Since scientists started identifying lots of planets orbiting distant stars in the past decade, one of the weirder types is the surprisingly common "hot Jupiter" -- a gas giant like the ones we know that orbits very close to its star. Now NASA's Spitzer space telescope has spotted a particularly weird hot Jupiter that's more of a "very hot and cold Jupiter."
The planet, called HD 80606b, is about 190 light-years from Earth and has a highly eccentric orbit around its star that's more like that of a comet than the planets in our solar system. Every 111 days, the planet passes so close around its star that it almost touches it -- if it were able, it could probably reach out and high-five or fist-bump its sun. It then swings farther away from its star (a little less than the distance between our sun and Earth) before doing a U-turn at the other end of its elliptical orbit to repeat the cycle.
Radio Emission from Red-Giant Hot Jupiters
Authors:
Fuji et al
Abstract:
When planet-hosting stars evolve off the main sequence and go through the red-giant branch, the stars become orders of magnitudes more luminous and at the same time lose mass at much higher rates than their main-sequence counterparts. Accordingly, planetary companions around them at orbital distances of a few AU, if they exist, will be heated up to the level of canonical hot Jupiters and also subjected to a dense stellar wind. Given that magnetized planets interacting with stellar winds emit radio waves, such "Red-Giant Hot Jupiters" (RGHJs) may also be candidate radio emitters. We estimate the spectral auroral radio intensity of RGHJs based on the empirical relation with the stellar wind as well as a proposed scaling for planetary magnetic fields. RGHJs might be intrinsically as bright as or brighter than canonical hot Jupiters, and about 100 times brighter than equivalent objects around main-sequence stars. We examine the capabilities of low-frequency radio observatories to detect this emission and find that the signal from a RGHJ may be detectable at distances up to a few hundred parsecs with the Square Kilometer Array.
Axisymmetric Simulations of Hot Jupiter-Stellar Wind Hydrodynamic Interaction
Authors:
Christie et al
Abstract:
Gas giant exoplanets orbiting at close distances to the parent star are subjected to large radiation and stellar wind fluxes. In this paper, hydrodynamic simulations of the planetary upper atmosphere and its interaction with the stellar wind are carried out to understand the possible flow regimes and how they affect the Lyman-alpha transmission spectrum. Following Tremblin and Chiang, charge exchange reactions are included to explore the role of energetic atoms as compared to thermal particles. In order to understand the role of the tail as compared to the leading edge of the planetary gas, the simulations were carried out under axisymmetry, and photoionization and stellar wind electron impact ionization reactions were included to limit the extent of the neutrals away from the planet. By varying the planetary gas temperature, two regimes are found. At high temperature, a supersonic planetary wind is found, which is turned around by the stellar wind and forms a tail behind the planet. At lower temperatures, the planetary wind is shut off when the stellar wind penetrates inside where the sonic point would have been. In this regime mass is lost by viscous interaction at the boundary between planetary and stellar wind gases. Absorption by cold hydrogen atoms is large near the planetary surface, and decreases away from the planet as expected. The hot hydrogen absorption is in an annulus and typically dominated by the tail, at large impact parameter, rather than by the thin leading edge of the mixing layer near the substellar point.
HST hot-Jupiter transmission spectral survey: Clear skies for cool Saturn WASP-39b
Authors:
Fischer et al
Abstract:
We present HST STIS optical transmission spectroscopy of the cool (approximately 1116 K) Saturn-mass exoplanet WASP-39b from 0.29-1.025 micron, along with complementary transit observations from Spitzer IRAC at 3.6 and 4.5 micron. The low density and large atmospheric pressure scale height of WASP-39b make it particularly amenable to atmospheric characterization using this technique. We detect a Rayleigh scattering slope as well as sodium and potassium absorption features; this is the first exoplanet in which both alkali features are clearly detected with the extended wings predicted by cloud-free atmosphere models. The full transmission spectrum is well matched by a clear, H2- dominated atmosphere or one containing a weak contribution from haze, in good agreement with the preliminary reduction of these data presented in Sing et al. (2016). WASP-39b is predicted to have a pressure-temperature profile comparable to that of HD 189733b and WASP-6b, making it one of the coolest transiting gas giants observed in our HST STIS survey. Despite this similarity, WASP- 39b appears to be largely cloud-free while the transmission spectra of HD 189733b and WASP-6b both indicate the presence of high altitude clouds or hazes. These observations further emphasize the surprising diversity of cloudy and cloud-free gas giant planets in short-period orbits and the corresponding challenges associated with developing predictive cloud models for these atmospheres.
An international team of astronomers, led by the University of Cambridge, has obtained the most detailed 'fingerprint' of a rocky planet outside our solar system to date, and found a planet of two halves: one that is almost completely molten, and the other which is almost completely solid.
According to the researchers, conditions on the hot side of the planet are so extreme that it may have caused the atmosphere to evaporate, with the result that conditions on the two sides of the planet vary widely: temperatures on the hot side can reach 2500 degrees Celsius, while temperatures on the cool side are around 1100 degrees. The results are reported in the journal Nature.
Using data from NASA's Spitzer Space Telescope, the researchers examined a planet known as 55 Cancri e, which orbits a sun-like star located 40 light years away in the Cancer constellation, and have mapped how conditions on the planet change throughout a complete orbit, the first time this has been accomplished for such a small planet.
55 Cancri e is a 'super Earth': a rocky exoplanet about twice the size and eight times the mass of Earth, and orbits its parent star so closely that a year lasts just 18 hours. The planet is also tidally locked, meaning that it always shows the same face to its parent star, similar to the Moon, so there is a permanent 'day' side and a 'night' side. Since it is among the nearest super Earths whose composition can be studied, 55 Cancri e is among the best candidates for detailed observations of surface and atmospheric conditions on rocky exoplanets.
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Physicists of MIPT (Moscow Institute of Physics and Technology) and the Space Research Institute of the Russian Academy of Sciences developed optical technology for the "correction" of light coming from distant stars, which will significantly improve the "seeing" of telescopes and therefore will enable us to directly observe exoplanets as Earth-twins. Their work has been published in the Journal of Astronomical Telescopes, Instruments, and Systems (JATIS).
