Oxygen escape from the Earth during geomagnetic reversals: Implications to mass extinction
Authors:
Wei et al
Abstract:
The
evolution of life is affected by variations of atmospheric oxygen level
and geomagnetic field intensity. Oxygen can escape into interplanetary
space as ions after gaining momentum from solar wind, but Earth's strong
dipole field reduces the momentum transfer efficiency and the ion
outflow rate, except for the time of geomagnetic polarity reversals when
the field is significantly weakened in strength and becomes Mars-like
in morphology. The newest databases available for the Phanerozoic era
illustrate that the reversal rate increased and the atmospheric oxygen
level decreased when the marine diversity showed a gradual pattern of
mass extinctions lasting millions of years. We propose that accumulated
oxygen escape during an interval of increased reversal rate could have
led to the catastrophic drop of oxygen level, which is known to be a
cause of mass extinction. We simulated the oxygen ion escape rate for
the Triassic–Jurassic event, using a modified Martian ion escape model
with an input of quiet solar wind inferred from Sun-like stars. The
results show that geomagnetic reversal could enhance the oxygen escape
rate by 3–4 orders only if the magnetic field was extremely weak, even
without consideration of space weather effects. This suggests that our
hypothesis could be a possible explanation of a correlation between
geomagnetic reversals and mass extinction. Therefore, if this causal
relation indeed exists, it should be a “many-to-one” scenario rather the
previously considered “one-to-one”, and planetary magnetic field should
be much more important than previously thought for planetary
habitability.
Detecting industrial pollution in the atmospheres of earth-like exoplanets
Authors:
Lin et al
Abstract:
Detecting biomarkers, such as molecular oxygen, in the atmospheres of transiting exoplanets has been a major focus in the search for alien life. We point out that in addition to these generic indicators, anthropogenic pollution could be used as a novel biomarker for intelligent life. To this end, we identify pollutants in the Earth's atmosphere that have significant absorption features in the spectral range covered by the James Webb Space Telescope (JWST). We estimate that for an Earth-mass planet in the habitable zone of a white dwarf, methane (CH4) and nitrous oxide (N2O) can be detected at earth-like concentrations with an integration time of ~1.5 hrs and 12 hrs respectively. Detecting pollutants that are produced nearly exclusively by anthropogenic activities will be significantly more challenging. Of these pollutants, we focus on tetrafluoromethane (CF4) and trichlorofluoromethane (CCl3F), which will be the easiest to detect. We estimate that ~1.5 days (~3 days) of total integration time will be sufficient to detect or constrain the concentration of CCl3F (CF4) to ~100 times current terrestrial level.
Exoplanet population inference and the abundance of Earth analogs from noisy, incomplete catalogs
Authors:
Foreman-Mackey et al
Abstract:
No true extrasolar Earth analog is known. Hundreds of planets have been found around Sun-like stars that are either Earth-sized but on shorter periods, or else on year-long orbits but somewhat larger. Under strong assumptions, exoplanet catalogs have been used to make an extrapolated estimate of the rate at which Sun-like stars host Earth analogs. These studies are complicated by the fact that every catalog is censored by non-trivial selection effects and detection efficiencies, and every property (period, radius, etc.) is measured noisily. Here we present a general hierarchical probabilistic framework for making justified inferences about the population of exoplanets, taking into account survey completeness and, for the first time, observational uncertainties. We are able to make fewer assumptions about the distribution than previous studies; we only require that the occurrence rate density be a smooth function of period and radius (employing a Gaussian process). By applying our method to synthetic catalogs, we demonstrate that it produces more accurate estimates of the whole population than standard procedures based on weighting by inverse detection efficiency. We apply the method to an existing catalog of small planet candidates around G dwarf stars (Petigura et al. 2013). We confirm a previous result that the radius distribution changes slope near Earth's radius. We find that the rate density of Earth analogs is about 0.02 (per star per natural logarithmic bin in period and radius) with large uncertainty. This number is much smaller than previous estimates made with the same data but stronger assumptions.
Evolution of linear warps in accretion discs and applications to protoplanetary discs in binaries
Authors:
Foucart et al
Abstract:
The existence of warped accretion discs is expected in a wide variety of astrophysical systems, including circumstellar discs in binaries and discs around binary protostars. A common feature of these discs is that they are perturbed by a misaligned external potential. In this paper, we study the long-term evolution of the disc warp and precession in the case of thick discs (with the dimensionless thickness H/r larger than the viscosity parameter α) in which bending waves can propagate. For small warps, such discs undergo approximately rigid-body precession with a coherent global frequency. We derive the analytical expressions for the warp/twist profiles of the disc and the alignment timescale for a variety of disc models/parameters. Applying our results to circumbinary discs, we find that these discs align with the orbital plane of the binary on a timescale comparable to the global precession time of the disc, and typically much smaller than its viscous timescale. The development of parametric instabilities associated with disc warp may further increase the inclination damping rate. We discuss the implications of our finding for the observations of misaligned circumbinary discs (such as KH 15D) and circumbinary planetary systems (such as Kepler-413); these observed misalignments provide useful constraints on the uncertain aspects of the disc warp theory. On the other hand, we find that circumstellar discs can maintain large misalignments with respect to the plane of the binary companion over their entire lifetime. Even when the enhanced damping effect due to parametric instabilities associated with the disc warp are taken into account, we estimate that inclination angles of ∼20∘ can be maintained for equal mass circular binaries and typical disc parameters (α=0.01, H/r=0.1). [Abridged]
Debris disc formation induced by planetary growth
Authors:
Kobayashi et al
Abstract:
Several hundred stars older than 10 million years have been observed to have infrared excesses. These observations are explained by dust grains formed by the collisional fragmentation of hidden planetesimals. Such dusty planetesimal discs are known as debris discs. In a dynamically cold planetesimal disc, collisional coagulation of planetesimals produces planetary embryos which then stir the surrounding leftover planetesimals. Thus, the collisional fragmentation of planetesimals that results from planet formation forms a debris disc. We aim to determine the properties of the underlying planetesimals in debris discs by numerically modelling the coagulation and fragmentation of planetesimal populations. The brightness and temporal evolution of debris discs depend on the radial distribution of planetesimal discs, the location of their inner and outer edges, their total mass, and the size of planetesimals in the disc. We find that a radially narrow planetesimal disc is most likely to result in a debris disc that can explain the trend of observed infrared excesses of debris discs around G-type stars, for which planet formation occurs only before 100 million years. Early debris disc formation is induced by planet formation, while the later evolution is explained by the collisional decay of leftover planetesimals around planets that have already formed. Planetesimal discs with underlying planetesimals of radii ∼100km at ≈30 AU most readily explain the Spitzer Space Telescope 24 and 70μm fluxes from debris discs around G-type stars.
Characterizing the Cool KOIs. VI. H- and K-band Spectra of Kepler M Dwarf Planet-Candidate Hosts
Authors:
Muirhead et al
Abstract:
We present H- and K-band spectra for late-type Kepler Objects of Interest (the "Cool KOIs"): low-mass stars with transiting-planet candidates discovered by NASA's Kepler Mission that are listed on the NASA Exoplanet Archive. We acquired spectra of 103 Cool KOIs and used the indices and calibrations of Rojas-Ayala et al. to determine their spectral types, stellar effective temperatures and metallicities, significantly augmenting previously published values. We interpolate our measured effective temperatures and metallicities onto evolutionary isochrones to determine stellar masses, radii, luminosities and distances, assuming the stars have settled onto the main-sequence. As a choice of isochrones, we use a new suite of Dartmouth predictions that reliably include mid-to-late M dwarf stars. We identify five M4V stars: KOI-961 (confirmed as Kepler 42), KOI-2704, KOI-2842, KOI-4290, and the secondary component to visual binary KOI-1725, which we call KOI-1725 B. We also identify a peculiar star, KOI-3497, which has a Na and Ca lines consistent with a dwarf star but CO lines consistent with a giant. Visible-wavelength adaptive optics imaging reveals two objects within a 1 arc second diameter; however, the objects' colors are peculiar. The spectra and properties presented in this paper serve as a resource for prioritizing follow-up observations and planet validation efforts for the Cool KOIs, and are all available for download online using the "data behind the figure" feature.
Probing the presence of planets in transition discs' cavities via warps: the case of TW Hya
Authors:
Facchini et al
Abstract:
We are entering the era in which observations of protoplanetary discs properties can indirectly probe the presence of massive planets or low mass stellar companions interacting with the disc. In particular, the detection of warped discs can provide important clues to the properties of the star-disc system. In this paper we show how observations of warped discs can be used to infer the dynamical properties of the systems. We concentrate on circumbinary discs, where the mass of the secondary can be planetary. First, we provide some simple relations that link the amplitude of the warp in the linear regime to the parameters of the system. Secondly, we apply our method to the case of TW Hya, a transition disc for which a warp has been proposed based on spectroscopic observations. Assuming values for the disc and stellar parameters from observations, we conclude that, in order for a warp induced by a planetary companion to be detectable, the planet mass should be large (Mp≈10−14MJ) and the disc should be viscous (α≈0.15−0.25). We also apply our model to LkCa 15 and T Cha, where a substellar companion has been detected within the central cavity of the transition discs.
