet and the adjacent start on the plane of the sky coincide once in a while). This means that conic-helical planetary orbits at binary stars can be detected photometrically. We consider, as an example, Kepler-16 binary stars to provide illustrative numerical data on the possible parameters and the stability of the conic-helical planetary orbits, as well as on the transitability. Then for the general case, we also show that the power of the gravitational radiation due to this planet can be comparable or even exceed the power of the gravitational radiation due to the stars in the binary. This means that in the future, with a progress of gravitational wave detectors, the presence of a planet in a conic-helical orbit could be revealed by the noticeably enhanced gravitational radiation from the binary star system.

The Occurrence and Architecture of Exoplanetary Systems

The Occurrence and Architecture of Exoplanetary Systems

Authors:

Winn et al

Abstract:

The basic geometry of the Solar System -- the shapes, spacings, and orientations of the planetary orbits -- has long been a subject of fascination as well as inspiration for planet formation theories. For exoplanetary systems, those same properties have only recently come into focus. Here we review our current knowledge of the occurrence of planets around other stars, their orbital distances and eccentricities, the orbital spacings and mutual inclinations in multiplanet systems, the orientation of the host star's rotation axis, and the properties of planets in binary-star systems.

How Exoplanetary Systems end up With Orbital Resonances

Capture of Planets Into Mean Motion Resonances and the Origins of Extrasolar Orbital Architectures

Author:

Batygin

Abstract:

The early stages of dynamical evolution of planetary systems are often shaped by dissipative processes that drive orbital migration. In multi-planet systems, convergent amassing of orbits inevitably leads to encounters with rational period ratios, which may result in establishment of mean motion resonances. The success or failure of resonant capture yields exceedingly different subsequent evolutions, and thus plays a central role in determining the ensuing orbital architecture of planetary systems. In this work, we employ an integrable Hamiltonian formalism for first order planetary resonances that allows both secondary bodies to have finite masses and eccentricities, and construct a comprehensive theory for resonant capture. Particularly, we derive conditions under which orbital evolution lies within the adiabatic regime, and provide a generalized criterion for guaranteed resonant locking as well as a procedure for calculating capture probabilities when capture is not certain. Subsequently, we utilize the developed analytical model to examine the evolution of Jupiter and Saturn within the protosolar nebula, and investigate the origins of the dominantly non-resonant orbital distribution of sub-Jovian extrasolar planets. Our calculations show that the commonly observed extrasolar orbital structure can be understood if planet pairs encounter mean motion commensurabilities on slightly eccentric (e~0.02) orbits. Accordingly, we speculate that resonant capture among low-mass planets is typically rendered unsuccessful due to subtle axial asymmetries inherent to the global structure of protoplanetary disks.

Wednesday, May 27, 2015

Studying and Differentiating Dwarf, Subgiant & Giant FGK Stars With and Without Giant Exoplanets

Homogeneous abundance analysis of dwarf, subgiant and giant FGK stars with and without giant planets

Authors:

da Silva et al

Abstract:

We have analyzed high-resolution and high signal-to-noise ratio optical spectra of nearby FGK stars with and without detected giant planets in order to homogeneously measure their photospheric parameters, mass, age, and the abundances of volatile (C, N, and O) and refractory (Na, Mg, Si, Ca, Ti, V, Mn, Fe, Ni, Cu, and Ba) elements. Our sample contains 309 stars from the solar neighborhood (up to the distance of 100 pc), out of which 140 are dwarfs, 29 are subgiants, and 140 are giants. The photospheric parameters are derived from the equivalent widths of Fe I and Fe II lines. Masses and ages come from the interpolation in evolutionary tracks and isochrones on the HR diagram. The abundance determination is based on the equivalent widths of selected atomic lines of the refractory elements and on the spectral synthesis of C_2, CN, C I, O I, and Na I features. We apply a set of statistical methods to analyze the abundances derived for the three subsamples. Our results show that: i) giant stars systematically exhibit underabundance in [C/Fe] and overabundance in [N/Fe] and [Na/Fe] in comparison with dwarfs, a result that is normally attributed to evolution-induced mixing processes in the envelope of evolved stars; ii) for solar analogs only, the abundance trends with the condensation temperature of the elements are correlated with age and anticorrelated with the surface gravity, which is in agreement with recent studies; iii) as in the case of [Fe/H], dwarf stars with giant planets are systematically enriched in [X/H] for all the analyzed elements, except for O and Ba (the former due to limitations of statistics), confirming previous findings in the literature that not only iron has an important relation with the planetary formation; and iv) giant planet hosts are also significantly overabundant for the same metallicity when the elements from Mg to Cu are combined together.

GSC 6214-210 ABb: a 15 Jupiter Mass Orbiting a Binary Stellar System at 330 AU

An ALMA Constraint on the GSC 6214-210 B Circum-Substellar Accretion Disk Mass

Authors:

Bowler et al

Abstract:

We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of GSC 6214-210 A and B, a solar-mass member of the 5-10 Myr Upper Scorpius association with a 15 ± 2 Mjup companion orbiting at ≈330 AU (2.2"). Previous photometry and spectroscopy spanning 0.3-5 μm revealed optical and thermal excess as well as strong Hα and Pa~β emission originating from a circum-substellar accretion disk around GSC 6214-210 B, making it the lowest mass companion with unambiguous evidence of a subdisk. Despite ALMA's unprecedented sensitivity and angular resolution, neither component was detected in our 880 μm (341 GHz) continuum observations down to a 3-σ limit of 0.22 mJy/beam. The corresponding constraints on the dust mass and total mass are less than 0.15 Mearth and less than 0.05 Mjup, respectively, or less than 0.003% and less than 0.3% of the mass of GSC 6214-210 B itself assuming a 100:1 gas-to-dust ratio and characteristic dust temperature of 10-20 K. If the host star possesses a putative circum-stellar disk then at most it is a meager 0.0015% of the primary mass, implying that giant planet formation has certainly ceased in this system. Considering these limits and its current accretion rate, GSC 6214-210 B appears to be at the end stages of assembly and is not expected to gain any appreciable mass over the next few Myr.

Orbital Evolution of Asteroids and Exoplanets Around Giant Stars

The orbital evolution of asteroids, pebbles and planets from giant branch stellar radiation and winds

Authors:

Veras et al

Abstract:

The discovery of over 50 planets around evolved stars and more than 35 debris discs orbiting white dwarfs highlight the increasing need to understand small body evolution around both early and asymptotic giant branch (GB) stars. Pebbles and asteroids are susceptible to strong accelerations from the intense luminosity and winds of GB stars. Here, we establish equations that can model time-varying GB stellar radiation, wind drag and mass loss. We derive the complete three-dimensional equations of motion in orbital elements due to (1) the Epstein and Stokes regimes of stellar wind drag, (2) Poynting-Robertson drag, and (3) the Yarkovsky drift with seasonal and diurnal components. We prove through averaging that the potential secular eccentricity and inclination excitation due to Yarkovsky drift can exceed that from Poynting-Robertson drag and radiation pressure by at least three orders of magnitude, possibly flinging asteroids which survive YORP spin-up into a widely dispersed cloud around the resulting white dwarf. The GB Yarkovsky effect alone may change an asteroid's orbital eccentricity by ten per cent in just one Myr. Damping perturbations from stellar wind drag can be just as extreme, but are strongly dependent on the highly uncertain local gas density and mean free path length. We conclude that GB radiative and wind effects must be considered when modelling the post-main-sequence evolution of bodies smaller than about 1000 km.

HD 11560's Kuiper Belt Directly Imaged, Slightly Blue to Grey Color, Probably Water Ice

Direct Imaging and Spectroscopy of a Young Extrasolar Kuiper Belt in the Nearest OB Association

Authors:

Currie et al

Abstract:

We describe the discovery of a bright, young Kuiper belt-like debris disk around HD 115600, a ∼ 1.4--1.5 M⊙, ∼ 15 Myr old member of the Sco-Cen OB Association. Our H-band coronagraphy/integral field spectroscopy from the \textit{Gemini Planet Imager} shows the ring has a (luminosity scaled) semi major axis of (∼ 22 AU) ∼ 48 AU, similar to the current Kuiper belt. The disk appears to have neutral scattering dust, is eccentric (e ∼ 0.1--0.2), and could be sculpted by analogues to the outer solar system planets. Spectroscopy of the disk ansae reveal a slightly blue to gray disk color, consistent with major Kuiper belt chemical constituents, where water-ice is a very plausible dominant constituent. Besides being the first object discovered with the next generation of extreme adaptive optics systems (i.e. SCExAO, GPI, SPHERE), HD 115600's debris ring and planetary system provides a key reference point for the early evolution of the solar system, the structure and composition of the Kuiper belt, and the interaction between debris disks and planets.