The first exoplanets (extra solar planets), which are the planets outside our solar system, had been discovered in the late 20th century, and now we have detected of more than two thousand of them. It is almost impossible to see the faint light of the planets themselves without special tools -- it is saturated "overshadowed" by the radiation of parent star. Therefore exoplanets are discovered by indirect methods: by registration of the weak periodic fluctuations in the luminosity of the star when a planet passes in front of its disk (the transit method), or by spectral translational vibrations of the star itself from the impact of the planet's gravity (the radial-velocity method). For the first time, in the late 2000s, astronomers were able to directly obtain images of exoplanets. So far we have about 65 of such images. To obtain them, the scientists use stellar coronagraphs first created in 1930s for observations of the solar corona outside eclipses known as solar coronagraphs. These devices have a focal mask - an "artificial moon" inside them, which blocks some part of the field of view -- ultimately, it covers the solar disk, allowing you to see the dim solar corona.
To repeat this technique for the stars, we need a much higher level of accuracy and much higher resolution of the telescope, which accommodates a coronagraph. Apparent size of the orbit of Earth-type planets, nearest to us, is about 0.1 arcseconds. This is close to the resolution limit of modern space telescopes (for example, the resolution of the space telescope Hubble is about 0.05 seconds). To remove the effects of atmospheric distortions in ground-based telescopes, scientists use adaptive optics -- mirrors that can change shape while adjusting to the state of the atmosphere. In some cases, the mirror shape can be maintained with an accuracy of 1 nanometer, but such systems do not keep pace with the dynamics of atmospheric changes and are extremely expensive.
A team led by Alexander Tavrov, an associate professor at MIPT and the Head of the Planetary Astronomy Laboratory at the Space Research Institute of the Russian Academy of Sciences, has found a way to obtain the highest resolution, while using relatively simple and inexpensive systems of adaptive optics.
Warm Jupiters are less lonely than hot Jupiters: close neighbours
Authors:
Huang et al
Abstract:
Exploiting the Kepler transit data, we uncover a dramatic distinction in the prevalence of sub-Jovian companions, between systems that contain hot Jupiters (periods inward of 10 days) and those that host warm Jupiters (periods between 10 and 200 days). Hot Jupiters as a whole, with the singular exception of WASP-47b, do not have any detectable inner or outer planetary companions (with periods inward of 50 days and sizes down to 2REarth). Restricting ourselves to inner companions, our limits reach down to 1REarth. In stark contrast, half of the warm Jupiters are closely flanked by small companions. Statistically, the companion fractions for hot and warm Jupiters are mutually exclusive, in particular in regard to inner companions.
The high companion fraction of warm Jupiters also yields clue to their formation. The warm Jupiters that have close-by siblings should have low orbital eccentricities and low mutual inclinations. The orbital configurations of these systems are reminiscent of those of the low-mass, close-in planetary systems abundantly discovered by the Kepler mission. This, and other arguments, lead us to propose that these warm Jupiters are formed in-situ. There are indications that there may be a second population of warm Jupiters with different characteristics. In this picture, WASP-47b could be regarded as the extending tail of the in-situ warm Jupiters into the hot Jupiter region, and does not represent the generic formation route for hot Jupiters.
Liberating exomoons in white dwarf planetary systems
Authors:
Payne et al
Abstract:
Previous studies indicate that more than a quarter of all white dwarf (WD) atmospheres are polluted by remnant planetary material, with some WDs being observed to accrete the mass of Pluto in 106 yr. The short sinking time-scale for the pollutants indicates that the material must be frequently replenished. Moons may contribute decisively to this pollution process if they are liberated from their parent planets during the post-main-sequence evolution of the planetary systems. Here, we demonstrate that gravitational scattering events amongst planets in WD systems easily trigger moon ejection. Repeated close encounters within tenths of planetary Hill radii are highly destructive to even the most massive, close-in moons. Consequently, scattering increases both the frequency of perturbing agents in WD systems, as well as the available mass of polluting material in those systems, thereby enhancing opportunities for collision and fragmentation and providing more dynamical pathways for smaller bodies to reach the WD. Moreover, during intense scattering, planets themselves have pericentres with respect to the WD of only a fraction of an astronomical unit, causing extreme Hill-sphere contraction, and the liberation of moons into WD-grazing orbits. Many of our results are directly applicable to exomoons orbiting planets around main-sequence stars.
The SPHERE view of the planet-forming disk around HD100546
Authors:
Garufi et al
Abstract:
We image with unprecedented spatial resolution and sensitivity disk features that could be potential signs of planet-disk interaction. Two companion candidates have been claimed in the disk around the young Herbig Ae/Be star HD100546. Thus, this object serves as an excellent target for our investigation of the natal environment of giant planets. We exploit the power of extreme adaptive optics operating in conjunction with the new high-contrast imager SPHERE to image HD100546 in scattered light. We obtain the first polarized light observations of this source in the visible (with resolution as fine as 2 AU) and new H and K band total intensity images that we analyze with the Pynpoint package. The disk shows a complex azimuthal morphology, where multiple scattering of photons most likely plays an important role. High brightness contrasts and arm-like structures are ubiquitous in the disk. A double-wing structure (partly due to ADI processing) resembles a morphology newly observed in inclined disks. Given the cavity size in the visible (11 AU), the CO emission associated to the planet candidate 'c' might arise from within the circumstellar disk. We find an extended emission in the K band at the expected location of 'b'. The surrounding large-scale region is the brightest in scattered light. There is no sign of any disk gap associated to 'b'.
A pragmatic Bayesian perspective on correlation analysis: The exoplanetary gravity - stellar activity case
Authors:
Figueira et al
Abstract:
We apply the Bayesian framework to assess the presence of a correlation between two quantities. To do so, we estimate the probability distribution of the parameter of interest, ρ, characterizing the strength of the correlation. We provide an implementation of these ideas and concepts using python programming language and the pyMC module in a very short (∼130 lines of code, heavily commented) and user-friendly program.
We used this tool to assess the presence and properties of the correlation between planetary surface gravity and stellar activity level as measured by the log(R′HK) indicator. The results of the Bayesian analysis are qualitatively similar to those obtained via p-value analysis, and support the presence of a correlation in the data. The results are more robust in their derivation and more informative, revealing interesting features such as asymmetric posterior distributions or markedly different credible intervals, and allowing for a deeper exploration.
We encourage the reader interested in this kind of problem to apply our code to his/her own scientific problems. The full understanding of what the Bayesian framework is can only be gained through the insight that comes by handling priors, assessing the convergence of Monte Carlo runs, and a multitude of other practical problems. We hope to contribute so that Bayesian analysis becomes a tool in the toolkit of researchers, and they understand by experience its advantages and limitations.