Lunar and Terrestrial Planet Formation in the Grand Tack Scenario
Authors:
Jacobson et al
Abstract:
We present conclusions from a large number of N-body simulations of the giant impact phase of terrestrial planet formation. We focus on new results obtained from the recently proposed Grand Tack model, which couples the gas-driven migration of giant planets to the accretion of the terrestrial planets. The giant impact phase follows the oligarchic growth phase, which builds a bi-modal mass distribution within the disc of embryos and planetesimals. By varying the ratio of the total mass in the embryo population to the total mass in the planetesimal population and the mass of the individual embryos, we explore how different disc conditions control the final planets. The total mass ratio of embryos to planetesimals controls the timing of the last giant (Moon forming) impact and its violence. The initial embryo mass sets the size of the lunar impactor and the growth rate of Mars. After comparing our simulated outcomes with the actual orbits of the terrestrial planets (angular momentum deficit, mass concentration) and taking into account independent geochemical constraints on the mass accreted by the Earth after the Moon forming event and on the timescale for the growth of Mars, we conclude that the protoplanetary disc at the beginning of the giant impact phase must have had most of its mass in Mars-sized embryos and only a small fraction of the total disc mass in the planetesimal population. From this, we infer that the Moon forming event occurred between ∼60 and ∼130 My after the formation of the first solids, and was caused most likely by an object with a mass similar to that of Mars.
Evolution of eccentricity and orbital inclination of migrating planets in 2:1 mean motion resonance
Authors:
Teyssandier et al
Abstract:
We determine, analytically and numerically, the conditions needed for a system of two migrating planets trapped in a 2:1 mean motion resonance to enter an inclination-type resonance. We provide an expression for the asymptotic equilibrium value that the eccentricity ei of the inner planet reaches under the combined effects of migration and eccentricity damping. We also show that, for a ratio q of inner to outer masses below unity, ei has to pass through a value ei,res of order 0.3 for the system to enter an inclination-type resonance. Numerically, we confirm that such a resonance may also be excited at another, larger, value ei,res≃0.6, as found by previous authors. A necessary condition for onset of an inclination-type resonance is that the asymptotic equilibrium value of ei is larger than ei,res. We find that, for q≤1, the system cannot enter an inclination-type resonance if the ratio of eccentricity to semimajor axis damping timescales te/ta is smaller than 0.2. This result still holds if only the eccentricity of the outer planet is damped and q≲1. As the disc/planet interaction is characterized by te/ta∼10−2, we conclude that excitation of inclination through the type of resonance described here is very unlikely to happen in a system of two planets migrating in a disc.
Detecting the spin-orbit misalignment of the super-Earth 55 Cnc e
Authors:
Bourrier et al
Abstract:
We present time-resolved spectroscopy of transits of the super-Earth 55 Cnc e using HARPS-N observations. We devised an empirical correction for the "color effect" affecting the radial velocity residuals from the Keplerian fit, which significantly improves their dispersion with respect to the HARPS-N pipeline standard data-reduction. Using our correction, we were able to detect the smallest Rossiter-McLaughlin anomaly amplitude of an exoplanet so far (~60 cm/s). The super-Earth 55 Cnc e is also the smallest exoplanet with a Rossiter-McLaughlin anomaly detection. We measured the sky-projected obliquity lambda = 72.4 (+12.7 -11.5 deg), indicating that the planet orbit is prograde, highly misaligned and nearly polar compared to the stellar equator. The entire 55 Cancri system may have been highly tilted by the presence of a stellar companion.
The curious case of HD41248. A pair of static signals buried behind red-noise
Authors:
Jenkins et al
Abstract:
Gaining a better understanding of the effects of stellar induced radial velocity noise is critical for the future of exoplanet studies, since the discovery of the lowest-mass planets using this method will require us to go below the intrinsic stellar noise limit. An interesting test case in this respect is that of the southern solar analogue HD41248. The radial velocity time series of this star has been proposed to contain either a pair of signals with periods of around 18 and 25 days, that could be due to a pair of resonant super-Earths, or a single and varying 25 day signal that could arise due to a complex interplay between differential rotation and modulated activity. In this letter we build-up more evidence for the former scenario, showing that the signals are still clearly significant even after more than 10 years of observations and they likely do not change in period, amplitude, or phase as a function of time, the hallmarks of static Doppler signals. We show that over the last two observing seasons this star was more intrinsically active and the noise reddened, highlighting why better noise models are needed to find the lowest amplitude signals, in particular models that consider noise correlations. This analysis shows that there is still sufficient evidence for the existence of two super-Earths on the edge of, or locked into, a 7:5 mean motion resonance orbiting HD41248.
Ancient dynamos of terrestrial planets more sensitive to core-mantle boundary heat flows
Authors:
Hori et al
Abstract:
The early dynamos of Earth and Mars probably operated without an inner core being present. They were thus exclusively driven by secular cooling and radiogenic heating, whereas the present geodynamo is thought to be predominantly driven by buoyancy fluxes which arise from the release of latent heat and the compositional enrichment associated with inner core solidification. The impact of the inner core growth on the ancient geodynamo has been discussed extensively but is still controversial. The Martian dynamo stopped operating more than 4 Gyr ago but left its signature in the form of a strong crustal magnetization that is much stronger in the southern than in the northern hemisphere. This dichotomy can, for example, be explained by a dynamo predominantly operating in the southern hemisphere due to a heterogeneous heat flux through the core-mantle boundary (CMB). The early Martian dynamo may also have operated without an inner core being present. Here we explore the impact of lateral CMB heat flux variations on dynamos with and without an inner core by comparing numerical dynamos driven by homogeneous internal sources or by bottom buoyancy sources, arising from the inner core boundary (ICB). Three different CMB heat-flux patterns are tested that either break the northern/southern or the azimuthal symmetry. In the dynamos driven by internal heating a rather small CMB heat-flux heterogeneity suffices to break internal symmetries and leads to boundary-induced structures and different field strengths. The effect is much smaller for dynamos driven by ICB buoyancy sources. Our results indicate that the field intensity and morphology of the ancient dynamos of Earth or Mars were more variable and more sensitive to the thermal CMB structure than the geodynamo after onset of inner core growth.
On the edge of habitability and the extremes of liquidity
Authors:
Hansen-Goos et al
Abstract:
The physical and biological mechanisms that extend the equilibrium domain of liquid water into the ice region of the bulk phase diagram are examined in view of their importance for the enhancement of planetary habitability. The physical phenomena studied are the premelting of ice, which allows for films of liquid water at temperatures well below freezing, and the wetting of hygroscopic salts with the persistence of briny films even for thermodynamic conditions remote from those of bulk liquid water. Organisms are known to produce a variety of frost-suppressing substances, one of which, the anti-freeze protein, is described here. In this article, we provide a synthesis of theoretical and experimental studies whilst extending ideas into new territory as we address the question of habitability.
PLANET HUNTERS. VI. AN INDEPENDENT CHARACTERIZATION OF KOI-351 AND SEVERAL LONG PERIOD PLANET CANDIDATES FROM THE KEPLER ARCHIVAL DATA*
Authors:
Schmitt et al
Abstract:
We report the discovery of 14 new transiting planet candidates in the Kepler field from the Planet Hunters citizen science program. None of these candidates overlapped with Kepler Objects of Interest (KOIs) at the time of submission. We report the discovery of one more addition to the six planet candidate system around KOI-351, making it the only seven planet candidate system from Kepler. Additionally, KOI-351 bears some resemblance to our own solar system, with the inner five planets ranging from Earth to mini-Neptune radii and the outer planets being gas giants; however, this system is very compact, with all seven planet candidates orbiting lsim 1 AU from their host star. A Hill stability test and an orbital integration of the system shows that the system is stable. Furthermore, we significantly add to the population of long period transiting planets; periods range from 124 to 904 days, eight of them more than one Earth year long. Seven of these 14 candidates reside in their host star's habitable zone.
A high-contrast coronagraph for earth-like exoplanets direct imaging: design and test
Authors:
Liu et al
Abstract:
The high-contrast coronagraph for direct imaging earth-like exoplanets at the visible needs a contrast of 10^(-10) at a small angular separation of 4 lambda/D or less. Here we report our recent laboratory experiment that is close to the limits. The test of the high-contrast imaging coronagraph is based on our step-transmission apodized filter. To achieve the goal, we use a liquid crystal array (LCA) as a phase corrector to create a dark hole based on our dedicated focal dark algorithm. We have suppressed the diffracted and speckle noise near the star point image to a level of 1.68 x 10^(-9) at 4 lambda/D, which can be immediately used for the direct imaging of Jupiter like exoplanets. This demonstrates that high-contrast coronagraph telescope in space has the potentiality to detect and characterize earth-like planets.