Tuesday, May 26, 2015

Photosynthesis has a Unique Biosignature

Biological signatures in clumped isotopes of O2

Authors:

Yeung et al

Abstract:

The abundances of molecules containing more than one rare isotope have been applied broadly to determine formation temperatures of natural materials. These applications of “clumped” isotopes rely on the assumption that isotope-exchange equilibrium is reached, or at least approached, during the formation of those materials. In a closed-system terrarium experiment, we demonstrate that biological oxygen (O2) cycling drives the clumped-isotope composition of O2 away from isotopic equilibrium. Our model of the system suggests that unique biological signatures are present in clumped isotopes of O2—and not formation temperatures. Photosynthetic O2 is depleted in 18O18O and 17O18O relative to a stochastic distribution of isotopes, unlike at equilibrium, where heavy-isotope pairs are enriched. Similar signatures may be widespread in nature, offering new tracers of biological and geochemical cycling.

Characterizing the Habitable Zones of Exoplanetary Systems with a Large Ultraviolet/Visible/Near-IR Space Observatory

Characterizing the Habitable Zones of Exoplanetary Systems with a Large Ultraviolet/Visible/Near-IR Space Observatory

Authors:

France et al

Abstract:

Understanding the surface and atmospheric conditions of Earth-size, rocky planets in the habitable zones (HZs) of low-mass stars is currently one of the greatest astronomical endeavors. Knowledge of the planetary effective surface temperature alone is insufficient to accurately interpret biosignature gases when they are observed in the coming decades. The UV stellar spectrum drives and regulates the upper atmospheric heating and chemistry on Earth-like planets, is critical to the definition and interpretation of biosignature gases, and may even produce false-positives in our search for biologic activity. This white paper briefly describes the scientific motivation for panchromatic observations of exoplanetary systems as a whole (star and planet), argues that a future NASA UV/Vis/near-IR space observatory is well-suited to carry out this work, and describes technology development goals that can be achieved in the next decade to support the development of a UV/Vis/near-IR flagship mission in the 2020s.

Estimating the Success of Biosignature Hunting Missions

Impact of ηearth on the capabilities of affordable space missions to detect biosignatures on extrasolar planets

Authors:

Leger et al

Abstract:

We present an analytic model to estimate the capabilities of space missions dedicated to the search for biosignatures in the atmosphere of rocky planets located in the habitable zone of nearby stars. Relations between performance and mission parameters such as mirror diameter, distance to targets, and radius of planets, are obtained. Two types of instruments are considered: coronagraphs observing in the visible, and nulling interferometers in the thermal infrared. Missions considered are: single-pupil coronagraphs with a 2.4 m primary mirror, and formation flying interferometers with 4 x 0.75 m collecting mirrors. The numbers of accessible planets are calculated as a function of {\eta}earth. When Kepler gives its final estimation for {\eta}earth, the model will permit a precise assessment of the potential of each instrument. Based on current estimations, {\eta}earth = 10% around FGK stars and 50% around M stars, the coronagraph could study in spectroscopy only ~1.5 relevant planets, and the interferometer ~14.0. These numbers are obtained under the major hypothesis that the exozodiacal light around the target stars is low enough for each instrument. In both cases, a prior detection of planets is assumed and a target list established. For the long-term future, building both types of spectroscopic instruments, and using them on the same targets, will be the optimal solution because they provide complementary information. But as a first affordable space mission, the interferometer looks the more promising in term of biosignature harvest.

Monday, May 25, 2015

Helium may be Present in Warm Neptune Gliese 436b's Atmosphere

Helium Atmospheres on Warm Neptune- and Sub-Neptune-Sized Exoplanets and Applications to GJ 436 b

Authors:

Hu et al

Abstract:

Warm Neptune- and sub-Neptune-sized exoplanets in orbits smaller than Mercury's are thought to have experienced extensive atmospheric evolution. Here we propose that a potential outcome of this atmospheric evolution is the formation of helium-dominated atmospheres. The hydrodynamic escape rates of Neptune- and sub-Neptune-sized exoplanets are comparable to the diffusion-limited escape rate of hydrogen, and therefore the escape is heavily affected by diffusive separation between hydrogen and helium. A helium atmosphere can thus be formed -- from a primordial hydrogen-helium atmosphere -- via atmospheric hydrodynamic escape from the planet. The helium atmosphere has very different abundances of major carbon and oxygen species from those of a hydrogen atmosphere, leading to distinctive transmission and thermal emission spectral features. In particular, the hypothesis of a helium-dominated atmosphere can explain the thermal emission spectrum of GJ 436 b, a warm Neptune-sized exoplanet, while also consistent with the transmission spectrum. This model atmosphere contains trace amounts of hydrogen, carbon, and oxygen, with the predominance of CO over CH4 as the main form of carbon. With our atmospheric evolution model, we find that if the mass of the initial atmosphere envelope is 1E-3 planetary mass, hydrodynamic escape can reduce the hydrogen abundance in the atmosphere by several orders of magnitude in ~10 billion years. Observations of exoplanet transits may thus detect signatures of helium atmospheres and probe the evolutionary history of small exoplanets.

Forming Orbital Resonances With two SuperEarths During Exoplanetary Migration

On the migration of two planets in a disc and the formation of mean motion resonances

Author:

Migaszewski

Abstract:

We study the dynamics of a system of two super-Earths embedded in a protoplanetary disc. Depending on the disc parameters, planets' masses and positions in the disc, the migration of a planet can be inward or outward and the migration of a two-planet system can be convergent or divergent. The convergent migration means that the period ratio P2/P1 decreases in time. In such a case mean motion resonance (MMR) can be formed when the period ratio reaches a resonant value of a first order MMR (p+1)/p, where p is a small integer. When the divergent migration occurs, P2/P1 increases in time and a system initially close to MMR moves away from the resonance. We build a simple model of an irradiated viscous disc and use analytical prescriptions for the planet-disc interactions. We performed 3500 simulations of the migration of two-planet systems with various masses and initial orbits. We found that approximately half of the systems end up as configurations involved in one of the first order MMRs such as 2:1, 3:2, 4:3 and 5:4. In all these cases both resonant angles of a given MMR librate. The first angle librates around 0, the second around pi, when the period ratio is larger than (p+1)/p. The situation is reversed when P2/P1 less than (p+1)/p. The amplitudes of the librations depend on the period ratio and does not depend on the planets' masses. The amplitudes are minimal and ~0, when P2/P1~(p+1)/p and increase when P2/P1 deviates from the nominal value. We found the range of the period ratios for which the angles of 2:1 MMR librate to be [1.72, 2.12]. The range for 3:2 MMR is [1.4, 1.7]. The upper limit of P2/P1 for which the resonant angles of 4:3 MMR librate is 1.4. The lower limit for this resonance as well as the range for 5:4 MMR could not be determined due to too few solutions with P2/P1 less than 1.33. We found that almost all the systems evolve periodically.

Hot SuperEarth Gliese 1214b is Cloudy

An LBT view of the atmosphere of GJ1214b

Authors:

Nascimbeni et al

Abstract:

The atmospheric composition and vertical structure of the super-Earth GJ1214b has been questioned since its discovery in 2009. Recent studies point towards the presence of high-altitude clouds masking the lower layers. Anyway, some data points, gathered at different times and facilities, do not fit well into this picture, probably because of a combination of stellar activity and systematic errors. We observed two transits of GJ1214b with the LBC dual-channel camera at the Large Binocular Telescope. For the first time, we measured the relative planetary radius k=Rp/R⋆ at blue and red optical wavelengths simultaneously (B+R), thus constraining the presence of Rayleigh scattering on GJ1214b after correcting for stellar activity effects. To the same purpose, a long-term photometric follow-up of the host star was carried out with WiFSIP@STELLA, revealing a rotational period significantly longer than that previously reported. Our new unbiased estimates of k yield a flat transmission spectra extending to shorter wavelengths, thus confirming the "cloudy atmosphere" scenario for GJ1214b.