THE EVOLUTION OF PLANET–DISK SYSTEMS THAT ARE MILDLY INCLINED TO THE ORBIT OF A BINARY COMPANION
Authors:
Lubow et al
Abstract:
We determine the evolution of a giant planet–disk system that orbits a member of a binary star system and is mildly inclined with respect to the binary orbital plane. The planet orbit and disk are initially mutually coplanar. We analyze the evolution of the planet and the disk by analytic means and hydrodynamic simulations. We generally find that the planet and the disk do not remain coplanar unless the disk mass is very large or the gap that separates the planet from the disk is very small. The relative planet–disk tilt undergoes secular oscillations whose initial amplitudes are typically of the order of the initial disk tilt relative to the binary orbital plane for disk masses ~1% of the binary mass or less. The effects of a secular resonance and the disk tilt decay enhance the planet–disk misalignment. The secular resonance plays an important role for disk masses greater than the planet mass. At later times, the accretion of disk gas by the planet causes its orbit to evolve toward alignment, if the disk mass is sufficiently large. The results have several implications for the evolution of massive planets in binary systems.
HORSESHOE DRAG IN THREE-DIMENSIONAL GLOBALLY ISOTHERMAL DISKS
Authors:
Masset et al
Abstract:
We study the horseshoe dynamics of a low-mass planet in a three-dimensional, globally isothermal, inviscid disk. We find, as reported in previous work, that the boundaries of the horseshoe region (separatrix sheets) have cylindrical symmetry about the disk's rotation axis. We interpret this feature as arising from the fact that the whole separatrix sheets have a unique value of Bernoulli's constant, and that this constant does not depend on altitude, but only on the cylindrical radius, in barotropic disks. We next derive an expression for the torque exerted by the horseshoe region on the planet, or horseshoe drag. Potential vorticity is not materially conserved as in two-dimensional flows, but it obeys a slightly more general conservation law (Ertel's theorem) that allows an expression for the horseshoe drag identical to the expression in a two-dimensional disk to be obtained. Our results are illustrated and validated by three-dimensional numerical simulations. The horseshoe region is found to be slightly narrower than previously extrapolated from two-dimensional analyses with a suitable softening length of the potential. We discuss the implications of our results for the saturation of the corotation torque, and the possible connection to the flow at the Bondi scale, which the present analysis does not resolve.
Kardashev's Classification at 50+: A Fine Vehicle with Room for Improvement
Authors:
Cirkovic et al
Abstract:
We review the history and status of the famous classification of extraterrestrial civilizations given by the great Russian astrophysicist Nikolai Semenovich Kardashev, roughly half a century after it has been proposed. While Kardashev's classification (or Kardashev's scale) has often been seen as oversimplified, and multiple improvements, refinements, and alternatives to it have been suggested, it is still one of the major tools for serious theoretical investigation of SETI issues. During these 50+ years, several attempts at modifying or reforming the classification have been made; we review some of them here, together with presenting some of the scenarios which present difficulties to the standard version. Recent results in both theoretical and observational SETI studies, especially the G-hat infrared survey (2014-2015), have persuasively shown that the emphasis on detectability inherent in Kardashev's classification obtains new significance and freshness. Several new movements and conceptual frameworks, such as the Dysonian SETI, tally extremely well with these developments. So, the apparent simplicity of the classification is highly deceptive: Kardashev's work offers a wealth of still insufficiently studied methodological and epistemological ramifications and it remains, in both letter and spirit, perhaps the worthiest legacy of the SETI "founding fathers".
VARIATIONS ON DEBRIS DISKS. III. COLLISIONAL CASCADES AND GIANT IMPACTS IN THE TERRESTRIAL ZONES OF SOLAR-TYPE STARS
Authors:
Kenyon et al
Abstract:
We analyze two new sets of coagulation calculations for solid particles orbiting within the terrestrial zone of a solar-type star. In models of collisional cascades, numerical simulations demonstrate that the total mass, the mass in 1 mm and smaller particles, and the dust luminosity decline with time more rapidly than predicted by analytic models, $\propto {t}^{-n}$ with n ≈ 1.1–1.2 instead of 1. Size distributions derived from the numerical calculations follow analytic predictions at r lesssim 0.1 km but are shallower than predicted at larger sizes. In simulations of planet formation, the dust luminosity declines more slowly than in pure collisional cascades, with n ≈ 0.5–0.8 instead of 1.1–1.2. Throughout this decline, giant impacts produce large, observable spikes in dust luminosity that last ~0.01–0.1 Myr and recur every 1–10 Myr. If most solar-type stars have Earth mass planets with a lesssim1–2 AU, observations of debris around 1–100 Myr stars allow interesting tests of theory. Current data preclude theories where terrestrial planets form out of 1000 km or larger planetesimals. Although the observed frequency of debris disks among gsim30 Myr old stars agrees with our calculations, the observed frequency of warm debris among 5–20 Myr old stars is smaller than predicted.
The longevity of habitable planets and the development of intelligent life
Authors:
Simpson et al
Abstract:
Why did the emergence of our species require a timescale similar to the entire habitable period of our planet? Our late appearance has previously been interpreted by Carter (2008) as evidence that observers typically require a very long development time, implying that intelligent life is a rare occurrence. Here we present an alternative explanation, which simply asserts that many planets possess brief periods of habitability. We also propose that the rate-limiting step for the formation of observers is the enlargement of species from an initially microbial state. In this scenario the development of intelligent life is a slow but almost inevitable process, greatly enhancing the prospects of future SETI experiments such as the Breakthrough Listen project.
Hiding in the Shadows II: Collisional Dust as Exoplanet Markers
Authors:
Dobinson et al
Abstract:
Observations of the youngest planets (∼1-10 Myr for a transitional disk) will increase the accuracy of our planet formation models. Unfortunately, observations of such planets are challenging and time-consuming to undertake even in ideal circumstances. Therefore, we propose the determination of a set of markers that can pre-select promising exoplanet-hosting candidate disks. To this end, N-body simulations were conducted to investigate the effect of an embedded Jupiter mass planet on the dynamics of the surrounding planetesimal disk and the resulting creation of second generation collisional dust. We use a new collision model that allows fragmentation and erosion of planetesimals, and dust-sized fragments are simulated in a post process step including non-gravitational forces due to stellar radiation and a gaseous protoplanetary disk. Synthetic images from our numerical simulations show a bright double ring at 850 μm for a low eccentricity planet, whereas a high eccentricity planet would produce a characteristic inner ring with asymmetries in the disk. In the presence of first generation primordial dust these markers would be difficult to detect far from the orbit of the embedded planet, but would be detectable inside a gap of planetary origin in a transitional disk.