HIP 114328: a new refractory-poor and Li-poor solar twin
Authors:
Melendez et al
Abstract:
The standard solar model fails to predict the very low lithium abundance in the Sun, which is much lower than the proto-solar nebula. This Li problem has been debated for decades, and it has been ascribed either to planet formation or to secular stellar depletion. In order to test the evolution of Li, it is important to find solar twins in a range of ages. Also, the study of stars similar to the Sun is relevant in relation to the signature of terrestrial planet formation around the Sun. [Methods]. We acquired high-resolution (R=110,000), high S/N (~300) ESO/VLT UVES spectra of several solar twin candidates and the Sun (as reflected from the asteroid Juno). Among the solar twin candidates we identify HIP 114328 as a solar twin and perform a differential line-by-line abundance analysis of this star relative to the Sun. [Results]. HIP 114328 has stellar parameters Teff = 5785+/-10 K, log g = 4.38+/-0.03, [Fe/H] = -0.022+/-0.009, and a microturbulent velocity 0.05+/-0.03 km/s higher than solar. The differential analysis shows that this star is chemically very similar to the Sun. The refractory elements seem even slightly more depleted than in the Sun, meaning that HIP 114328 may be as likely to form terrestrial planets as the Sun. HIP 114328 is about 2 Gyr older than the Sun, and is thus the second oldest solar twin analyzed at high precision. It has a Li abundance of A(Li)NLTE <= 0.46, which is about 4 times lower than in the Sun (A(Li)NLTE = 1.07 dex), but close to the oldest solar twin known, HIP 102152. [Conclusions]. Based on the lower abundances of refractory elements when compared to other solar twins, HIP 114328 seems an excellent candidate to host rocky planets. The low Li abundance of this star is consistent with its old age and fits very well the emerging Li-age relation among solar twins of different ages.
SOPHIE velocimetry of Kepler transit candidates XII. KOI-1257 b: a highly-eccentric 3-month period transiting exoplanet
Authors:
Santerne et al
Abstract:
In this paper we report a new transiting warm giant planet: KOI-1257 b. It was first detected in photometry as a planet-candidate by the Kepler space telescope and then validated thanks to a radial velocity follow-up with the SOPHIE spectrograph. It orbits its host star with a period of 86.647661 d ± 3 s and a high eccentricity of 0.772 ± 0.045. The planet transits the main star of a metal-rich, relatively old binary system with stars of mass of 0.99 ± 0.05 Msun and 0.70 ± 0.07 Msun for the primary and secondary (respectively). This binary system is constrained thanks to a self-consistent modelling of the Kepler transit light curve, the SOPHIE radial velocities, line bisector and full-width half maximum (FWHM) variations as well as the spectral energy distribution. However, future observations are needed to confirm it. The PASTIS fully-Bayesian software was used to validate the nature of the planet and to determine which star of the binary system is the transit host. By accounting for the dilution from the binary both in photometry and in radial velocity, we find that the planet has a mass of 1.45 ± 0.35 Mjup, and a radius of 0.94 ± 0.12 Rjup, and thus a bulk density of 2.1 ± 1.2 g.cm−3. The planet has an equilibrium temperature of 511 ± 50 K, making it one of the few known membre of the warm-jupiter population. The HARPS-N spectrograph was also used to observe a transit of KOI-1257 b, simultaneously with a joint amateur and professional photometric follow-up, with the aims at constraining the orbital obliquity of the planet. However, the Rossiter-McLaughlin was not clearly detected, resulting in poor constraints on the orbital obliquity of the planet.
A Statistical Reconstruction of the Planet Population Around Kepler Solar-Type Stars
Authors:
Silburt et al
Abstract:
Using the most recent Kepler catalog, we reconstruct the occurrence rate of small (Neptune-sized or below) planets as a function of orbital period and planet radius, taking careful account of various detection biases. We analyze a sample of 76,000 Sun-like stars and their associated planet candidates with periods between 20 and 200 days, and sizes between 1 and 4R⊕. Such planets have likely experienced little photoevaporation, and may reflect the "primordial" planet population. Assuming that the size distribution of planets are independent of their orbital periods (and vice versa), we conclude that Kepler planets are preferentially peaked at 2−2.8R⊕, with their numbers decreasing gradually toward smaller sizes. These planets are found roughly uniformly in logarithmic period. The average number of planets per star, in the stated period and size ranges, is 0.46±0.03. This number rises by ∼0.2 if one includes planets inward of 20 days. Upon extrapolation we obtain an occurrence rate, for Earth-like planets within the "habitable zone" (as calculated by 1-D climate models), of 6.4+3.4−1.1%. We discuss the astrophysical implications of our results.
In our study, we introduce a number of novel statistical approaches, including the adoption of the "iterative simulation" technique (in addition to the standard MCMC technique), incorporation of uncertainties in planet radii, and an improved consideration of detection bias. Our results largely agree with those from an earlier work by Petigura et al. (2013), based on different statistical treatments and noise models. However, this agreement masks two substantial underlying discrepancies that (to first order) cancel each other out.
Dust from collisions: A way to probe the composition of exo-planets?
Authors:
Morlok et al
Abstract:
In order to link infrared observations of dust formed during planet formation in debris disks to mid-infrared spectroscopic data of planetary materials from differentiated terrestrial and asteroidal bodies, we obtained absorption spectra of a representative suite of terrestrial crustal and mantle materials, and of typical martian meteorites.
A series of debris disk spectra characterized by a strong feature in the 9.0–9.5 μm range (HD23514, HD15407a, HD172555 and HD165014), is comparable to materials that underwent shock, collision or high temperature events. These are amorphous materials such as tektites, SiO2-glass, obsidian, and highly shocked shergottites as well as inclusions from mesosiderites (group A).
A second group (BD+20307, Beta Pictoris, HD145263, ID8, HD113766, HD69830, P1121, and Eta Corvi) have strong pyroxene and olivine bands in the 9–12 μm range and is very similar to ultramafic rocks (e.g. harzburgite, dunite) (group B).
This could indicate the occurrence of differentiated materials similar to those in our Solar System in these other systems.
However, mixing of projectile and target material, as well as that of crustal and mantle material has to be taken into account in large scale events like hit-and-run and giant collisions or even large-scale planetary impacts. This could explain the olivine-dominated dust of group B.
The crustal-type material of group A would possibly require the stripping of upper layers by grazing-style hit-and run encounters or high energy events like evaporation/condensation in giant collisions. In tidal disruptions or the involvement of predominantly icy/water bodies the resulting mineral dust would originate mainly in one of the involved planetesimals. This could allow attributing the observed composition to a specific body (such as e.g. Eta Corvi).
Spin evolution of Earth-sized exoplanets, including atmospheric tides and core-mantle friction
Authors:
Cunha et al
Abstract:
Planets with masses between 0.1 - 10 M_earth are believed to host dense atmospheres. These atmospheres can play an important role on the planet's spin evolution, since thermal atmospheric tides, driven by the host star, may counterbalance gravitational tides. In this work we study the long-term spin evolution of Earth-sized exoplanets. We generalize previous works by including the effect of eccentric orbits and obliquity. We show that under the effect of tides and core-mantle friction, the obliquity of the planets evolve either to 0 or 180 degrees. The rotation of these planets is also expected to evolve into a very restricted number of equilibrium configurations. In general, none of this equilibria is synchronous with the orbital mean motion. The role of thermal atmospheric tides becomes more important for Earth-sized planets in the habitable zones of their systems, so they cannot be neglected when we search for their potential habitability.
GJ 832c: A super-earth in the habitable zone
Authors:
Wittenmeyer et al
Abstract:
We report the detection of GJ 832c, a super-Earth orbiting near the inner edge of the habitable zone of GJ 832, an M dwarf previously known to host a Jupiter analog in a nearly-circular 9.4-year orbit. The combination of precise radial-velocity measurements from three telescopes reveals the presence of a planet with a period of 35.68+/-0.03 days and minimum mass (m sin i) of 5.4+/-1.0 Earth masses. GJ 832c moves on a low-eccentricity orbit (e=0.18+/-0.13) towards the inner edge of the habitable zone. However, given the large mass of the planet, it seems likely that it would possess a massive atmosphere, which may well render the planet inhospitable. Indeed, it is perhaps more likely that GJ 832c is a "super-Venus," featuring significant greenhouse forcing. With an outer giant planet and an interior, potentially rocky planet, the GJ 832 planetary system can be thought of as a miniature version of our own Solar system.