Sunday, May 24, 2015

Modeling Massive Rings

Gravito-electromagnetic Effects of Massive Rings

Author:

Ruggiero et al

Abstract:

The Einstein field equations in linear post-Newtonian approximation can be written in analogy with electromagnetism, in the so-called gravito-electromagnetic formalism. We use this analogy to study the gravitational field of a massive ring: in particular, we consider a continuous mass distribution on Keplerian orbit around a central body, and we work out the gravitational field generated by this mass distribution in the intermediate zone between the central body and the ring, focusing on the gravito-magnetic component that originates from the rotation of the ring. In doing so, we generalize and complement some previous results that focused on the purely Newtonian effects of the ring (thus neglecting its rotation) or that were applied to the case of rotating spherical shells. Eventually, we study in some simple cases the effect of the the rotation of the ring, and suggest that, in principle, this approach could be used to infer information about the angular momentum of the ring.

Maximizing Sensitivity and Yield from Space-Based Microlens Parallax Surveys for Exoplanets

Criteria for Sample Selection to Maximize Planet Sensitivity and Yield from Space-Based Microlens Parallax Surveys

Authors:

Yee et al

Abstract:

Space-based microlens parallax measurements are a powerful tool for understanding planet populations, especially their distribution throughout the Galaxy. However, if space-based observations of the microlensing events must be specifically targeted, it is crucial that microlensing events enter the parallax sample without reference to the known presence or absence of planets. Hence, it is vital to define objective criteria for selecting events where possible and to carefully consider and minimize the selection biases where not possible so that the final sample represents a controlled experiment.

We present objective criteria for initiating observations and determining their cadence for a subset of events, and we define procedures for isolating subjective decision making from information about detected planets for the remainder of events. We also define procedures to resolve conflicts between subjective and objective selections. These procedures maximize planet sensitivity of the sample as a whole by allowing for planet detections even if they occur before satellite observations for objectively-selected events and by helping to trigger fruitful follow-up observations for subjectively-chosen events. This paper represents our public commitment to these procedures, which is a necessary component of enforcing objectivity on the experimental protocol.

Using Asteroseismology to Determine Kepler Exoplanetary Host Stars' Ages and Fundamental Properties

Ages and fundamental properties of Kepler exoplanet host stars from asteroseismology

Authors:

Aguirre et al

Abstract:

We present a study of 33 planet-candidate host stars for which asteroseismic observations have sufficiently high signal-to-noise ratio to allow extraction of individual pulsation frequencies. We implement a new Bayesian scheme that is flexible in its input to process individual oscillation frequencies, combinations of them, and average asteroseismic parameters, and derive robust fundamental properties for these targets. Applying this scheme to grids of evolutionary models yields stellar properties with median statistical uncertainties of 1.1% (radius), 1.7% (density), 3.3% (mass), 4.4% (distance), and 14% (age), making this the exoplanet host-star sample with the most precise and uniformly determined fundamental parameters to date. We assess the systematics from changes in the solar abundances and mixing-length parameter, showing that they are smaller than the statistical errors. We also determine the stellar properties with three other fitting algorithms and explore the systematics arising from using different evolution and pulsation codes, resulting in 1% in density and radius, and 2% and 7% in mass and age, respectively. We identify the initial helium abundance as a source of systematics comparable to our statistical uncertainties, and discuss future prospects for constraining this parameter by combining asteroseismology and data from space missions. Finally we compare our derived properties with those obtained using the global average asteroseismic observables along with effective temperature and metallicity, finding an excellent level of agreement. Owing to selection effects, our results show that the majority of the high signal-to-noise ratio asteroseismic Kepler host stars are older than the Sun.

Saturday, May 23, 2015

Dust Trapped in Protostellar Disk Spiral Arms due to Gravitational Instability

Dust trapping by spiral arms in gravitationally unstable protostellar discs

Authors:

Dipierro et al

Abstract:

In this paper we discuss the influence of gravitational instabilities in massive protostellar discs on the dynamics of dust grains. Starting from a Smoothed Particle Hydrodynamics (SPH) simulation, we have computed the evolution of the dust in a quasi-static gas density structure typical of self-gravitating disc. For different grain size distributions we have investigated the capability of spiral arms to trap particles. We have run 3D radiative transfer simulations in order to construct maps of the expected emission at (sub-)millimetre and near-infrared wavelengths. Finally, we have simulated realistic observations of our disc models at (sub-)millimetre and near-infrared wavelengths as they may appear with the Atacama Large Millimetre/sub-millimetre Array (ALMA) and the High-Contrast Coronographic Imager for Adaptive Optics (HiCIAO) in order to investigate whether there are observational signatures of the spiral structure. We find that the pressure inhomogeites induced by gravitational instabilities produce a non-negligible dynamical effect on centimetre sized particles leading to significant overdensities in spiral arms. We also find that the spiral structure is readily detectable by ALMA over a wide range of (sub-)millimetre wavelengths and by HiCIAO in near-infrared scattered light for non-face-on discs located in the Ophiucus star-forming region. In addition, we find clear spatial spectral index variations across the disc, revealing that the dust trapping produces a migration of large grains that can be potentially investigated through multi-wavelenghts observations in the (sub-)millimetric. Therefore, the spiral arms observed to date in protoplanetary disc might be interpreted as density waves induced by the development of gravitational instabilities.

Particle Trapping in Protoplanetary Disk Vortices

PARTICLE TRAPPING AND STREAMING INSTABILITY IN VORTICES IN PROTOPLANETARY DISKS

Authors:

Raettig et al

Abstract:

We analyze the concentration of solid particles in vortices created and sustained by radial buoyancy in protoplanetary disks, e.g., baroclinic vortex growth. Besides the gas drag acting on particles, we also allow for back-reaction from dust onto the gas. This becomes important when the local dust-to-gas ratio approaches unity. In our two-dimensional, local, shearing sheet simulations, we see high concentrations of grains inside the vortices for a broad range of Stokes numbers, St. An initial dust-to-gas ratio of 1:100 can easily be reversed to 100:1 for St = 1.0. The increased dust-to-gas ratio triggers the streaming instability, thus counter-intuitively limiting the maximal achievable overdensities. We find that particle trapping inside vortices opens the possibility for gravity assisted planetesimal formation even for small particles (St = 0.01) and a low initial dust-to-gas ratio of 1:104, e.g., much smaller than in the previously studied magnetohydrodynamic zonal flow case.

A Ring of C2H in the Molecular Disk Orbiting TW Hya

A Ring of C2H in the Molecular Disk Orbiting TW Hya

Authors:

Kastner et al

Abstract:

We have used the Submillimeter Array to image, at ~1.5" resolution, C2H (3-2) emission from the circumstellar disk orbiting the nearby (D = 54 pc), ~8 Myr-old, ~0.8 Msun classical T Tauri star TW Hya. The SMA imaging reveals that the C2H emission exhibits a ring-like morphology. Based on a model in which the C2H column density follows a truncated radial power-law distribution, we find that the inner edge of the ring lies at ~45 AU, and that the ring extends to at least ~120 AU. Comparison with previous (single-dish) observations of C2H (4-3) emission indicates that the C2H molecules are subthermally excited and, hence, that the emission arises from the relatively warm, tenuous upper atmosphere of the disk. We propose that the C2H emission most likely traces particularly efficient photo-destruction of small grains and/or photodesorption and photodissociation of hydrocarbons derived from grain ice mantles in the surface layers of the outer disk. The presence of a C2H ring in the TW Hya disk hence likely serves as a marker of dust grain processing and radial and vertical grain size segregation within the disk.

Friday, May 22, 2015

Turbulent Wakes Around High Mass Exoplanets/Brown Dwarfs can Destroy Protoplanetary Disk Gaps

TURBULENT WAKES DESTROY GAPS AROUND HIGH-MASS PLANETS OR BROWN DWARFS IN RADIATIVELY INEFFICIENT DISKS

Authors:

Richert et al

Abstract:

Recent observations of gaps and non-axisymmetric features in the dust distributions of transition disks have been interpreted as evidence of embedded massive protoplanets. However, comparing the predictions of planet–disk interaction models to the observed features has shown far from perfect agreement. This may be due to the strong approximations used for the predictions. For example, spiral arm fitting typically uses results that are based on low-mass planets in an isothermal gas. In this work, we describe two-dimensional, global, hydrodynamical simulations of disks with embedded protoplanets, with and without the assumption of local isothermality, for a range of planet-to-star mass ratios 1–10 ${{M}_{{\rm J}}}$ for a 1 ${{M}_{\odot }}$ star. We use the Pencil Code in polar coordinates for our models. We find that the inner and outer spiral wakes of massive protoplanets ($M\gtrsim 5\;{{M}_{{\rm J}}}$) produce significant shock heating that can trigger buoyant instabilities. These drive sustained turbulence throughout the disk when they occur. The strength of this effect depends strongly on the mass of the planet and the thermal relaxation timescale; for a $10\;{{M}_{{\rm J}}}$ planet embedded in a thin, purely adiabatic disk, the spirals, gaps, and vortices typically associated with planet–disk interactions are disrupted. We find that the effect is only weakly dependent on the initial radial temperature profile. The spirals that form in disks heated by the effects we have described may fit the spiral structures observed in transition disks better than the spirals predicted by linear isothermal theory.