Augmenting WFIRST Microlensing with a Ground-based Optical Telescope Network
Authors:
Zhu et al
Abstract:
Augmenting the WFIRST microlensing campaigns with intensive observations from a ground-based network of wide-field survey telescopes would have several major advantages. First, it would enable one-dimensional (1-D) microlens parallax measurements over the entire mass range M≳M⊕. For luminous lenses, such 1-D parallax measurements can be promoted to complete solutions (mass, distance, transverse velocity) by high-resolution imaging a few years after the observations. This would provide crucial information not only about the hosts of planets and other lenses, but also enable a much more precise Galactic model. The addition of such a ground-based survey would also yield full 2-D vector parallax measurements, with largest sensitivity to low-mass lenses, which (being non-luminous) are not subject to followup imaging. These 2-D parallax measurements will directly yield mass and distance measurements for most planetary and binary events. It would also yield additional complete solutions for single-lens events, especially those with low-mass lenses. Other benefits of such a survey include improved understanding of binaries (particularly with low mass primaries), and sensitivity to distant ice-giant and gas-giant companions of WFIRST lenses that cannot be detected by WFIRST itself due to its restricted observing windows. Such a ground-based survey can only be conducted in the optical and therefore requires that WFIRST be pointed at lower-extinction fields than is currently envisaged. This would come at some cost to the event rate. Therefore the benefits of improved characterization of lenses must be weighed against these costs.
The VLA Nascent Disk and Multiplicity Survey: First Look at Resolved Candidate Disks around Class 0 and I Protostars in the Perseus Molecular Cloud
Authors:
Segura-Cox et al
Abstract:
We present the first dust emission results toward a sample of seven protostellar disk candidates around Class 0 and I sources in the Perseus molecular cloud from the VLA Nascent Disk and Multiplicity (VANDAM) survey with ~0.05'' or 12 AU resolution. To examine the surface brightness profiles of these sources, we fit the Ka-band 8 mm dust-continuum data in the u,v-plane to a simple, parametrized model based on the Shakura-Sunyaev disk model. The candidate disks are well-fit by a model with a disk-shaped profile and have masses consistent with known Class 0 and I disks. The inner-disk surface densities of the VANDAM candidate disks have shallower density profiles compared to disks around more evolved Class II systems. The best-fit model radii of the seven early-result candidate disks are R_c greater than 10 AU; at 8 mm, the radii reflect lower limits on the disk size since dust continuum emission is tied to grain size and large grains radially drift inwards. These relatively large disks, if confirmed kinematically, are inconsistent with theoretical models where the disk size is limited by strong magnetic braking to less than 10 AU at early times.
Protoplanetary Disk Heating and Evolution Driven by the Spiral Density Waves
Author:
Rafikov
Abstract:
High-resolution imaging of some protoplanetary disks in scattered light reveals presence of the global spiral arms of significant amplitude, likely excited by massive planets or stellar companions. Assuming that these arms are density waves, evolving into spiral shocks, we assess their effect on the thermodynamics, accretion, and global evolution of the disk. We derive analytical expressions for the direct (irreversible) heating, angular momentum transport, and mass accretion rate induced by the disk shocks of arbitrary strength. We find these processes to be very sensitive to the shock amplitude. Focusing on the waves of moderate strength (density jump at the shock ΔΣ/Σ∼1) we show the associated disk heating to be negligible (contributing at ∼1% level to the energy budget) in passive, irradiated protoplanetary disks on ∼100 AU scales, but becoming important within several AU from the star. At the same time, shock heating can be a significant (or even dominant) energy source in disks of cataclysmic variables, stellar X-ray binaries, and supermassive black hole binaries, heated mainly by viscous dissipation. Mass accretion induced by the global spiral shocks is comparable to (or exceeds) the mass inflow due to viscous stresses. Protoplanetary disks featuring prominent global spirals must be evolving rapidly, in ≲0.5 Myr at ∼100 AU. A direct upper limit on the disk evolution timescale can be established via the measurement of the gravitational torque due to the spiral arms from the imaging data. Our findings suggest that, regardless of their origin, global spiral waves must be important agents of the protoplanetary disk evolution. They may serve as an effective mechanism of disk dispersal and could be related to the transitional disk phenomenon.
Resolved Gas Interior to the Dust Rings of the HD 141569 Disk
Authors:
Flaherty et al
Abstract:
The disk around HD 141569 is one of a handful of systems whose weak infrared emission is consistent with a debris disk, but still has a significant reservoir of gas. Here we report spatially resolved mm observations of the CO(3-2) and CO(1-0) emission as seen with the SMA and CARMA. We find that the excitation temperature for CO is lower than expected from cospatial blackbody grains, similar to previous observations of analogous systems, and derive a gas mass that lies between that of gas-rich primordial disks and gas-poor debris disks. The data also indicate a large inner hole in the CO gas distribution and an outer radius that lies interior to the outer scattered light rings. This spatial distribution, with the dust rings just outside the gaseous disk, is consistent with the expected interactions between gas and dust in an optically thin disk. This indicates that gas can have a significant effect on the location of the dust within debris disks.
Vortex formation in protoplanetary discs induced by the vertical shear instability
Authors:
Richard et al
Abstract:
We present the results of 2D and 3D hydrodynamic simulations of idealized protoplanetary discs that examine the formation and evolution of vortices by the vertical shear instability (VSI). In agreement with recent work, we find that discs with radially decreasing temperature profiles and short thermal relaxation time-scales, are subject to the axisymmetric VSI. In three dimensions, the resulting velocity perturbations give rise to quasi-axisymmetric potential vorticity perturbations that break-up into discrete vortices, in a manner that is reminiscent of the Rossby wave instability. Discs with very short thermal evolution time-scales (i.e. {\tau} less than 0.1 local orbit periods) develop strong vorticity perturbations that roll up into vortices that have small aspect ratios ({\chi} less than 2) and short lifetimes (~ a few orbits). Longer thermal time-scales give rise to vortices with larger aspect ratios (6 less than {\chi} less than 10), and lifetimes that depend on the entropy gradient. A steeply decreasing entropy profile leads to vortex lifetimes that exceed the simulation run times of hundreds of orbital periods. Vortex lifetimes in discs with positive or weakly decreasing entropy profiles are much shorter, being 10s of orbits at most, suggesting that the subcritical baroclinic instability plays an important role in sustaining vortices against destruction through the elliptical instability. Applied to the outer regions of protoplanetary discs, where the VSI is most likely to occur, our results suggest that vortices formed by the VSI are likely to be short lived structures.