The Metallicity Dependence of Giant Planet Incidence
Authors:
Gonzalez et al
Abstract:
We describe three corrections that should be applied to the observed relative incidence of nearby stars hosting giant planets. These are diffusion in the stellar atmosphere, use of the [Ref] index in place of [Fe/H] for metallicity, and correction for local sampling with the W velocity. We have applied these corrections to a subset of the SPOCS exoplanet survey with uniform giant planet detectability. Fitting the binned data to a power law of the form, α10β[Fe/H], we derived α=0.022±0.007 and β=3.0±0.5; this value of β is 50\% larger than the value determined by \citet{fv05}. While the statistical significance of this difference is marginal, given the small number statistics, these corrections should be included in future analyses that include larger samples.
Formation of Isothermal Disks around Protoplanets. I. Introductory Three-Dimensional Global Simulations for Sub-Neptune-Mass Protoplanets
Authors:
Wang et al
Abstract:
The regular satellites found around Neptune ($\approx 17~M_{\Earth}$) and Uranus ($\approx 14.5~M_{\Earth}$) suggest that past gaseous circumplanetary disks may have co-existed with solids around sub-Neptune-mass protoplanets ($ less than 17~M_{\Earth}$). These disks have been shown to be cool, optically thin, quiescent, with low surface density and low viscosity. Numerical studies of the formation are difficult and technically challenging. As an introductory attempt, three-dimensional global simulations are performed to explore the formation of circumplanetary disks around sub-Neptune-mass protoplanets embedded within an isothermal protoplanetary disk at the inviscid limit of the fluid in the absence of self-gravity. Under such conditions, a sub-Neptune-mass protoplanet can reasonably have a rotationally supported circumplanetary disk. The size of the circumplanetary disk is found to be roughly one-tenth of the corresponding Hill radius, which is consistent with the orbital radii of irregular satellites found for Uranus. The protoplanetary gas accretes onto the circumplanetary disk vertically from high altitude and returns to the protoplanetary disk again near the midplane. This implies an open system in which the circumplanetary disk constantly exchanges angular momentum and material with its surrounding prenatal protoplanetary gas.
Precise determination of fundamental parameters of six exoplanet host stars and their planets
Authors:
Liu et al
Abstract:
The aim of this paper is to determinate the fundamental parameters of six exoplanet host (EH) stars and their planets. While techniques for detecting exoplanets yield properties of the planet only as a function of the properties of the host star, hence, we must accurately determine parameters of EH stars at first. For this reason, we constructed a grid of stellar models including diffusion and rotation-induced extra-mixing with given ranges of input parameters (i.e. mass, metallicity, and initial rotation rate). In addition to the commonly used observational constraints such as the effective temperature T_{eff}, luminosity L and metallicity [Fe/H], we added two observational constraints, the lithium abundance log N (Li) and the rotational period P_{rot}. These two additional observed parameters can make further constrains on the model due to their correlations with mass, age and other stellar properties. Hence, our estimations of fundamental parameters for these EH stars and their planets are with higher precision than previous works. Therefore, the combination of rotational period and lithium help us to obtain more accurate parameters for stars, leading to an improvement of the knowledge of the physical state about the EH stars and their planets.
Gaia, Non-Single Stars, Brown Dwarfs, and Exoplanets
Authors:
Sozzetti et al
Abstract:
In its all-sky survey, Gaia will monitor astrometrically and photometrically millions of main-sequence stars with sufficient sensitivity to brown dwarf companions within a few AUs from their host stars and to transiting brown dwarfs on very short periods, respectively. Furthermore, thousands of detected ultra-cool dwarfs in the backyard of the Sun will have direct (absolute) distance estimates from Gaia, and for these Gaia astrometry will be of sufficient precision to reveal any orbiting companions with masses as low as that of Jupiter. Gaia observations thus bear the potential for critical contributions to many important questions in brown dwarfs astrophysics (how do they form in isolation and as companions to stars? Can planets form around them? What are their fundamental parameters such as ages, masses, and radii? What is their atmospheric physics?), and their connection to stars and planets. The full legacy potential of Gaia in the realm of brown dwarf science will be realized when combined with other detection and characterization programs, both from the ground and in space.
Europe’s powerful Milky Way mapper is facing some problems as controllers ready the Gaia telescope for operations. It turns out that there is “stray light” bleeding into the telescope, which will affect how well it can see the stars around it. Also, the telescope optics are also not transmitting as efficiently as the design predicted.
Controllers emphasize the light problem would only affect the faintest visible stars, and that tests are ongoing to minimize the impact on the mission. Still, there will be some effect on how well Gaia can map the stars around it due to this issue.
“While there will likely be some loss relative to Gaia’s pre-launch performance predictions, we already know that the scientific return from the mission will still be immense, revolutionizing our understanding of the formation and evolution of our Milky Way galaxy and much else,” wrote the Gaia project team in a blog post.
Both of these problems have been known publicly since April, and the team has been working hard in recent months to pinpoint the cause. Of the two of them, it appears the team is having the most success with the optics transmission problems. They have traced the issue to water vapor in the telescope that freezes (no surprise since Gaia operates between -100 degrees Celsius and -150 Celsius, or -148 Fahrenheit and -238 Fahrenheit.)
Analysing weak orbital signals in Gaia data
Author:
Lucy
Abstract:
Anomalous orbits are found when minimum-chi^{2} estimation is applied to synthetic Gaia data for weak orbital signals - i.e., orbits whose astrometric signatures are comparable to the single-scan measurement error (Pourbaix 2002). These orbits are nearly parabolic, edge-on, and their major axes align with the line-of-sight to the observer. Such orbits violate the Copernican principle (CPr) and as such could be rejected. However, the preferred alternative is to develop a statistical technique that incorporates the CPr as a fundamental postulate. This can be achieved in the context of Bayesian estimation by defining a Copernican prior. With this development, Pourbaix's anomalous orbits no longer arise. Instead, orbits with a somewhat higher chi^{2} but which do not violate the CPr are selected.
Other areas of astronomy where the investigator must analyse data from 'imperfect experiments' might similarly benefit from appropriately- defined Copernican priors.
Planet formation in Binaries
Authors:
Thebault et al
Abstract:
Spurred by the discovery of numerous exoplanets in multiple systems, binaries have become in recent years one of the main topics in planet formation research. Numerous studies have investigated to what extent the presence of a stellar companion can affect the planet formation process. Such studies have implications that can reach beyond the sole context of binaries, as they allow to test certain aspects of the planet formation scenario by submitting them to extreme environments. We review here the current understanding on this complex problem. We show in particular how each of the different stages of the planet-formation process is affected differently by binary perturbations. We focus especially on the intermediate stage of kilometre-sized planetesimal accretion, which has proven to be the most sensitive to binarity and for which the presence of some exoplanets observed in tight binaries is difficult to explain by in-situ formation following the "standard" planet-formation scenario. Some tentative solutions to this apparent paradox are presented. The last part of our review presents a thorough description of the problem of planet habitability, for which the binary environment creates a complex situation because of the presence of two irradation sources of varying distance.
Planetesimal formation in self-gravitating discs – the effects of particle self-gravity and back-reaction
Authors:
Gibbons et al
Abstract:
We study particle dynamics in self-gravitating gaseous discs with a simple cooling law prescription via two-dimensional simulations in the shearing sheet approximation. It is well known that structures arising in the gaseous component of the disc due to a gravitational instability can have a significant effect on the evolution of dust particles. Previous results have shown that spiral density waves can be highly efficient at collecting dust particles, creating significant local overdensities of particles. The degree of such concentrations has been shown to be dependent on two parameters: the size of the dust particles and the rate of gas cooling. We expand on these findings, including the self-gravity of dust particles, to see how these particle overdensities evolve. We use the pencil code to solve the local shearing sheet equations for gas on a fixed grid together with the equations of motion for solids coupled to the gas through an aerodynamic drag force. We find that the enhancements in the surface density of particles in spiral density wave crests can reach levels high enough to allow the solid component of the disc to collapse under its own self-gravity. This produces many gravitationally bound collections of particles within the spiral structure. The total mass contained in bound structures appears nearly independent of the cooling time, suggesting that the formation of planetesimals through dust particle trapping by self-gravitating density waves may be possible at a larger range of radii within a disc than previously thought. So, density waves due to gravitational instabilities in the early stages of star formation may provide excellent sites for the rapid formation of many large, planetesimal-sized objects.
Modeling dust growth in protoplanetary disks: The breakthrough case
Authors:
Drazkowska et al
Abstract:
Simple toy models are often not sufficient to cover the complexity of the dust coagulation process, and a number of numerical approaches are therefore used, among which integration of the Smoluchowski equation and various versions of Monte Carlo algorithm are the most popular. In this paper, we directly compare the Smoluchowski and Monte Carlo approaches and we find a general agreement for most of the coagulation problems. However, for the sweep-up growth driven by the "lucky" breakthrough mechanism, the methods exhibit very different resolution dependencies. With too few mass bins, the Smoluchowski algorithm tends to overestimate the growth rate and the probability of breakthrough. The Monte Carlo method is less resolution dependent in the growth timescale aspect but it tends to underestimate the breakthrough chance due to its limited dynamic mass range. We discuss the features and drawbacks of both the approaches, which may limit their astrophysical applications.