Spectroscopy of Numerous Brown Dwarfs Found by WISE Using Hubble

HUBBLE SPACE TELESCOPE SPECTROSCOPY OF BROWN DWARFS DISCOVERED WITH THE WIDE-FIELD INFRARED SURVEY EXPLORER

Authors:

Schneider et al

Abstract:

We present a sample of brown dwarfs identified with the Wide-field Infrared Survey Explorer (WISE) for which we have obtained Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) near-infrared grism spectroscopy. The sample (22 in total) was observed with the G141 grism covering 1.10–1.70 μm, while 15 were also observed with the G102 grism, which covers 0.90–1.10 μm. The additional wavelength coverage provided by the G102 grism allows us to (1) search for spectroscopic features predicted to emerge at low effective temperatures (e.g.,ammonia bands) and (2) construct a smooth spectral sequence across the T/Y boundary. We find no evidence of absorption due to ammonia in the G102 spectra. Six of these brown dwarfs are new discoveries, three of which are found to have spectral types of T8 or T9. The remaining three, WISE J082507.35+280548.5 (Y0.5), WISE J120604.38+840110.6 (Y0), and WISE J235402.77+024015.0 (Y1), are the 19th, 20th, and 21st spectroscopically confirmed Y dwarfs to date. We also present HST grism spectroscopy and reevaluate the spectral types of five brown dwarfs for which spectral types have been determined previously using other instruments.

J125601.92-125723.9B: an L Class Brown Dwarf Nemesis ~41 Light Years Away

Discovery of a young planetary mass companion to the nearby M dwarf VHS J125601.92-125723.9

Authors:

Gauza et al

Abstract:

In a search for common proper motion companions using the VISTA Hemisphere Survey and 2MASS catalogs we have identified a very red (J-Ks=2.47 mag) late-L dwarf companion of a previously unrecognized M dwarf VHS J125601.92-125723.9, located at a projected angular separation of 8.06"+/-0.03". From low-resolution optical and near-IR spectroscopy we classified the primary and the companion as an M7.5+/-0.5 and L7+/-1.5, respectively. The primary shows weaker alkali lines than field dwarfs of similar spectral type, but still consistent with either a high-gravity dwarf or a younger object of hundreds of millions of years. The secondary shows spectral features characteristic for low surface gravity objects at ages below several hundred Myr, like the triangular shape of the H-band continuum and alkali lines weaker than in field dwarfs of the same spectral type. The absence of lithium in the atmosphere of the primary and the likely membership to the Local Association allowed us to constrain the age of the system to the range of 150-300 Myr. We report a measurement of the trigonometric parallax pi=78.8+/-6.4 mas, which translates into a distance of 12.7+/-1.0 pc; the pair thus has a projected physical separation of 102+/-9 AU. We derived the Lbol of the components and compared them with theoretical evolutionary models to estimate the masses and effective temperatures. For the primary, we determined log(Lbol/LSun)=-3.14+/-0.10, and a mass of 73 (+20,-15} MJup at the boundary between stars and brown dwarfs and Teff of 2620+/-140 K. For the companion we obtained log(Lbol/LSun)=-5.05+/-0.22 and a mass of 11.2 (+9.7,-1.8) MJup placing it near the deuterium-burning mass limit. The effective temperature derived from evolutionary models is 880 (+140,-110) K, about 400-700 K cooler than expected for field late-L dwarfs.

Thursday, May 21, 2015

Temperature Inversion Detected in Hot Jupiter WASP-33b's Dayside Atmosphere

Spectroscopic Evidence for a Temperature Inversion in the Dayside Atmosphere of the Hot Jupiter WASP-33b

Authors:

Haynes et al

Abstract:

We present observations of two occultations of the extrasolar planet WASP-33b using the Wide Field Camera 3 (WFC3) on the HST, which allow us to constrain the temperature structure and composition of its dayside atmosphere. WASP-33b is the most highly irradiated hot Jupiter discovered to date, and the only exoplanet known to orbit a delta-Scuti star. We observed in spatial scan mode to decrease instrument systematic effects in the data, and removed fluctuations in the data due to stellar pulsations. The RMS for our final, binned spectrum is approximately 1.05 times the photon noise. We compare our final spectrum, along with previously published photometric data, to atmospheric models of WASP-33b spanning a wide range in temperature profiles and chemical compositions. We find that the data require models with an oxygen-rich chemical composition and a temperature profile that increases at high altitude. We also find that our spectrum displays an excess in the measured flux towards short wavelengths that is best explained as emission from TiO. If confirmed by additional measurements at shorter wavelengths, this planet would become the first hot Jupiter with a temperature inversion that can be definitively attributed to the presence of TiO in its dayside atmosphere.

Hot Jupiter Kepler-7b Case Study: The Effects of Longitude-Dependent Cloud Coverage on Light Phase Curves

EFFECT OF LONGITUDE-DEPENDENT CLOUD COVERAGE ON EXOPLANET VISIBLE WAVELENGTH REFLECTED-LIGHT PHASE CURVES

Authors:

Webber et al

Abstract:

We use a planetary albedo model to investigate variations in visible wavelength phase curves of exoplanets. Thermal and cloud properties for these exoplanets are derived using one-dimensional radiative-convective and cloud simulations. The presence of clouds on these exoplanets significantly alters their planetary albedo spectra. We confirm that non-uniform cloud coverage on the dayside of tidally locked exoplanets will manifest as changes to the magnitude and shift of the phase curve. In this work, we first investigate a test case of our model using a Jupiter-like planet, at temperatures consistent to 2.0 AU insolation from a solar type star, to consider the effect of H2O clouds. We then extend our application of the model to the exoplanet Kepler-7b and consider the effect of varying cloud species, sedimentation efficiency, particle size, and cloud altitude. We show that, depending on the observational filter, the largest possible shift of the phase curve maximum will be ~2°–10° for a Jupiter-like planet, and up to ~30° (~0.08 in fractional orbital phase) for hot-Jupiter exoplanets at visible wavelengths as a function of dayside cloud distribution with a uniformly averaged thermal profile. The models presented in this work can be adapted for a variety of planetary cases at visible wavelengths to include variations in planet–star separation, gravity, metallicity, and source-observer geometry. Finally, we tailor our model for comparison with, and confirmation of, the recent optical phase-curve observations of Kepler-7b with the Kepler space telescope. The average planetary albedo can vary between 0.1 and 0.6 for the 1300 cloud scenarios that were compared to the observations. Many of these cases cannot produce a high enough albedo to match the observations. We observe that smaller particle size and increasing cloud altitude have a strong effect on increasing albedo. In particular, we show that a set of models where Kepler-7b has roughly half of its dayside covered in small-particle clouds high in the atmosphere, made of bright minerals like MgSiO3 and Mg2SiO4, provide the best fits to the observed offset and magnitude of the phase-curve, whereas Fe clouds are found to be too dark to fit the observations.

XO-2b: a hot Jupiter With a Variable Host Star

XO-2b: a hot Jupiter with a variable host star that potentially affects its measured transit depth

Authors:

Zellem et al

Abstract:

The transiting hot Jupiter XO-2b is an ideal target for multi-object photometry and spectroscopy as it has a relatively bright (V-mag = 11.25) K0V host star (XO-2N) and a large planet-to-star contrast ratio (Rp/Rs≈0.015). It also has a nearby (31.21") binary stellar companion (XO-2S) of nearly the same brightness (V-mag = 11.20) and spectral type (G9V), allowing for the characterization and removal of shared systematic errors (e.g., airmass brightness variations). We have therefore conducted a multiyear (2012--2015) study of XO-2b with the University of Arizona's 61" (1.55~m) Kuiper Telescope and Mont4k CCD in the Bessel U and Harris B photometric passbands to measure its Rayleigh scattering slope to place upper limits on the pressure-dependent radius at, e.g., 10~bar. Such measurements are needed to constrain its derived molecular abundances from primary transit observations. We have also been monitoring XO-2N since the 2013--2014 winter season with Tennessee State University's Celestron-14 (0.36~m) automated imaging telescope to investigate stellar variability, which could affect XO-2b's transit depth. Our observations indicate that XO-2N is variable, potentially due to spots, with a peak-to-peak amplitude of 0.00455±0.00090~R-mag and period of 29.87±0.19 days. Due to the likely influence of XO-2N's variability on the derivation of XO-2b's transit depth, we cannot bin multiple nights of data to decrease our uncertainties, preventing us from constraining its gas abundances. This study demonstrates that long-term monitoring programs of exoplanet host stars are crucial for understanding host star variability.