Bar instability in disk-halo systems
Authors:
Sellwood et al
Abstract:
We show that the exponential growth rate of a bar in a stellar disk is substantially greater when the disk is embedded in a live halo than in a rigid one having the same mass distribution. We also find that the vigor of the instability in disk-halo systems varies with the shape of the halo velocity ellipsoid. Disks in rigid halos that are massive enough to be stable by the usual criteria, quickly form bars in isotropic halos and much greater halo mass is needed to avoid a strong bar; thus stability criteria derived for disks in rigid halos do not apply when the halo is responsive. The study presented here is of an idealized family of models with near uniform central rotation and that lack an extended halo; we present more realistic models with extended halos in a companion paper. The puzzle presented by the absence of strong bars in some galaxies having gently rising inner rotation curves is compounded by the results presented here.
Effects of photophoresis on the dust distribution in a 3D protoplanetary disc
Authors:
Cuello et al
Abstract:
Photophoresis is a physical process based on momentum exchange between an illuminated dust particle and its gaseous environment. Its net effect in protoplanetary discs (PPD) is the outward transport of solid bodies from hot to cold regions. This process naturally leads to the formation of ring-shaped features where dust piles up. In this work, we study the dynamical effects of photophoresis in PPD by including the photophoretic force in the two-fluid (gas+dust) smoothed particle hydrodynamics (SPH) code developed by Barri\`ere-Fouchet et al. (2005). We find that the conditions of pressure and temperature encountered in the inner regions of PPD result in important photophoretic forces, which dramatically affect the radial motion of solid bodies. Moreover, dust particles have different equilibrium locations in the disc depending on their size and their intrinsic density. The radial transport towards the outer parts of the disc is more efficient for silicates than for iron particles, which has important implications for meteoritic composition. Our results indicate that photophoresis must be taken into account in the inner regions of PPD to fully understand the dynamics and the evolution of the dust composition.
A Method for Selecting M dwarfs with an Increased Likelihood of Unresolved Ultra-cool Companionship
Authors:
Cook et al
Abstract:
Locating ultra-cool companions to M dwarfs is important for constraining low-mass formation models, the measurement of sub-stellar dynamical masses and radii, and for testing ultra-cool evolutionary models. We present an optimised method for identifying M dwarfs which may have unresolved ultra-cool companions. We construct a catalogue of 440,694 candidates, from WISE, 2MASS and SDSS, based on optical and near-infrared colours and reduced proper motion. With strict reddening, photometric and quality constraints we isolate a sub-sample of 36,898 M dwarfs and search for possible mid-infrared M dwarf + ultra-cool dwarf candidates by comparing M dwarfs which have similar optical/near-infrared colours (chosen for their sensitivity to effective temperature and metallicity). We present 1,082 M dwarf + ultra-cool dwarf candidates for follow-up. Using simulated ultra-cool dwarf companions to M dwarfs, we estimate that the occurrence of unresolved ultra-cool companions amongst our M dwarf + ultra-cool dwarf candidates should be at least four times the average for our full M dwarf catalogue. We discuss possible contamination and bias and predict yields of candidates based on our simulations.
Two Transiting Low Density Sub-Saturns from K2
Authors:
Petigura et al
Abstract:
We report the discovery and confirmation of two sub-Saturn planets orbiting a bright (V = 11.3), metal-rich ([Fe/H] = 0.42 ± 0.04 dex) G3 dwarf in the K2 Campaign 2 field. The planets are 5.68 ± 0.56 Earth-radii and 7.82 ± 0.72 Earth-radii and have orbital periods of 20.8851 ± 0.0003 d and 42.3633±0.0006 d, near to the 2:1 mean-motion resonance. We obtained 32 radial velocities (RVs) with Keck/HIRES and detected the reflex motion due to EPIC-203771098b and c. These planets have masses of 21.0 ± 5.4 Earth-masses and 27.0 ± 6.9 Earth-masses, respectively. With low densities of 0.63 ± 0.25 g/cc and 0.31 ± 0.12 g/cc, respectively, the planets require thick envelopes of H/He to explain their large sizes and low masses. Interior structure models predict that the planets have fairly massive cores of 17.6 ± 4.3 Earth-masses and 16.1 ± 4.2 Earth-masses, respectively. They may have formed exterior to their present locations, accreted their H/He envelopes at large orbital distances, and migrated in as a resonant pair. The proximity to resonance, large transit depths, and host star brightness offer rich opportunities for TTV follow-up. Finally, the low surface gravities of the EPIC-203771098 planets make them favorable targets for transmission spectroscopy by HST, Spitzer, and JWST.
Photo-ionization of planetary winds: case study HD209458b
Authors:
Schneiter et al
Abstract:
Close-in hot Jupiters are exposed to a tremendous photon flux that ionizes the neutral escaping material from the planet leaving an observable imprint that makes them an interesting laboratory for testing theoretical models. In this work we present 3D hydrodynamic simulations with radiation transfer calculations of a close-in exoplanet in a blow-off state. We calculate the Ly-α absorption and compare it with observations of HD 209458b an previous simplified model results.Our results show that the hydrodynamic interaction together with a proper calculation of the photoionization proccess are able to reproduce the main features of the observed Ly-α absorption, in particular at the blue-shifted wings of the line. We found that the ionizing stellar flux produce an almost linear effect on the amount of absorption in the wake. Varying the planetary mass loss rate and the radiation flux, we were able to reproduce the 10% absorption observed at −100 km s−1.