Calibrating Ultracool Dwarfs: Optical Template Spectra, Bolometric Corrections, and χ Values
Authors:
Schmidt et al
Abstract:
We present optical template spectra, bolometric corrections, and χ values for ultracool dwarfs. The templates are based on spectra from the Sloan Digital Sky Survey (SDSS) and the Astrophysical Research Consortium 3.5-m telescope. The spectral features and overall shape of the L dwarf templates are consistent with previous spectroscopic standards and the templates have a radial velocity precision of ∼10--20 km s−1. We calculate bolometric fluxes (accurate to 10--20\%) for 101 late-M and L dwarfs from SDSS, 2MASS, and WISE photometry, SDSS spectra, and BT-Settl model spectra. We find that the z- and J-band bolometric corrections for late-M and L dwarfs have a strong correlation with z−J and J−KS colors respectively. The new χ values, which can be used to convert Hα equivalent widths to activity strength, are based on spectrophotometrically calibrated SDSS spectra and the new bolometric fluxes. While the measured χ values have typical uncertainties of ∼20\%, ultracool dwarf models show the continuum surrounding Hα can vary by up to an order of magnitude with changing surface gravity. Our semi-empirical χ values are one to two orders of magnitude larger than previous χ values for mid- to late-L dwarfs, indicating that the upper limits for Hα activity strength on the coolest L dwarfs have been underestimated.
New Evidence for a Substellar Luminosity Problem: Dynamical Mass for the Brown Dwarf Binary Gl 417BC
Authors:
Dupuy et al
Abstract:
We present new evidence for a problem with cooling rates predicted by substellar evolutionary models that implies model-derived masses in the literature for brown dwarfs and directly imaged planets may be too high. Based on our dynamical mass for Gl 417BC (L4.5+L6) and a gyrochronology system age from its young, solar-type host star, commonly used models predict luminosities 0.2−0.4 dex lower than we observe. This corroborates a similar luminosity−age discrepancy identified in our previous work on the L4+L4 binary HD 130948BC, which coincidentally has nearly identical component masses (≈50−55 MJup) and age (≈800 Myr) as Gl 417BC. Such a luminosity offset would cause systematic errors of 15%−25% in model-derived masses at this age. After comparing different models, including cloudless models that should not be appropriate for mid-L dwarfs like Gl 417BC and HD 130948BC but actually match their luminosities better, we speculate the observed over-luminosity could be caused by opacity holes (i.e., patchy clouds) in these objects. Moreover, from hybrid substellar evolutionary models that account for cloud disappearance we infer the corresponding phase of over-luminosity may extend from a few hundred Myr up to a few Gyr and cause masses to to be over-estimated by up to 25%, even well after clouds disappear from view entirely. Thus, the range of of ages and spectral types affected by this potential systematic shift in luminosity evolution would encompass most known directly imaged gas-giants and field brown dwarfs.
Spectral Variability from the Patchy Atmospheres of T and Y Dwarfs
Authors:
Morley et al
Abstract:
Brown dwarfs of a variety of spectral types have been observed to be photometrically variable. Previous studies have focused on objects at the L/T transition, where the iron and silicate clouds in L dwarfs break up or dissipate. However, objects outside of this transitional effective temperature regime also exhibit variability. Here, we present models for mid-late T dwarfs and Y dwarfs. We present models that include patchy salt and sulfide clouds as well as water clouds for the Y dwarfs. We find that for objects over 375 K, patchy cloud opacity would generate the largest amplitude variability within near-infrared spectral windows. For objects under 375 K, water clouds also become important and generate larger amplitude variability in the mid-infrared. We also present models in which we perturb the temperature structure at different pressure levels of the atmosphere to simulate hot spots. These models show the most variability in the absorption features between spectral windows. The variability is strongest at wavelengths that probe pressure levels at which the heating is the strongest. The most illustrative types of observations for understanding the physical processes underlying brown dwarf variability are simultaneous, multi-wavelength observations that probe both inside and outside of molecular absorption features.
Trigonometric parallaxes of young field L dwarfs
Authors:
Zapatero Osorio et al
Abstract:
We aim to determine the trigonometric parallaxes and proper motions of a sample of ten field L0-L5 dwarfs with spectroscopic evidence for low-gravity atmospheres. We obtained J and Ks imaging data using 2-4-m class telescopes with a typical cadence of one image per month between 2010 January and 2012 December. We also obtained low resolution optical spectra (R~300, 500-1100 nm) using the 10-m GTCs to assess the presence of lithium absorption in four targets and confirm their young age. Trigonometric parallaxes and proper motions were derived to typical accuracies of 1 mas and +/-10 mas/yr. All ten L dwarfs have large motions, and are located at distances between 9 and 47 pc. They lie above and on the sequence of field dwarfs in the absolute J and K_s magnitude versus spectral type and luminosity versus Teff diagrams, implying ages similar to or smaller than those typical of the field. The detection of atomic lithium in the atmosphere of 2MASS J00452143+1634446 is reported for the first time. Three dwarfs have locations in the HR diagram indicative of old ages and high masses consistent with the observed lithium depletion previously published. We did not find evidence for the presence of astrometric companions with minimum detectable masses typically >=25 Mjup and face-on, circular orbits with periods between 60-90 d and 3 yr around eight targets. The astrometric and spectroscopic data indicate that about 60-70% of the field L-type dwarfs in our sample with evidence for low-gravity atmospheres are indeed young-to-intermediate-age brown dwarfs of the solar neighborhood with expected ages and masses in the intervals 10-500 Myr and 11-45 Mjup. The peaked-shape of the H-band spectra of L dwarfs, a signpost of youth, appears to be present up to ages of 120-500 Myr and intermediate-to-high gravities.
Signatures of Cloud, Temperature, and Gravity From Spectra of the Closest Brown Dwarfs
Authors:
Faherty et al
Abstract:
We present medium resolution optical and NIR spectral data for components of the newly discovered WISE J104915.57-531906.1AB (Luhman 16AB) brown dwarf binary. The optical spectra reveal strong 6708 A Li I absorption in both Luhman 16A (8.0+/-0.4 A) and Luhman 16B (3.8+/-0.4 A). Interestingly, this is the first detection of Li I absorption in a T dwarf. Combined with the lack of surface gravity features, the Li I detection constrains the system age to 0.1 - 3 Gyr. In the NIR data, we find strong KI absorption at 1.168, 1.177, 1.243, and 1.254 {\mu}m in both components. Compared to the strength of KI line absorption in equivalent spectral subtype brown dwarfs, Luhman 16A is weaker while Luhman 16B is stronger. Analyzing the spectral region around each doublet in distance scaled flux units and comparing the two sources, we confirm the J band flux reversal and find that Luhman 16B has a brighter continuum in the 1.17 {\mu}m and 1.25 {\mu}m regions than Luhman 16A. Converting flux units to a brightness temperature we interpret this to mean that the secondary is ~ 50 K warmer than the primary in regions dominated by condensate grain scattering. One plausible explanation for this difference is that Luhman 16B has thinner clouds or patchy holes in its atmosphere allowing us to see to deeper, hotter regions. We also detect comparably strong FeH in the 0.9896 {\mu}m Wing-Ford band for both components. Traditionally, a signpost of changing atmosphere conditions from late-type L to early T dwarfs, the persistence and similarity of FeH at 0.9896 {\mu}m in both Luhman 16A and Luhman 16B is an indication of homogenous atmosphere conditions. We calculate bolometric luminosities from observed data supplemented with best fit models for longer wavelengths and find the components are consistent within 1{\sigma} with resultant Teffs of 1310+/-30 K and 1280+/-75 K for Luhman 16AB respectively.
Kinematics and Luminosities of Brown Dwarfs with the BDNYC group
Author:
Riedel
Abstract:
Due to magnitude limits, the Gaia survey will not delve as deeply into the local population of brown dwarfs as it will other stellar populations. While hundreds or thousands of brown dwarfs will be measured by Gaia, we propose a different, indirect method wherein studies using Gaia data will help teach us about brown dwarfs: Identifying moving groups that contain brown dwarfs. This use of Gaia data will directly help attempts to disentangle the effects of age and mass on brown dwarf spectra, which opens the possibilities for determining empirical constraints on brown dwarf evolution.
Tidal dissipation in stars and giant planets
Authors:
Ogilvie et al
Abstract:
Astrophysical fluid bodies that orbit close to one another induce tidal distortions and flows that are subject to dissipative processes. The spin and orbital motions undergo a coupled evolution over astronomical timescales, which is relevant for many types of binary star, short-period extrasolar planetary systems and the satellites of the giant planets in the solar system. I review the principal mechanisms that have been discussed for tidal dissipation in stars and giant planets in both linear and nonlinear regimes. I also compare the expectations based on theoretical models with recent observational findings.