Wednesday, May 20, 2015

Why Hart Found Narrow Ecospheres—A Minor Science Mystery Solved

Why Hart Found Narrow Ecospheres—A Minor Science Mystery Solved

Author:

Levenson

Abstract:

Aims:

To explain why two NASA computer simulation studies in the 1970s (Hart, 1978, 1979) briefly rocked the subfield of astrobiology and SETI studies by showing very narrow habitable zones (HZs) for solar-type stars. Although other studies later supported wider HZs, it was never clear why the Hart simulations found the narrow limits they did.

Results:

Investigation of the state of climate studies and radiative transfer models in the period 1960–1970 provides a likely explanation.

Conclusion:

Hart's findings were in line with earlier results, preventing him from noticing that his radiation model was inadequate.

Modeling Methane Exoplanets

Methane Planets and their Mass-Radius Relation

Authors:

Helled et al

Abstract:

Knowledge of both the mass and radius of an exoplanet allows us to estimate its mean density, and therefore, its composition. Exoplanets seem to fill a very large parameter space in terms of mass and composition, and unlike the solar-system's planets, exoplanets also have intermediate masses (~5-50 M_Earth) with various densities. In this letter, we investigate the behavior of the Mass-Radius relation for methane (CH_4) planets and show that when methane planets are massive enough (M_planet greater than ~15 M_Earth) the methane can dissociate and lead to a differentiated planet with a carbon core, a methane envelope, and a hydrogen atmosphere. The contribution of a rocky core to the behavior of CH_4 planet is considered as well. We also develop interior models for several detected intermediate-mass planets that could, in principle, be methane/methane-rich planets. The example of methane planets emphasizes the complexity of the Mass-Radius relation and the challenge in inferring the planetary composition uniquely.

Update: Forming Giant Moons Around SuperJovian Exoplanets

Water ice lines and the formation of giant moons around super-Jovian planets

Authors:

Heller et al

Abstract:

Most of the exoplanets with known masses at Earth-like distances to Sun-like stars are heavier than Jupiter, which raises the question of whether such planets are accompanied by detectable, possibly habitable moons. Here we simulate the accretion disks around super-Jovian planets and find that giant moons with masses similar to Mars can form. Our results suggest that the Galilean moons formed during the final stages of accretion onto Jupiter, when the circumjovian disk was sufficiently cool. But in contrast to other studies, with our assumptions, we show that Jupiter was still feeding from the circumsolar disk and that its principal moons cannot have formed after the complete photoevaporation of the circumsolar nebula. To counteract the steady loss of moons into the planet due to type I migration, we propose that the water ice line around Jupiter and super-Jovian exoplanets acted as a migration trap for moons. Heat transitions, however, cross the disk during the gap opening within 10^4 yr, which makes them inefficient as moon traps. This indicates a fundamental difference between planet and moon formation. We find that icy moons larger than the smallest known exoplanet can form at about 15 - 30 Jupiter radii around super-Jovian planets. Their size implies detectability by the Kepler and PLATO space telescopes as well as by the European Extremely Large Telescope.

Original.

Tuesday, May 19, 2015

Nonphotosynthetic Pigments as Potential Biosignatures

Nonphotosynthetic Pigments as Potential Biosignatures

Authors:

Schwieterman et al

Abstract:

Previous work on possible surface reflectance biosignatures for Earth-like planets has typically focused on analogues to spectral features produced by photosynthetic organisms on Earth, such as the vegetation red edge. Although oxygenic photosynthesis, facilitated by pigments evolved to capture photons, is the dominant metabolism on our planet, pigmentation has evolved for multiple purposes to adapt organisms to their environment. We present an interdisciplinary study of the diversity and detectability of nonphotosynthetic pigments as biosignatures, which includes a description of environments that host nonphotosynthetic biologically pigmented surfaces, and a lab-based experimental analysis of the spectral and broadband color diversity of pigmented organisms on Earth. We test the utility of broadband color to distinguish between Earth-like planets with significant coverage of nonphotosynthetic pigments and those with photosynthetic or nonbiological surfaces, using both 1-D and 3-D spectral models. We demonstrate that, given sufficient surface coverage, nonphotosynthetic pigments could significantly impact the disk-averaged spectrum of a planet. However, we find that due to the possible diversity of organisms and environments, and the confounding effects of the atmosphere and clouds, determination of substantial coverage by biologically produced pigments would be difficult with broadband colors alone and would likely require spectrally resolved data.

The Impact of Disk Winds on Terrestrial Exoplanet Formation

Formation of terrestrial planets in disks evolving via disk winds and implications for the origin of the solar system's terrestrial planets

Authors:

Ogihara et al

Abstract:

Recent three-dimensional magnetohydrodynamical simulations have identified a disk wind by which gas materials are lost from the surface of a protoplanetary disk, which can significantly alter the evolution of the inner disk and the formation of terrestrial planets. A simultaneous description of the realistic evolution of the gaseous and solid components in a disk may provide a clue for solving the problem of the mass concentration of the terrestrial planets in the solar system. We simulate the formation of terrestrial planets from planetary embryos in a disk that evolves via magnetorotational instability and a disk wind. The aim is to examine the effects of a disk wind on the orbital evolution and final configuration of planetary systems. We perform N-body simulations of sixty 0.1 Earth-mass embryos in an evolving disk. The evolution of the gas surface density of the disk is tracked by solving a one-dimensional diffusion equation with a sink term that accounts for the disk wind. We find that even in the case of a weak disk wind, the radial slope of the gas surface density of the inner disk becomes shallower, which slows or halts the type I migration of embryos. If the effect of the disk wind is strong, the disk profile is significantly altered (e.g., positive surface density gradient, inside-out evacuation), leading to outward migration of embryos inside ~ 1 AU. Disk winds play an essential role in terrestrial planet formation inside a few AU by changing the disk profile. In addition, embryos can undergo convergent migration to ~ 1 AU in certainly probable conditions. In such a case, the characteristic features of the solar system's terrestrial planets (e.g., mass concentration around 1 AU, late giant impact) may be reproduced.

Multiple Kepler "Terrestrial" Exoplanets are Really SuperEarths and Mini Neptunes

REVISION OF EARTH-SIZED KEPLER PLANET CANDIDATE PROPERTIES WITH HIGH-RESOLUTION IMAGING BY THE HUBBLE SPACE TELESCOPE

Authors:

Cartier et al

Abstract:

We present the results of our Hubble Space Telescope program and describe how our analysis methods were used to re-evaluate the habitability of some of the most interesting Kepler planet candidates. Our program observed 22 Kepler Object of Interest (KOI) host stars, several of which were found to be multiple star systems unresolved by Kepler. We use our high-resolution imaging to spatially resolve the stellar multiplicity of Kepler-296, KOI-2626, and KOI-3049, and develop a conversion to the Kepler photometry (Kp) from the F555W and F775W filters on WFC3/UVIS. The binary system Kepler-296 (five planets) has a projected separation of $0\buildrel{\prime\prime}\over{.} 217$ (80 AU); KOI-2626 (one planet candidate) is a triple star system with a projected separation of $0\buildrel{\prime\prime}\over{.} 201$ (70 AU) between the primary and secondary components and $0\buildrel{\prime\prime}\over{.} 161$ (55 AU) between the primary and tertiary; and the binary system KOI-3049 (one planet candidate) has a projected separation of $0\buildrel{\prime\prime}\over{.} 464$ (225 AU). We use our measured photometry to fit the separated stellar components to the latest Victoria–Regina Stellar Models with synthetic photometry to conclude that the systems are coeval. The components of the three systems range from mid-K dwarf to mid-M dwarf spectral types.We solved for the planetary properties of each system analytically and via an MCMC algorithm using our independent stellar parameters. The planets range from $\sim 1.6\;\;{\rm to}\;\sim 4.2\;{{R}_{\oplus }}$, mostly Super Earths and mini-Neptunes. As a result of the stellar multiplicity, some planets previously in the Habitable Zone are, in fact, not, and other planets may be habitable depending on their assumed stellar host.