Exoplanet Transits Registered at the Universidad de Monterrey Observatory. Part I: HAT-P-12b, HAT-P-13b, HAT-P-16b, HAT-P-23b and WASP-10b
Authors:
Sada et al
Abstract:
Forty transits of the exoplanets HAT-P-12b, HAT-P-13b, HAT-P-16b, HAT-P-23b and WASP-10b were recorded with the 0.36m telescope at the Universidad de Monterrey Observatory. The images were captured with a standard Johnson-Cousins Rc and Ic and Sloan z' filters and processed to obtain individual light curves of the events. These light curves were successfully combined for each system to obtain a resulting one of higher quality, but with a slightly larger time sampling rate. A reduction by a factor of about four in per-point scatter was typically achieved, resulting in combined light curves with a scatter of ~1 mmag. The noise characteristics of the combined light curves were verified by comparing Allan variance plots of the residuals. The combined light curves for each system, along with radial velocity measurements from the literature when available, were modeled using a Monte Carlo method to obtain the essential parameters that characterize the systems. Our results for all these systems confirm the derived transit parameters (the planet-to-star radius ratio, Rp/R*; the scaled semi-major axis, a/R*; the orbital inclination, i; in some cases the eccentricity, e; and argument of periastron of the orbit, {\omega}), validating the methodology. This technique can be used by small college observatories equipped with modest-sized telescopes to help characterize known extrasolar planet systems. In some instances, the uncertainties of the essential transit parameters are also reduced. For HAT-P-23b, in particular, we derive a planet size 4.5 +- 1.0% smaller. We also derive improved linear periods for each system, useful for scheduling observations.
Is there an exoplanet in the Solar System?
Authors:
Mustill et al
Abstract:
We
investigate the prospects for the capture of the proposed Planet 9 from
other stars in the Sun's birth cluster. Any capture scenario must
satisfy three conditions: the encounter must be more distant than ~150
au to avoid perturbing the Kuiper belt; the other star must have a
wide-orbit planet (a>~100au); the planet must be captured onto an
appropriate orbit to sculpt the orbital distribution of wide-orbit
Solar System bodies. Here we use N-body simulations to show that these
criteria may be simultaneously satisfied. In a few percent of slow close
encounters in a cluster, bodies are captured onto heliocentric, Planet
9-like orbits. During the ~100 Myr cluster phase, many stars are likely
to host planets on highly-eccentric orbits with apastron distances
beyond 100 au if Neptune-sized planets are common and susceptible to
planet--planet scattering. While the existence of Planet 9 remains
unproven, we consider capture from one of the Sun's young brethren a
plausible route to explain such an object's orbit. Capture appears to
predict a large population of Trans-Neptunian Objects (TNOs) whose
orbits are aligned with the captured planet, and we propose that
different formation mechanisms will be distinguishable based on their
imprint on the distribution of TNOs.
Using the gravitational microlensing technique, astronomers have recently detected what appears to be a Saturn-like planet residing near the Milky Way's bulge. The newly discovered exoplanet has a mass somewhere between Saturn and Jupiter and is orbiting a star with half the mass of the sun. A paper detailing the finding was published online on Mar. 21 on the arXiv pre-print server.
If a star moves in front of an another star, the light from the distant star is bent by the gravitational pull of the nearer star and the more distant star is magnified. Microlensing does not rely on the light from the host stars; thus, it can detect planets, even when the host stars cannot be detected. This technique is very useful for detecting alien worlds in the inner galactic disk and bulge, where it is difficult to search for planets with other methods.
An international team of researchers, led by Aparna Bhattacharyaha of the University of Notre Dame used the gravitational microlensing method to detect a gas giant planet orbiting the lens stars of a microlensing event. This gravity lens, discovered in August 2014, was designated OGLE-2014-BLG-1760 and is the 1,760th microlensing event detected by the Optical Gravitational Lensing Experiment (OGLE) collaboration. OGLE is a Polish astronomical project based at the University of Warsaw, searching for dark matter and extrasolar planets. It utilizes the 1.3 meter Warsaw telescope mounted at the Las Campanas observatory in Chile.
Follow-up observations were carried out by the Microlensing Observation in Astrophysics (MOA) collaboration, the Microlensing Follow-Up Network (μFUN) and the RoboNet project. MOA uses the 1.8 meter MOA-II telescope at the Mount John Observatory at Lake Tekapo, New Zealand, while μFUN and RoboNet are global groups employing a network of telescopes worldwide.
The Radial and Rotational Velocities of PSO J318.5338−22.8603, a Newly Confirmed Planetary-Mass Member of the β Pictoris Moving Group
Authors:
Allers et al
Abstract:
PSO J318.5338−22.8603 is an extremely-red planetary-mass object that has been identified as a candidate member of the β Pictoris moving group based on its spatial position and tangential velocity. We present a high resolution K-band spectrum of PSO J318.5338−22.8603. Using a forward-modeling Markov Chain Monte Carlo approach, we report the first measurement of the radial velocity and v sin(i) of PSO J318.5−22, −6.0+0.8−1.1 km s−1 and 17.5+2.3−2.8 km s−1, respectively. We calculate the space velocity and position of PSO J318.5−22 and confirm that it is a member of the β Pictoris moving group. Adopting an age of 23±3 Myr for PSO J318.5−22, we determine a mass of 8.3±0.5 MJup and effective temperature of 1127+24−26 K using evolutionary models. PSO J318.5338−22.8603 is intermediate in mass and temperature to the directly-imaged planets β Pictoris b and 51 Eridani b, making it an important benchmark object in the sequence of planetary-mass members of the β Pictoris moving group. Combining our v sin(i) measurement with recent photometric variability data, we constrain the inclination of PSO J318.5−22 to greater than 29∘ and its rotational period to 5-10.2 hours. The equatorial velocity of PSO J318.5−22 indicates that its rotation is consistent with an extrapolation of the velocity-mass relationship for solar system planets.
Giant planet formation via pebble accretion
Author:
Guilera
Abstract:
In the standard model of core accretion, the formation of giant planets occurs by two main processes: first, a massive core is formed by the accretion of solid material; then, when this core exceeds a critical value (typically greater than 10 Earth masses) a gaseous runaway growth is triggered and the planet accretes big quantities of gas in a short period of time until the planet achieves its final mass. Thus, the formation of a massive core has to occur when the nebular gas is still available in the disk. This phenomenon imposes a strong time-scale constraint in giant planet formation due to the fact that the lifetimes of the observed protoplanetary disks are in general lower than 10 Myr. The formation of massive cores before 10 Myr by accretion of big planetesimals (with radii greater than 10 km) in the oligarchic growth regime is only possible in massive disks. However, planetesimal accretion rates significantly increase for small bodies, especially for pebbles, particles of sizes between mm and cm, which are strongly coupled with the gas. In this work, we study the formation of giant planets incorporating pebble accretion rates in our global model of planet formation.