Unravelling tidal dissipation in gaseous giant planets
Authors:
Guenel et al
Abstract:
Tidal dissipation in planetary interiors is one of the key physical mechanisms that drive the evolution of star-planet and planet-moon systems. New constraints are now obtained both in the Solar and exoplanetary systems. Tidal dissipation in planets is intrinsically related to their internal structure. In particular, fluid and solid layers behave differently under tidal forcing. Therefore, their respective dissipation reservoirs have to be compared. In this letter, we compute separately the contributions of the potential dense rocky/icy core and the convective fluid envelope of gaseous giant planets, as a function of core size and mass. We then compare the associated dissipation reservoirs, by evaluating the frequency-average of the imaginary part of the Love numbers k22 in each region. In the case of Jupiter and Saturn-like planets, we show that the viscoelastic dissipation in the core could dominate the turbulent friction acting on tidal inertial waves in the envelope. However, the fluid dissipation would not be negligible. This demonstrates that it is necessary to build complete models of tidal dissipation in planetary interiors from their deep interior to their surface without any arbitrary a-priori.
Transiting exoplanets from the CoRoT space mission: XXIV. CoRoT-24: A transiting multi-planet system
Authors:
Alonso et al
Abstract:
We present the discovery of a candidate multiply-transiting system, the first one found in the CoRoT mission. Two transit-like features with periods of 5.11 and 11.76d are detected in the CoRoT light curve, around a main sequence K1V star of r=15.1. If the features are due to transiting planets around the same star, these would correspond to objects of 3.7±0.4 and 5.0±0.5 R_earth respectively. Several radial velocities serve to provide an upper limit of 5.7 M_earth for the 5.11~d signal, and to tentatively measure a mass of 28+11−11 M_earth for the object transiting with a 11.76~d period. These measurements imply low density objects, with a significant gaseous envelope. The detailed analysis of the photometric and spectroscopic data serve to estimate the probability that the observations are caused by transiting Neptune-sized planets as >26× higher than a blend scenario involving only one transiting planet, and >900× higher than a scenario involving two blends and no planets. The radial velocities show a long term modulation that might be attributed to a 1.5 M_jup planet orbiting at 1.8~A.U. from the host, but more data are required to determine the precise orbital parameters of this companion.
Dynamical structure of the inner 100 AU of the deeply embedded protostar IRAS 16293-2422
Authors:
Favre et al
Abstract:
A fundamental question about the early evolution of low-mass protostars is when circumstellar disks may form. High angular resolution observations of molecular transitions in the (sub)millimeter wavelength windows make it possible to investigate the kinematics of the gas around newly-formed stars, for example to identify the presence of rotation and infall. IRAS 16293-2422 was observed with the extended Submillimeter Array (eSMA) resulting in subarcsecond resolution (0.46" x 0.29", i.e. ∼ 55 × 35~AU) images of compact emission from the C17O (3-2) and C34S (7-6) transitions at 337~GHz (0.89~mm). To recover the more extended emission we have combined the eSMA data with SMA observations of the same molecules. The emission of C17O (3-2) and C34S (7-6) both show a velocity gradient oriented along a northeast-southwest direction with respect to the continuum marking the location of one of the components of the binary, IRAS16293A. Our combined eSMA and SMA observations show that the velocity field on the 50--400~AU scales is consistent with a rotating structure. It cannot be explained by simple Keplerian rotation around a single point mass but rather needs to take into account the enclosed envelope mass at the radii where the observed lines are excited. We suggest that IRAS 16293-2422 could be among the best candidates to observe a pseudo-disk with future high angular resolution observations.
A search for eclipsing binaries that host discs
Authors:
Meng et al
Abstract:
We search for systems hosting eclipsing discs using a complete sample of eclipsing binaries (EBs); those previously identified in the third phase of the Optical Gravitational Lensing Experiment (OGLE-III). Within a subsample of 2823 high-cadence, high-photometric precision and large eclipsing depth detached EBs previously identified in the Large Magellanic Cloud (LMC), we find that the skewness and kurtosis of the light-curve magnitude distribution within the primary eclipse can distinguish EBs with a complex-shaped eclipse from those without. Two systems with previously identified eclipsing discs (OGLE-LMC-ECL-11893 and OGLE-LMC-ECL-17782) are identified with near zero skewness (|S| < 0.5) and positive kurtosis. No additional eclipsing disc systems were found in the OGLE-III LMC, Small Magellanic Cloud or Galactic Disc EB light curves. We estimate that the fraction of detached early-type LMC EBs (which have a primary with an I-band magnitude brighter than ≃19 mag) that exhibit atypical eclipses and so could host a disc is approximately 1/1000. As circumstellar disc lifetimes are short, we expected to primarily find eclipsing discs around young stars. In addition, as there is more room for a disc in a widely separated binary and because a disc close to a luminous star would be above the dust sublimation temperature, we expected to primarily find eclipsing discs in long-period binaries. However, OGLE-LMC-ECL-17782 is a 13.3 d period B star system with a transient and hot (∼6000 K, ∼0.1 au radius) disc and Scott et al. estimate an age of 150 Myr for OGLE-LMC-ECL-11893. Both discs are unexpected in the EB sample and impel explanation.
OGLE-LMC-ECL-11893: THE DISCOVERY OF A LONG-PERIOD ECLIPSING BINARY WITH A CIRCUMSTELLAR DISK
Authors:
Dong et al
Abstract:
We report the serendipitous discovery of a disk-eclipse system OGLE-LMC-ECL-11893. The eclipse occurs with a period of 468 days, a duration of about 15 days, and a deep (up to ΔmI ≈ 1.5), peculiar, and asymmetric profile. A possible origin of such an eclipse profile involves a circumstellar disk. The presence of the disk is confirmed by the H-α line profile from the follow-up spectroscopic observations, and the star is identified as Be/Ae type. Unlike the previously known disk-eclipse candidates, the eclipses of OGLE-LMC-ECL-11893 retain the same shape throughout the span of ~17 yr (13 orbital periods), indicating no measurable orbital precession of the disk.
CONSTRAINING THE ATMOSPHERIC COMPOSITION OF THE DAY-NIGHT TERMINATORS OF HD 189733b: ATMOSPHERIC RETRIEVAL WITH AEROSOLS
Authors:
Lee et al
Abstract:
A number of observations have shown that Rayleigh scattering by aerosols dominates the transmission spectrum of HD 189733b at wavelengths shortward of 1 μm. In this study, we retrieve a range of aerosol distributions consistent with transmission spectroscopy between 0.3-24 μm that were recently re-analyzed by Pont et al. To constrain the particle size and the optical depth of the aerosol layer, we investigate the degeneracies between aerosol composition, temperature, planetary radius, and molecular abundances that prevent unique solutions for transit spectroscopy. Assuming that the aerosol is composed of MgSiO3, we suggest that a vertically uniform aerosol layer over all pressures with a monodisperse particle size smaller than about 0.1 μm and an optical depth in the range 0.002-0.02 at 1 μm provides statistically meaningful solutions for the day/night terminator regions of HD 189733b. Generally, we find that a uniform aerosol layer provide adequate fits to the data if the optical depth is less than 0.1 and the particle size is smaller than 0.1 μm, irrespective of the atmospheric temperature, planetary radius, aerosol composition, and gaseous molecules. Strong constraints on the aerosol properties are provided by spectra at wavelengths shortward of 1 μm as well as longward of 8 μm, if the aerosol material has absorption features in this region. We show that these are the optimal wavelengths for quantifying the effects of aerosols, which may guide the design of future space observations. The present investigation indicates that the current data offer sufficient information to constrain some of the aerosol properties of HD189733b, but the chemistry in the terminator regions remains uncertain.
The thermal emission of the exoplanet WASP-3b
Authors:
Rostron et al
Abstract:
We report the detection of thermal emission from the transiting hot Jupiter WASP-3b at 3.6, 4.5 and 8.0 μm using the Spitzer Space Telescope. We obtain planet-to-star flux ratios of 0.209+0.040−0.028, 0.282 ± 0.012 and 0.328+0.086−0.055 per cent at these wavelengths, respectively, implying infrared brightness temperatures of T3.6μm=2280+210−150K, T4.5 μm = 2400 ± 80 K and T8.0μm=2210+390−250K. We find that WASP-3b falls into an emerging class of highly irradiated planets whose measured temperatures suggest that the planets are dark and redistribute heat around the planet inefficiently. The latter is similarly concluded from 1D atmospheric model comparisons, which also favour the presence of an atmospheric temperature inversion. We compare the WASP-3 system to the proposed inversion–activity relation, finding that it hints at a more complex relation than a simple cut-off in activity implied by previous data. Using eclipse timings we also constrain e cos ω to be −0.0006+0.0010−0.0006, suggesting that the eccentricity of WASP-3b can only be large for a narrow range of ω.