Monday, May 18, 2015

λ Orionis Cluster Circumstellar Disks Show Gas Giants and SuperEarths Must Form in Less Than 5 Million Years

A SCUBA-2 850-μm survey of circumstellar disks in the λ Orionis cluster

Authors:

Ansdell et al

Abstract:

We present results from an 850-μm survey of the ∼ 5 Myr old λ Orionis star-forming region. We used the SCUBA-2 camera on the James Clerk Maxwell Telescope to survey a ∼0.5-diameter circular region containing 36 (out of 59) cluster members with infrared excesses indicative of circumstellar disks. We detected only one object at >3σ significance, the Herbig Ae star HD 245185, with a flux density of ∼74 mJy beam−1 corresponding to a dust mass of ∼150 M⊕. Stacking the individually undetected sources did not produce a significant mean signal but gives an upper limit on the average dust mass for λ Orionis disks of ∼3 M⊕. Our follow-up observations of HD 245185 with the Submillimeter Array found weak CO 2--1 line emission with an integrated flux of ∼170 mJy km s−1 but no 13CO or C18O isotopologue emission at 30 mJy km s−1 sensitivity, suggesting a gas mass of ≲1 MJup. The implied gas-to-dust ratio is thus ≳50 times lower than the canonical interstellar medium value, setting HD 245185 apart from other Herbig Ae disks of similar age, which have been found to be gas rich; as HD 245185 also shows signs of accretion, we may be catching it in the final phases of disk clearing. Our study of the λ Orionis cluster places quantitative constraints on planet formation timescales, indicating that at ∼5 Myr the average disk no longer has sufficient dust and gas to form giant planets and perhaps even super Earths; the bulk material has been mostly dispersed or is locked in pebbles/planetesimals larger than a few mm in size.

Giant Planet Zonal Jets

A dynamo driven by zonal jets at the upper surface: Applications to giant planets

Authors:

Guervilly et al

Abstract:

We present a dynamo mechanism arising from the presence of barotropically unstable zonal jet currents in a rotating spherical shell. The shear instability of the zonal flow develops in the form of a global Rossby mode, whose azimuthal wavenumber depends on the width of the zonal jets. We obtain self-sustained magnetic fields at magnetic Reynolds numbers greater than 1000. We show that the propagation of the Rossby waves is crucial for dynamo action. The amplitude of the axisymmetric poloidal magnetic field depends on the wavenumber of the Rossby mode, and hence on the width of the zonal jets. We discuss the plausibility of this dynamo mechanism for generating the magnetic field of the giant planets. Our results suggest a possible link between the topology of the magnetic field and the profile of the zonal winds observed at the surface of the giant planets. For narrow Jupiter-like jets, the poloidal magnetic field is dominated by an axial dipole whereas for wide Neptune-like jets, the axisymmetric poloidal field is weak.

Nemesis Class Giant Planet HD 106906b's Interaction With the Stellarward Dusty Debris Disk

A debris disk under the influence of a wide planetary mass companion: The system of HD106906

Authors:

Jilkova et al

Abstract:

The 13 Myr old star HD106906 is orbited by a debris disk of at least 0.067 M_Moon with an inner and outer radius of 20 AU and 120 AU, respectively, and by a planet at a distance of 650 AU. We use this curious combination of a close low-mass disk and a wide planet to motivate our simulations of this system. We study the parameter space of the initial conditions to quantify the mass loss from the debris disk and its lifetime under the influence of the planet. We find that when the planet orbits closer to the star than about 50 AU and with low inclination relative to the disk (less than about 10 degrees), more disk material is perturbed outside than inside the region constrained by observations on timescales shorter than 1 Myr. Considering the age of the system, such a short lifetime of the disk is incompatible with the timescale for planet--planet scattering which is one of the scenarios suggested to explain the wide separation of the planet. For some configurations when the planet's orbit is inclined with respect to the disk, the latter will start to wobble. We argue that this wobbling is caused by a mechanism similar to the Kozai--Lidov oscillations. We also observe various resonant structures (such as rings and spiral arms) induced in the disk by the planet.

Sunday, May 17, 2015

HD 91962: A Weird Quad Stellar System With 3 Stars Forming in a Protoplanetary-like Circumstellar Disk

The unusual quadruple system HD 91962 with a "planetary" architecture

Authors:

Tokovinin et al

Abstract:

The young nearby solar-type star HD 91962 is a rare quadruple system where three companions revolve around the main component with periods of 170.3 days, 8.85 years, and 205 years. The two outer orbits are nearly co-planar, and all orbits have small eccentricities. We refine the visual orbit of the outer pair, determine the combined spectro-interferometric orbit of the middle 8.8-yr pair and the spectroscopic orbit of the inner binary. The middle and inner orbits are likely locked in a 1:19 resonance, the ratio of the outer and middle periods is ~23. The masses of all components are estimated (inside-out: 1.14, 0.32, 0.64, 0.64 solar mass), the dynamical parallax is 27.4+-0.6 mas. We speculate that this multiple system originated from collapse of an isolated core and that the companions migrated in a dissipative disk. Other multiple systems with similar features (coplanarity, small eccentricity, and period ratio around 20) are known.

A Short Review of the Variability of Young Stellar Objects

Variability of young stellar objects: accretion, disks, outflows and magnetic activity

Author:

Stelzer

Abstract:

This article represents a short review of the variability characteristics of young stellar objects. Variability is a key property of young stars. Two major origins may be distinguished: a scaled-up version of the magnetic activity seen on main-sequence stars and various processes related to circumstellar disks, accretion and outflows.

A Technical Review of Optical Gravitational Lensing Experiment/OGLE-IV

OGLE-IV: Fourth Phase of the Optical Gravitational Lensing Experiment

Authors:

Udalski et al

Abstract:

We present both the technical overview and main science drivers of the fourth phase of the Optical Gravitational Lensing Experiment (hereafter OGLE-IV). OGLE-IV is currently one of the largest sky variability surveys worldwide, targeting the densest stellar regions of the sky. The survey covers over 3000 square degrees in the sky and monitors regularly over a billion sources.

The main targets include the inner Galactic Bulge and the Magellanic System. Their photometry spans the range of 12 less than I less than 21 mag and 13 less than I less than 21.7 mag, respectively. Supplementary shallower Galaxy Variability Survey covers the extended Galactic bulge and 2/3 of the whole Galactic disk within the magnitude range of 10 less than I less than 19 mag. All OGLE-IV surveys provide photometry with milli-magnitude accuracy at the bright end. The cadence of observations varies from 19-60 minutes in the inner Galactic bulge to 1-3 days in the remaining Galactic bulge fields, Magellanic System and the Galactic disk.

OGLE-IV provides the astronomical community with a number of real time services. The Early Warning System (EWS) contains information on two thousand gravitational microlensing events being discovered in real time annually, the OGLE Transient Detection System (OTDS) delivers over 200 supernovae a year. We also provide the real time photometry of unpredictable variables such as optical counterparts to the X-ray sources and R CrB stars.

Hundreds of thousands new variable stars have already been discovered and classified by the OGLE survey. The number of new detections will be at least doubled during the current OGLE-IV phase. The survey was designed and optimized primarily to conduct the second generation microlensing survey for exoplanets. It has already contributed significantly to the increase of the discovery rate of microlensing exoplanets and free-floating planets.

Saturday, May 16, 2015

A Survey of IC 348 Cluster's Protoplanetary Disks

A SCUBA-2 850 micron Survey of Protoplanetary Discs in the IC 348 Cluster

Authors:

Cieza et al

Abstract:

We present 850 micron observations of the 2-3 Myr cluster IC 348 in the Perseus molecular cloud using the SCUBA-2 camera on the James Clerk Maxwell Telescope. Our SCUBA-2 map has a diameter of 30 arcmin and contains ~370 cluster members, including ~200 objects with IR excesses. We detect a total of 13 discs. Assuming standard dust properties and a gas to dust mass ratio of 100, we derive disc masses ranging from 1.5 to 16 M_JUP . We also detect 8 Class 0/I protostars. We find that the most massive discs (M_Disc greater than 3 MJUP ; 850 micron fux greater than 10 mJy) in IC 348 tend to be transition objects according to the characteristic "dip" in their infrared Spectral Energy Distributions (SEDs). This trend is also seen in other regions. We speculate that this could be an initial conditions effect (e.g., more massive discs tend to form giant planets that result in transition disc SEDs) and/or a disc evolution effect (the formation of one or more massive planets results in both a transition disc SED and a reduction of the accretion rate, increasing the lifetime of the outer disc). A stacking analysis of the discs that remain undetected in our SCUBA-2 observations suggests that their median 850 micron flux should be ~1 mJy, corresponding to a disc mass ~0.3 M_JUP (gas plus dust) or ~1 M_Earth of dust. While the available data are not deep enough to allow a meaningful comparison of the disc luminosity functions between IC 348 and other young stellar clusters, our results imply that disc masses exceeding the Minimum Mass Solar Nebula are very rare (~1%) at the age of IC 348, specially around very low-mass stars.