EUV-driven ionospheres and electron transport on extrasolar giant planets orbiting active stars
Authors:
Chadney et al
Abstract:
The composition and structure of the upper atmospheres of Extrasolar Giant Planets (EGPs) are affected by the high-energy spectrum of their host stars from soft X-rays to EUV. This emission depends on the activity level of the star, which is primarily determined by its age. We focus upon EGPs orbiting K- and M-dwarf stars of different ages. XUV spectra for these stars are constructed using a coronal model. These spectra are used to drive both a thermospheric model and an ionospheric model, providing densities of neutral and ion species. Ionisation is included through photo-ionisation and electron-impact processes. We find that EGP ionospheres at all orbital distances considered and around all stars selected are dominated by the long-lived H+ ion. In addition, planets with upper atmospheres where H2 is not substantially dissociated have a layer in which H+3 is the major ion at the base of the ionosphere. For fast-rotating planets, densities of short-lived H+3 undergo significant diurnal variations, with the maximum value being driven by the stellar X-ray flux. In contrast, densities of longer-lived H+ show very little day/night variability and the magnitude is driven by the level of stellar EUV flux. The H+3 peak in EGPs with upper atmospheres where H2 is dissociated under strong stellar illumination is pushed to altitudes below the homopause, where this ion is likely to be destroyed through reactions with heavy species. The inclusion of secondary ionisation processes produces significantly enhanced ion and electron densities at altitudes below the main EUV ionisation peak, as compared to models that do not include electron-impact ionisation. We estimate infrared emissions from H+3, and while, in an H/H2/He atmosphere, these are larger from planets orbiting close to more active stars, they still appear too low to be detected with current observatories.
Simulating the Environment Around Planet-Hosting Stars - I. Coronal Structure
Authors:
Alvarado-Gómez et al
Abstract:
We present the results of a detailed numerical simulation of the circumstellar environment around three exoplanet-hosting stars. A state-of-the-art global magnetohydrodynamic (MHD) model is considered, including Alfv\'en wave dissipation as a self-consistent coronal heating mechanism. This paper contains the description of the numerical set-up, evaluation procedure, and the simulated coronal structure of each system (HD 1237, HD 22049 and HD 147513). The simulations are driven by surface magnetic field maps, recovered with the observational technique of Zeeman Doppler Imaging (ZDI). A detailed comparison of the simulations is performed, where two different implementations of this mapping routine are used to generate the surface field distributions. Quantitative and qualitative descriptions of the coronae of these systems are presented, including synthetic high-energy emission maps in the Extreme Ultra-Violet (EUV) and Soft X-rays (SXR) ranges. Using the simulation results, we are able to recover similar trends as in previous observational studies, including the relation between the magnetic flux and the coronal X-ray emission. Furthermore, for HD 1237 we estimate the rotational modulation of the high-energy emission due to the various coronal features developed in the simulation. We obtain variations, during a single stellar rotation cycle, up to 15\% for the EUV and SXR ranges. The results presented here will be used, in a follow-up paper, to self-consistently simulate the stellar winds and inner astrospheres of these systems.
Search for transiting exoplanets and variable stars in the open cluster NGC 7243
Authors:
Garai et al
Abstract:
We report results of the first five observing campaigns for the open stellar cluster NGC 7243 in the frame of project Young Exoplanet Transit Initiative (YETI). The project focuses on the monitoring of young and nearby stellar clusters, with the aim to detect young transiting exoplanets, and to study other variability phenomena on time-scales from minutes to years. After five observing campaigns and additional observations during 2013 and 2014, a clear and repeating transit-like signal was detected in the light curve of J221550.6+495611. Furthermore, we detected and analysed 37 new eclipsing binary stars in the studied region. The best fit parameters and light curves of all systems are given. Finally, we detected and analysed 26 new, presumably pulsating variable stars in the studied region. The follow-up investigation of these objects, including spectroscopic measurements of the exoplanet candidate, is currently planned.
On the formation of compact planetary systems via concurrent core accretion and migration
Authors:
Coleman et al
Abstract:
We present the results of planet formation N-body simulations based on a comprehensive physical model that includes planetary mass growth through mutual embryo collisions and planetesimal/boulder accretion, viscous disc evolution, planetary migration and gas accretion onto planetary cores. The main aim of this study is to determine which set of model parameters leads to the formation of planetary systems that are similar to the compact low mass multi-planet systems that have been discovered by radial velocity surveys and the Kepler mission. We vary the initial disc mass, solids-to-gas ratio and the sizes of the boulders/planetesimals, and for a restricted volume of the parameter space we find that compact systems containing terrestrial planets, super-Earths and Neptune-like bodies arise as natural outcomes of the simulations. Disc models with low values of the solids-to-gas ratio can only form short-period super-Earths and Neptunes when small planetesimals/boulders provide the main source of accretion, since the mobility of these bodies is required to overcome the local isolation masses for growing embryos. The existence of short-period super-Earths around low metallicity stars provides strong evidence that small, mobile bodies (planetesimals, boulders or pebbles) played a central role in the formation of the observed planets.
Globular Clusters as Cradles of Life and Advanced Civilizations
Authors:
Di Stefano et al
Abstract:
Globular clusters are ancient stellar populations with no star formation or core-collapse supernovae. Several lines of evidence suggest that globular clusters are rich in planets. If so, and if advanced civilizations can develop there, then the distances between these civilizations and other stars would be far smaller than typical distances between stars in the Galactic disk. The relative proximity would facilitate interstellar communication and travel. However, the very proximity that promotes interstellar travel also brings danger, since stellar interactions can destroy planetary systems. However, by modeling globular clusters and their stellar populations, we find that large regions of many globular clusters can be thought of as "sweet spots" where habitable-zone planetary orbits can be stable for long times. We also compute the ambient densities and fluxes in the regions within which habitable-zone planets can survive. Globular clusters are among the best targets for searches for extraterrestrial intelligence (SETI). We use the Drake equation to compare globular clusters to the Galactic disk, in terms of the likelihood of housing advanced communicating civilizations. We also consider free-floating planets, since wide-orbit planets can be ejected and travel freely through the cluster. A civilization spawned in a globular cluster may have opportunities to establish self-sustaining outposts, thereby reducing the probability that a single catastrophic event will destroy the civilization or its descendants. Although individual civilizations within a cluster may follow different evolutionary paths, or even be destroyed, the cluster may always host some advanced civilization, once a small number of them have managed to jump across interstellar space.