A Misaligned Prograde Orbit for Kepler-13 Ab via Doppler Tomography
Authors:
Johnson et al
Abstract:
Transiting planets around rapidly rotating stars are not amenable to precise radial velocity observations, such as are used for planet candidate validation, as they have wide, rotationally broadened stellar lines. Such planets can, however, be observed using Doppler tomography, wherein the stellar absorption line profile distortions during transit are spectroscopically resolved. This allows the validation of transiting planet candidates and the measurement of the stellar spin-planetary orbit (mis)alignment, an important statistical probe of planetary migration processes. We present Doppler tomographic observations which provide a direct confirmation of the hot Jupiter Kepler-13 Ab, and also show that the planet has a prograde, misaligned orbit, with lambda = 58.6 +/- 2.0 degrees. Our measured value of the spin-orbit misalignment is in significant disagreement with the value of lambda = 23 +/- 4 degrees previously measured by Barnes et al. (2011) from the gravity-darkened Kepler lightcurve. We also place an upper limit of 0.75 solar masses (95% confidence) on the mass of Kepler-13 C, the spectroscopic companion to Kepler-13 B, the proper motion companion of the planet host star Kepler-13 A.
The Transiting Exoplanet Survey Satellite
Authors:
Ricker et al
Abstract:
he Transiting Exoplanet Survey Satellite (TESS) will search for planets transiting bright and nearby stars. TESS has been selected by NASA for launch in 2017 as an Astrophysics Explorer mission. The spacecraft will be placed into a highly elliptical 13.7-day orbit around the Earth. During its two-year mission, TESS will employ four wide-field optical CCD cameras to monitor at least 200,000 main-sequence dwarf stars with I less than 13 for temporary drops in brightness caused by planetary transits. Each star will be observed for an interval ranging from one month to one year, depending mainly on the star's ecliptic latitude. The longest observing intervals will be for stars near the ecliptic poles, which are the optimal locations for follow-up observations with the James Webb Space Telescope. Brightness measurements of preselected target stars will be recorded every 2 min, and full frame images will be recorded every 30 min. TESS stars will be 10-100 times brighter than those surveyed by the pioneering Kepler mission. This will make TESS planets easier to characterize with follow-up observations. TESS is expected to find more than a thousand planets smaller than Neptune, including dozens that are comparable in size to the Earth. Public data releases will occur every four months, inviting immediate community-wide efforts to study the new planets. The TESS legacy will be a catalog of the nearest and brightest stars hosting transiting planets, which will endure as highly favorable targets for detailed investigations.
Complex organic molecules along the accretion flow in isolated and externally irradiated protoplanetary disks
Authors:
Walsh et al
Abstract:
The birth environment of the Sun will have influenced the conditions in the pre-solar nebula, including the attainable chemical complexity, important for prebiotic chemistry. The formation and distribution of complex organic molecules (COMs) in a disk around a T Tauri star is investigated for two scenarios: (i) an isolated disk, and (ii) a disk irradiated externally by a nearby massive star. The chemistry is calculated along the accretion flow from the outer disk inwards using a comprehensive network. Two simulations are performed, one beginning with complex ices and one with simple ices only. For the isolated disk, COMs are transported without major alteration into the inner disk where they thermally desorb into the gas reaching an abundance representative of the initial assumed ice abundance. For simple ices, COMs efficiently form on grain surfaces under the conditions in the outer disk. Gas-phase COMs are released into the molecular layer via photodesorption. For the irradiated disk, complex ices are also transported inwards; however, they undergo thermal processing caused by the warmer conditions in the irradiated disk which tends to reduce their abundance along the accretion flow. For simple ices, grain-surface chemistry cannot synthesise COMs in the outer disk because the necessary grain-surface radicals, which tend to be particularly volatile, are not sufficiently abundant on the grain surfaces. Gas-phase COMs are formed in the inner region of the irradiated disk via gas-phase chemistry induced by the desorption of strongly bound molecules such as methanol; hence, the abundances are not representative of the initial molecular abundances injected into the outer disk. These results suggest that the composition of comets formed in isolated disks may differ from those formed in externally irradiated disks with the latter composed of more simple ices.
Long Term Evolution of Planet-Induced Vortices in Protoplanetary Disks
Authors:
Fu et al
Abstract:
Recent observations of large-scale asymmetric features in protoplanetary disks suggest that large-scale vortices exist in such disks. Massive planets are known to be able to produce deep gaps in protoplanetary disks. The gap edges could become hydrodynamically unstable to the Rossby wave/vortex instability and form large-scale vortices. In this study we examine the long term evolution of these vortices by carrying out high-resolution two dimensional hydrodynamic simulations that last more than 104 orbits (measured at the planet's orbit). We find that the disk viscosity has a strong influence on both the emergence and lifetime of vortices. In the outer disk region where asymmetric features are observed, our simulation results suggest that the disk viscous α needs to be low ∼10−5 - 10−4 to sustain vortices to thousands and up to 104 orbits in certain cases. The chance of finding a vortex feature in a disk then decreases with smaller planet orbital radius. For α∼10−3 or larger, even planets with masses of 5 Jupiter-masses will have difficulty either producing or sustaining vortices. We have also studied the effects of different disk temperatures and planet masses. We discuss the implications of our findings on current and future protoplanetary disk observations.
Using Photometry to Probe the Circumstellar Environment of delta Scorpii
Authors:
Jones et al
Abstract:
We acquired Johnson BV photometry of the binary Be disk system delta~Scorpii during its 2009, 2010, 2011, and 2012 observing seasons and used it to probe the innermost regions of the disk. We found that several disk building events have occurred during this time, resulting in an overall brightening in the V-band and reddening of the system. In addition to these long-term trends, we found cyclical variability in each observing season on timescales between 60 and 100 days. We were able to reproduce the changes in the magnitude and colour of delta Sco using our theoretical models and found that variable mass-loss rates in the range 2.5-7.0x10^{-9} M_{sun}/yr over ~35 days can reproduce the observed increase in brightness.
The protoplanetary disk of FT Tauri: multi-wavelength data analysis and modeling
Authors:
Garufi et al
Abstract:
Investigating the evolution of protoplanetary disks is crucial for our understanding of star and planet formation. Several theoretical and observational studies have been performed in the last decades to advance this knowledge. FT Tauri is a young star in the Taurus star forming region that was included in a number of spectroscopic and photometric surveys. We investigate the properties of the star, the circumstellar disk, and the accretion and ejection processes and propose a consistent gas and dust model also as a reference for future observational studies. We performed a multi-wavelength data analysis to derive the basic stellar and disk properties, as well as mass accretion/outflow rate from TNG-Dolores, WHT-Liris, NOT-Notcam, Keck-Nirspec, and Herschel-Pacs spectra. From the literature, we compiled a complete Spectral Energy Distribution. We then performed detailed disk modeling using the MCFOST and ProDiMo codes. Multi-wavelengths spectroscopic and photometric measurements were compared with the reddened predictions of the codes in order to constrain the disk properties. This object can serve as a benchmark for primordial disks with significant mass accretion rate, high gas content and typical size.
The VAMPIRES instrument: Imaging the innermost regions of protoplanetary disks with polarimetric interferometr
Authors:
Norris et al
Abstract:
Direct imaging of protoplanetary disks promises to provide key insight into the complex sequence of processes by which planets are formed. However imaging the innermost region of such disks (a zone critical to planet formation) is challenging for traditional observational techniques (such as near-IR imaging and coronagraphy) due to the relatively long wavelengths involved and the area occulted by the coronagraphic mask. Here we introduce a new instrument -- VAMPIRES -- which combines non-redundnant aperture-masking interferometry with differential polarimetry to directly image this previously inaccessible innermost region. By using the polarization of light scattered by dust in the disk to provide precise differential calibration of interferometric visibilities and closure phases, VAMPIRES allows direct imaging at and beyond the telescope diffraction limit. Integrated into the SCExAO system at the Subaru telescope, VAMPIRES operates at visible wavelengths (where polarization is high) while allowing simultaneous infrared observations conducted by HICIAO. Here we describe the instrumental design and unique observing technique and present the results of the first on-sky commissioning observations, validating the excellent visibility and closure phase precision which are then used to project expected science performance metrics.
WISEA J064750.85-154616.4: a new nearby L/T transition dwarf
Authors:
Scholz et al
Abstract:
Aims:
Our aim is to detect and classify previously overlooked brown dwarfs in the solar neighbourhood.
Methods:
We performed a proper motion search among bright sources observed with the Wide-field Infrared Survey Explorer (WISE) that are also seen in the Two Micron All Sky Survey (2MASS). Our candidates appear according to their red J−Ks colours as nearby late-L dwarf candidates. Low-resolution near-infrared (NIR) classification spectroscopy in the HK band allowed us to get spectroscopic distance and tangential velocity estimates.