Stellar Formation Clusters Have Strong Effects on the Protoplanetary Disk Size

Strong effect of the cluster environment on the size of protoplanetary discs?

Authors:

Vincke et al

Abstract:

Context.

Most stars are born in clusters, thus the protoplanetary discs surrounding the newly formed stars might be influenced by this environment. Isolated star-disc encounters have previously been studied, and it was shown that very close encounters are necessary to completely destroy discs. However, relatively distant encounters are still able to change the disc size considerably.

Aims.

We quantify the importance of disc-size reduction that is due to stellar encounters in an entire stellar population.

Methods.

We modelled young, massive clusters of different densities using the code Nbody6 to determine the statistics of stellar encounter parameters. In a second step, we used these parameters to investigate the effect of the environments on the disc size. For this purpose, we performed a numerical experiment with an artificial initial disc size of 105 AU.

Results.

We quantify to which degree the disc size is more sensitive to the cluster environment than to the disc mass or frequency. We show that in all investigated clusters a large portion of discs is significantly reduced in size. After 5 Myr, the fraction of discs smaller than 1000 AU in ONC-like clusters with an average number density of 60pc3, the fraction of discs smaller than 1000 AU is 65%, while discs smaller than 100 AU make up 15%. These fractions increase to 84% and 39% for discs in denser clusters like IC 348 (500pc3). Even in clusters with a density four times lower than in the ONC (15pc3), about 43% of all discs are reduced to sizes below 1 000 AU and roughly 9% to sizes below 100 AU.

Conclusions.

For any disc in the ONC that initially was larger than 1 000 AU, the probability to be truncated to smaller disc sizes as a result of stellar encounters is quite high. Thus, among other effects, encounters are important in shaping discs and potentially forming planetary systems in stellar clusters.

Probing the Inner Protoplanetary Disks of AA Tauri Systems in NGC 2264

CSI 2264: Probing the inner disks of AA Tauri-like systems in NGC 2264

Authors:

McGinnis et al

Abstract:

Context.

The classical T Tauri star (CTTS) AA Tau has presented photometric variability that was attributed to an inner disk warp, caused by the interaction between the inner disk and an inclined magnetosphere. Previous studies of the young cluster NGC 2264 have shown that similar photometric behavior is common among CTTS.

Aims.

The goal of this work is to investigate the main causes of the observed photometric variability of CTTS in NGC 2264 that present AA Tau-like light curves, and verify if an inner disk warp could be responsible for their observed variability.

Methods.

In order to understand the mechanism causing these stars’ photometric behavior, we investigate veiling variability in their spectra and u − r color variations and estimate parameters of the inner disk warp using an occultation model proposed for AA Tau. We also compare infrared Spitzer IRAC and optical CoRoT light curves to analyze the dust responsible for the occultations.

Results.

AA Tau-like variability proved to be transient on a timescale of a few years. We ascribe this variability to stable accretion regimes and aperiodic variability to unstable accretion regimes and show that a transition, and even coexistence, between the two is common. We find evidence of hot spots associated with occultations, indicating that the occulting structures could be located at the base of accretion columns. We find average values of warp maximum height of 0.23 times its radial location, consistent with AA Tau, with variations of on average 11% between rotation cycles. We also show that extinction laws in the inner disk indicate the presence of grains larger than interstellar grains.

Conclusions.

The inner disk warp scenario is consistent with observations for all but one star with AA Tau-like variability in our sample. AA Tau-like systems are fairly common, comprising 14% of CTTS observed in NGC 2264, though this number increases to 35% among systems of mass 0.7 M⊙ ≲ M ≲ 2.0 M⊙. Assuming random inclinations, we estimate that nearly all systems in this mass range likely possess an inner disk warp. We attribute this to a possible change in magnetic field configurations among stars of lower mass.

Friday, May 15, 2015

Reanalysis of Microlensing Events From OGLE-III Finds Two Brown Dwarfs

REANALYSES OF ANOMALOUS GRAVITATIONAL MICROLENSING EVENTS IN THE OGLE-III EARLY WARNING SYSTEM DATABASE WITH COMBINED DATA

Authors:

Jeong et al

Abstract:

We reanalyze microlensing events in the published list of anomalous events that were observed from the Optical Gravitational Lensing Experiment (OGLE) lensing survey conducted during the 2004–2008 period. In order to check the existence of possible degenerate solutions and extract extra information, we conduct analyses based on combined data from other survey and follow-up observation and consider higher-order effects. Among the analyzed events, we present analyses of eight events for which either new solutions are identified or additional information is obtained. We find that the previous binary-source interpretations of five events are better interpreted by binary-lens models. These events include OGLE-2006-BLG-238, OGLE-2007-BLG-159, OGLE-2007-BLG-491, OGLE-2008-BLG-143, and OGLE-2008-BLG-210. With additional data covering caustic crossings, we detect finite-source effects for six events including OGLE-2006-BLG-215, OGLE-2006-BLG-238, OGLE-2006-BLG-450, OGLE-2008-BLG-143, OGLE-2008-BLG-210, and OGLE-2008-BLG-513. Among them, we are able to measure the Einstein radii of three events for which multi-band data are available. These events are OGLE-2006-BLG-238, OGLE-2008-BLG-210, and OGLE-2008-BLG-513. For OGLE-2008-BLG-143, we detect higher-order effects induced by the changes of the observer's position caused by the orbital motion of the Earth around the Sun. In addition, we present degenerate solutions resulting from the known close/wide or ecliptic degeneracy. Finally, we note that the masses of the binary companions of the lenses of OGLE-2006-BLG-450 and OGLE-2008-BLG-210 are in the brown-dwarf regime.

New Analysis Tools Developed for Studying Brown Dwarf Atmospheres, Gl 570D & HD 3651B Found to Have Ammonia

Uniform Atmospheric Retrieval Analysis of Ultracool Dwarfs I: Characterizing Benchmarks, Gl570D and HD3651B

Authors:

Line et al

Abstract:

Interpreting the spectra of brown dwarfs is key to determining the fundamental physical and chemical processes occurring in their atmospheres. Powerful Bayesian atmospheric retrieval tools have recently been applied to both exoplanet and brown dwarf spectra to tease out the thermal structures and molecular abundances to understand those processes. In this manuscript we develop a significantly upgraded retrieval method and apply it to the SpeX spectral library data of two benchmark late T-dwarfs, Gl570D and HD3651B, to establish the validity of our upgraded forward model parameterization and Bayesian estimator. Our retrieved metallicities, gravities, and effective temperature are consistent with the metallicity and presumed ages of the systems. We add the carbon-to-oxygen ratio as a new dimension to benchmark systems and find good agreement between carbon-to-oxygens ratio derived in the brown dwarfs and the host stars. Furthermore, we have for the first time unambiguously determined the presence of ammonia in the low-resolution spectra of these two late T-dwarfs. We also show that the retrieved results are not significantly impacted by the possible presence of clouds, though some quantities are significantly impacted by uncertainties in photometry. This investigation represents a watershed study in establishing the utility of atmospheric retrieval approaches on brown dwarf spectra.

What Much Radio and X Ray Activity Does Luhman 16AB Have?