Results:
We have discovered a new L9.5 dwarf, WISEA J064750.85-154616.4, at a spectroscopic distance of about 14 pc and with a tangential velocity of about 11 km/s, typical of the Galactic thin disc population. We have confirmed another recently found L/T transition object at about 10 pc, WISEA J140533.13+835030.7, which we classified as L8 (NIR).
Brown dwarf disks with ALMA
Authors:
Ricci et al
Abstract:
We present ALMA continuum and spectral line data at 0.89 mm and 3.2 mm for three disks surrounding young brown dwarfs and very low mass stars in the Taurus star forming region. Dust thermal emission is detected and spatially resolved for all the three disks, while CO(J=3-2) emission is seen in two disks. We analyze the continuum visibilities and constrain the disks physical structure in dust. The results of our analysis show that the disks are relatively large, the smallest one with an outer radius of about 70 AU. The inferred disk radii, radial profiles of the dust surface density and disk to central object mass ratios lie within the ranges found for disks around more massive young stars. We derive from our observations the wavelength dependence of the millimeter dust opacity. In all the three disks data are consistent with the presence of grains with at least millimeter sizes, as also found for disks around young stars, and confirm that the early stages of the solid growth toward planetesimals occur also around very low mass objects. We discuss the implications of our findings on models of solids evolution in protoplanetary disks, on the main mechanisms proposed for the formation of brown dwarfs and very low mass stars, as well as on the potential of finding rocky and giant planets around very low mass objects.
Cloud structure of brown dwarfs from spectroscopic variability observations
Authors:
Buenzli et al
Abstract:
Recent discoveries of variable brown dwarfs have provided us with a new window into their three-dimensional cloud structure. The highest variables are found at the L/T transition, where the cloud cover is thought to break up, but variability has been found to occur also for both cloudy L dwarfs and (mostly) cloud-free mid T dwarfs. We summarize results from recent HST programs measuring the spectral variability of brown dwarfs in the near-infrared and compare to results from ground-based programs. We discuss the patchy cloud structure of L/T transition objects, for which it is becoming increasingly certain that the variability does not arise from cloud holes into the deep hot regions but from varying cloud thickness. We present a new patchy cloud model to explain the spectral variability of 2MASSJ21392676+0220226. We also discuss the curious multi-wavelength variability behavior of the recently discovered very nearby early T dwarf WISE J104915.57-531906.1B (Luhman 16B) and the mid T dwarf 2MASS J22282889-431026.
TIDAL HEATING IN MULTILAYERED TERRESTRIAL EXOPLANETS
Authors:
Henning et al
Abstract:
The internal pattern and overall magnitude of tidal heating for spin-synchronous terrestrial exoplanets from 1 to 2.5 RE is investigated using a propagator matrix method for a variety of layer structures. Particular attention is paid to ice-silicate hybrid super-Earths, where a significant ice mantle is modeled to rest atop an iron-silicate core, and may or may not contain a liquid water ocean. We find multilayer modeling often increases tidal dissipation relative to a homogeneous model, across multiple orbital periods, due to the ability to include smaller volume low viscosity regions, and the added flexure allowed by liquid layers. Gradations in parameters with depth are explored, such as allowed by the Preliminary Earth Reference Model. For ice-silicate hybrid worlds, dramatically greater dissipation is possible beyond the case of a silicate mantle only, allowing non-negligible tidal activity to extend to greater orbital periods than previously predicted. Surface patterns of tidal heating are found to potentially be useful for distinguishing internal structure. The influence of ice mantle depth and water ocean size and position are shown for a range of forcing frequencies. Rates of orbital circularization are found to be 10-100 times faster than standard predictions for Earth-analog planets when interiors are moderately warmer than the modern Earth, as well as for a diverse range of ice-silicate hybrid super-Earths. Circularization rates are shown to be significantly longer for planets with layers equivalent to an ocean-free modern Earth, as well as for planets with high fractions of either ice or silicate melting.
Assessing the Possibility of Biological Complexity on Other Worlds, with an Estimate of the Occurrence of Complex Life in the Milky Way Galaxy
Authors:
Irwin et al
Abstract:
Rational speculation about biological evolution on other worlds is one of the outstanding challenges in astrobiology. With the growing confirmation that multiplanetary systems abound in the universe, the prospect that life occurs redundantly throughout the cosmos is gaining widespread support. Given the enormous number of possible abodes for life likely to be discovered on an ongoing basis, the prospect that life could have evolved into complex, macro-organismic communities in at least some cases merits consideration. Toward that end, we here propose a Biological Complexity Index (BCI), designed to provide a quantitative estimate of the relative probability that complex, macro-organismic life forms could have emerged on other worlds. The BCI ranks planets and moons by basic, first-order characteristics detectable with available technology. By our calculation only 11 (~1.7%) of the extrasolar planets known to date have a BCI above that of Europa; but by extrapolation, the total of such planets could exceed 100 million in our galaxy alone. This is the first quantitative assessment of the plausibility of complex life throughout the universe based on empirical data. It supports the view that the evolution of complex life on other worlds is rare in frequency but large in absolute number.
Chance and Necessity in Biochemistry: Implications for the Search for Extraterrestrial Biomarkers in Earth-like Environments
Authors:
Davila et al
Abstract:
In this paper, we examine a restricted subset of the question of possible alien biochemistries. That is, we look into how different life might be if it emerged in environments similar to that required for life on Earth. We advocate a principle of chance and necessity in biochemistry. According to this principle, biochemistry is in some fundamental way the sum of two processes: there is an aspect of biochemistry that is an endowment from prebiotic processes, which represents the necessity, plus an aspect that is invented by the process of evolution, which represents the chance. As a result, we predict that life originating in extraterrestrial Earth-like environments will share biochemical motifs that can be traced back to the prebiotic world but will also have intrinsic biochemical traits that are unlikely to be duplicated elsewhere as they are combinatorially path-dependent. Effective and objective strategies to search for biomarkers, and evidence for a second genesis, on planets with Earth-like environments can be built based on this principle.
Dancing with the stars: formation of the Fomalhaut triple system and its effect on the debris discs
Authors:
Shannon et al
Abstract:
Fomalhaut is a triple system, with all components widely separated (∼105 au). Such widely separated binaries are thought to form during cluster dissolution, but that process is unlikely to form such a triple system. We explore an alternative scenario, where A and C form as a tighter binary from a single molecular cloud core (with semimajor axis ∼104 au), and B is captured during cluster dispersal. We use N-body simulations augmented with the Galactic tidal forces to show that such a system naturally evolves into a Fomalhaut-like system in about half of cases, on a time-scale compatible with the age of Fomalhaut. From initial non-interacting orbits, Galactic tides drive cycles in B's eccentricity that lead to a close encounter with C. After several close encounters, typically lasting tens of millions of years, one of the stars is ejected. The Fomalhaut-like case with both components at large separations is almost invariably a precursor to the ejection of one component, most commonly Fomalhaut C. By including circumstellar debris in a subset of the simulations, we also show that such an evolution usually does not disrupt the coherently eccentric debris disc around Fomalhaut A, and in some cases can even produce such a disc. We also find that the final eccentricity of the disc around A and the disc around C are correlated, which may indicate that the dynamics of the three stars stirred C's disc, explaining its unusual brightness.
Planetesimal Interactions Can Explain the Mysterious Period Ratios of Small Near-Resonant Planets
Authors:
Chatterjee et al
Abstract:
An intriguing trend among the Kepler multi-planet systems is a clear over abundance of planet pairs with period ratios just wide of several mean motion resonances (MMR) and a dearth of systems just narrow of them. Traditional planet formation models such as gas-disk migration or planet-planet scattering are at odds with these observations. This is also in dramatic contrast to the period ratio distribution of multi-planet systems discovered via radial velocity (RV) surveys, where near-resonant planet pairs tend to pile up near exact integer ratio of orbital periods such as 2:1, as expected from gas-disk migration. We propose that gas-disk migration traps planets in a MMR. After gas dispersal, orbits of these trapped planets are altered through interaction with a residual planetesimal disk. We study the effects of planetesimal disk interactions on planet pairs trapped in 2:1 MMR using planets of mass typical of the KPCs and explore large ranges for the mass, and density profile of the planetesimal disk. We find that planet-planetesimal disk interactions naturally create the observed asymmetry in period ratio distribution for large ranges of planetesimal disk and planet properties. If the planetesimal disk mass is above a threshold of 0.2x the planet mass, these interactions typically disrupt MMR. Then the planets migrate in such a way that the ratio of their final orbital periods is slightly higher than the integer ratio corresponding to the initial MMR. Below this threshold these interactions typically cannot disrupt the resonance and the period ratio stays close to the integer ratio. The threshold explains why the more massive planet pairs found by RV surveys are still in resonance. These interactions can make significant changes in the planets' atmospheric and surface properties, especially for small planets typical of the KPCs, which may be observable in the near future.