The Deepest Constraints on Radio and X-ray Magnetic Activity in Ultracool Dwarfs from WISE J104915.57-531906.1

Authors:

Osten et al

Abstract:

We report upper limits to the radio and X-ray emission from the newly discovered ultracool dwarf binary WISE J104915.57−531906.1 (Luhman 16AB). As the nearest ultracool dwarf binary (2 pc), its proximity offers a hefty advantage to studying plasma processes in ultracool dwarfs which are more similar in gross properties (radius, mass, temperature) to the solar system giant planets than stars. The radio and X-ray emission upper limits from the Australia Telescope Compact Array (ATCA) and Chandra observations, each spanning multiple rotation periods, provide the deepest fractional radio and X-ray luminosities to date on an ultracool dwarf, with log(Lr,ν/Lbol)[Hz−1]less than −18.1 (5.5 GHz), log(Lr,ν/Lbol)[Hz−1] less than −17.9 (9 GHz), and log(Lx/Lbol) less than −5.7. While the radio upper limits alone do not allow for a constraint on the magnetic field strength, we limit the size of any coherently emitting region in our line of sight to less than 0.2\% of the radius of one of the brown dwarfs. Any source of incoherent emission must span less than about 20\% of the brown dwarf radius, assuming magnetic field strengths of a few tens to a few hundred Gauss. The fast rotation and large amplitude photometric variability exhibited by the T dwarf in the Luhman 16AB system are not accompanied by enhanced nonthermal radio emission, nor enhanced heating to coronal temperatures, as observed on some higher mass ultracool dwarfs, confirming the expected decoupling of matter and magnetic field in cool neutral atmospheres.

Thursday, May 14, 2015

Upper Limits on the Carbon-to-Oxygen Ratios of Eight Hot Jupiters' Atmospheres

Strict Upper Limits on the Carbon-to-Oxygen Ratios of Eight Hot Jupiters from Self-Consistent Atmospheric Retrieval

Author:

Benneke

Abstract:

The elemental compositions of hot Jupiters are informative relics of planet formation that can help us answer long-standing questions regarding the origin and formation of giant planets. Here, I present the main conclusions from a comprehensive atmospheric retrieval survey of eight hot Jupiters with detectable molecular absorption in their near-infrared transmission spectra. I analyze the eight transmission spectra using the newly-developed, self-consistent atmospheric retrieval framework, SCARLET. Unlike previous methods, SCARLET combines the physical and chemical consistency of complex atmospheric models with the statistical treatment of observational uncertainties known from atmospheric retrieval techniques. I find that all eight hot Jupiters consistently require carbon-to-oxygen ratios (C/O) below 0.9. The finding of C/O less than 0.9 is highly robust for HD209458b, WASP-12b, WASP-19b, HAT-P-1b, and XO-1b. For HD189733b, WASP-17b, and WASP-43b, I find that the published WFC3 transmission spectra favor C/O less than 0.9 at greater than 95% confidence. I further show that the water abundances on all eight hot Jupiters are consistent with solar composition. The relatively small depth of the detected water absorption features is due to the presence of clouds, not due to a low water abundance as previously suggested for HD209458b. The presence of a thick cloud deck is inferred for HD209458b and WASP-12b. HD189733b may host a similar cloud deck, rather than the previously suggested Rayleigh hazes, if star spots affect the observed spectrum. The approach taken in SCARLET can be regarded as a new pathway to interpreting spectral observations of planetary atmospheres. In this work, including our prior knowledge of H-C-N-O chemistry enables me to constrain the C/O ratio without detecting a single carbon-bearing molecule.

UV Driven Evaporation of Close-in Exoplanets

UV driven evaporation of close-in planets: energy-limited; recombination-limited and photon-limited flows

Authors:

Owen et al

Abstract:

We have investigated the evaporation of close-in exoplanets irradiated by ionizing photons. We find that the properties of the flow are controlled by the ratio of the recombination time to the flow time-scale. When the recombination time-scale is short compared to the flow time-scale the the flow is in approximate local ionization equilibrium with a thin ionization front, where the photon mean free path is short compared to flow scale. In this "recombination limited" flow the mass-loss scales roughly with the square root of the incident flux. When the recombination time is long compared to the flow time-scale the ionization front becomes thick and encompasses the entire flow, with the mass-loss rate scaling linearly with flux. If the planet's potential is deep the flow is approximately "energy-limited"; however, if the planet's potential is shallow we identify a new limiting mass-loss regime, which we term "photon-limited". In this scenario the mass-loss rate is purely limited by the incoming flux of ionizing photons. We have developed a new numerical approach that takes into account the frequency dependence of the incoming ionizing spectrum and performed a large suite of 1D simulations to characterise UV driven mass-loss around low mass planets. We find the flow is "recombination-limited" at high fluxes but becomes "energy-limited" at low fluxes; however, the transition is broad occurring over several order of magnitude in flux. Finally, we point out the transitions between the different flow types does not occur at a single flux value, but depends on the planet's properties, with higher mass planets becoming "energy-limited" at lower fluxes.

Radio Emission Variability of hot Jupiters

Time-scales of close-in exoplanet radio emission variability

Authors:

See et al

Abstract:

We investigate the variability of exoplanetary radio emission using stellar magnetic maps and 3D field extrapolation techniques. We use a sample of hot Jupiter hosting stars, focusing on the HD 179949, HD 189733 and tau Boo systems. Our results indicate two time-scales over which radio emission variability may occur at magnetised hot Jupiters. The first is the synodic period of the star-planet system. The origin of variability on this time-scale is the relative motion between the planet and the interplanetary plasma that is co-rotating with the host star. The second time-scale is the length of the magnetic cycle. Variability on this time-scale is caused by evolution of the stellar field. At these systems, the magnitude of planetary radio emission is anticorrelated with the angular separation between the subplanetary point and the nearest magnetic pole. For the special case of tau Boo b, whose orbital period is tidally locked to the rotation period of its host star, variability only occurs on the time-scale of the magnetic cycle. The lack of radio variability on the synodic period at tau Boo b is not predicted by previous radio emission models, which do not account for the co-rotation of the interplanetary plasma at small distances from the star.

Wednesday, May 13, 2015

A Model for the Evolution of SuperEarths & MiniNeptunes Applied to the Kepler-11 System

Evolutionary Models of Super-Earths and Mini-Neptunes Incorporating Cooling and Mass Loss

Authors:

Howe et al

Abstract:

We construct models of the structural evolution of super-Earth- and mini-Neptune-type exoplanets with hydrogen-helium envelopes, incorporating radiative cooling and XUV-driven mass loss. We conduct a parameter study of these models, focusing on initial mass, radius, and envelope mass fractions, as well as orbital distance, metallicity, and the specific prescription for mass loss. From these calculations, we investigate how the observed masses and radii of exoplanets today relate to the distribution of their initial conditions. Orbital distance and initial envelope mass fraction are the most important factors determining planetary evolution, particular radius evolution. Initial mass also becomes important below a "turnoff mass," which varies with orbital distance, with mass-radius curves being approximately flat for higher masses. Initial radius is the least important parameter we study, with very little difference between the hot start and cold start limits after an age of 100 Myr. Model sets with no mass loss fail to produce results consistent with observations, but a plausible range of mass loss scenarios is allowed. In addition, we present scenarios for the formation of the Kepler-11 planets. Our best fit to observations Kepler-11b and Kepler-11c involves formation beyond the snow line, after which they moved inward, circularized, and underwent a reduced degree mass loss.

Kepler's Small Exoplanets in Multi Planet Systems Have low Eccentricities

Eccentricity from transit photometry: small planets in Kepler multi-planet systems have low eccentricities

Authors:

Van Eylen et al

Abstract:

Solar system planets move on almost circular orbits. In strong contrast, many massive gas giant exoplanets travel on highly elliptical orbits, whereas the shape of the orbits of smaller, more terrestrial, exoplanets remained largely elusive. Knowing the eccentricity distribution in systems of small planets would be important as it holds information about the planet's formation and evolution, and influences its habitability. We make these measurements using photometry from the Kepler satellite and utilizing a method relying on Kepler's second law, which relates the duration of a planetary transit to its orbital eccentricity, if the stellar density is known. Our sample consists of 28 bright stars with precise asteroseismic density measurements. These stars host 74 planets with an average radius of 2.6 R⊕. We find that the eccentricity of planets in Kepler multi-planet systems is low and can be described by a Rayleigh distribution with σ = 0.049 ± 0.013. This is in full agreement with solar system eccentricities, but in contrast to the eccentricity distributions previously derived for exoplanets from radial velocity studies. Our findings are helpful in identifying which planets are habitable because the location of the habitable zone depends on eccentricity, and to determine occurrence rates inferred for these planets because planets on circular orbits are less likely to transit. For measuring eccentricity it is crucial to detect and remove Transit Timing Variations (TTVs), and we present some previously unreported TTVs. Finally transit durations help distinguish between false positives and true planets and we use our measurements to confirm six new exoplanets.