Two Small Temperate Planets Transiting Nearby M Dwarfs in K2 Campaigns 0 and 1
Authors:
Schlieder et al
Abstract:
The prime Kepler mission revealed that small planets (less than 4 R_earth) are common, especially around low-mass M dwarfs. K2, the re-purposed Kepler mission, continues this exploration of small planets around small stars. Here we combine K2 photometry with spectroscopy, adaptive optics imaging, and archival survey images to analyze two small planets orbiting the nearby, field age, M dwarfs K2-26 (EPIC 202083828) and K2-9. K2-26 is an M1.0 +/- 0.5 dwarf at 93 +/- 7 pc from K2 Campaign 0. We validate its 14.5665 d period planet and estimate a radius of 2.67^+0.46_-0.42 R_earth. K2-9 is an M2.5 +/- 0.5 dwarf at 110 +/- 12 pc from K2 Campaign 1. K2-9b was first identified by Montet et al. 2015; here we present spectra and adaptive optics imaging of the host star and independently validate and characterize the planet. Our analyses indicate K2-9b is a 2.25^+0.53_-0.96 R_earth planet with a 18.4498 d period. K2-26b exhibits a transit duration that is too long to be consistent with a circular orbit given the measured stellar radius. Thus, the long transits are likely due to the photoeccentric effect and our transit fits hint at an eccentric orbit. Both planets receive low incident flux from their host stars and have estimated equilibrium temperatures less than 500 K. K2-9b may receive approximately Earth-like insolation. However, its host star exhibits strong GALEX UV emission which could affect any atmosphere it harbors. K2-26b and K2-9b are representatives of a poorly studied class of small planets with cool temperatures that have radii intermediate to Earth and Neptune. Future study of these systems can provide key insight into trends in bulk composition and atmospheric properties at the transition from silicate dominated to volatile rich bodies.
Apodized pupil Lyot coronagraphs for arbitrary apertures. V. Hybrid Shaped Pupil designs for imaging Earth-like planets with future space observatories
Authors:
N'Diaye et al
Abstract:
We introduce a new class of solutions for Apodized Pupil Lyot Coronagraphs (APLC) with segmented aperture telescopes to remove broadband diffracted light from a star with a contrast level of 1010. These new coronagraphs provide a key advance to enabling direct imaging and spectroscopy of Earth twins with future large space missions. Building on shaped pupil (SP) apodization optimizations, our approach enables two-dimensional optimizations of the system to address any aperture features such as central obstruction, support structures or segment gaps. We illustrate the technique with a design that could reach 1010 contrast level at 34\,mas for a 12\,m segmented telescope over a 10\% bandpass centered at a wavelength λ0=500\,nm. These designs can be optimized specifically for the presence of a resolved star, and in our example, for stellar angular size up to 1.1\,mas. This would allow probing the vicinity of Sun-like stars located beyond 4.4\,pc, therefore fully retiring this concern. If the fraction of stars with Earth-like planets is $\eta_{\Earth}=0.1$, with 18\% throughput, assuming a perfect, stable wavefront and considering photon noise only, 12.5 exo-Earth candidates could be detected around nearby stars with this design and a 12\,m space telescope during a five-year mission with two years dedicated to exo-Earth detection (one total year of exposure time and another year of overheads). Our new hybrid APLC/SP solutions represent the first numerical solution of a coronagraph based on existing mask technologies and compatible with segmented apertures, and that can provide contrast compatible with detecting and studying Earth-like planets around nearby stars. They represent an important step forward towards enabling these science goals with future large space missions.
Observations and modelling of CO and [CI] in disks. First detections of [CI] and constraints on the carbon abundance
Authors:
Kama et al
Abstract:
The gas-solid budget of carbon in protoplanetary disks is related to the composition of the cores and atmospheres of the planets forming in them. The key gas-phase carbon carriers CO, C0 and C+ can now be observed in disks. The gas-phase carbon abundance in disks has not yet been well characterized, we aim to obtain new constraints on the [C]/[H] ratio in a sample of disks, and to get an overview of the strength of [CI] and warm CO emission.
We carried out a survey of the CO6--5 and [CI]1--0 and 2--1 lines towards 37 disks with APEX, and supplemented it with [CII] data from the literature. The data are interpreted using a grid of models produced with the DALI code. We also investigate how well the gas-phase carbon abundance can be determined in light of parameter uncertainties.
The CO6--5 line is detected in 13 out of 33 sources, the [CI]1--0 in 6 out of 12, and the [CI]2--1 in 1 out of 33. With deep integrations, the first unambiguous detections of [CI]~1--0 in disks are obtained, in TW~Hya and HD~100546.
Gas-phase carbon abundance reductions of a factor 5--10 or more can be identified robustly based on CO and [CI] detections. The atomic carbon detection in TW~Hya confirms a factor 100 reduction of [C]/[H]gas in that disk, while the data are consistent with an ISM-like carbon abundance for HD~100546. In addition, BP~Tau, T~Cha, HD~139614, HD~141569, and HD~100453 are either carbon-depleted or gas-poor disks. The low [CI]~2--1 detection rates in the survey mostly reflect insufficient sensitivity to detect T~Tauri disks. The Herbig~Ae/Be disks with CO and [CII] upper limits below the models are debris disk like systems. A roughly order of magnitude increase in sensitivity compared to our survey is required to obtain useful constraints on the gas-phase [C]/[H] ratio in most of the targeted systems.
Companions to APOGEE Stars I: A Milky Way-Spanning Catalog of Stellar and Substellar Companion Candidates and their Diverse Hosts
Authors:
Troup et al
Abstract:
In its three years of operation, the Sloan Digital Sky Survey (SDSS-III) Apache Point Observatory Galactic Evolution Experiment (APOGEE-1) observed greater than 14,000 stars with enough epochs over a sufficient temporal baseline for the fitting of Keplerian orbits. We present the custom orbit-fitting pipeline used to create this catalog, which includes novel quality metrics that account for the phase and velocity coverage of a fitted Keplerian orbit. With a typical RV precision of ∼100−200 m s−1, APOGEE can probe systems with small separation companions down to a few Jupiter masses. Here we present initial results from a catalog of 382 of the most compelling stellar and substellar companion candidates detected by APOGEE, which orbit a variety of host stars in diverse Galactic environments. Of these, 376 have no previously known small separation companion. The distribution of companion candidates in this catalog shows evidence for an extremely truncated brown dwarf (BD) desert with a paucity of BD companions only for systems with a less than 0.1−0.2 AU, with no indication of a desert at larger orbital separation. We propose a few potential explanations of this result, some which invoke this catalog's many small separation companion candidates found orbiting evolved stars. Furthermore, 16 BD and planet candidates have been identified around metal-poor ([Fe/H] less than demoapproximately 0.5) stars in this catalog, which may challenge the core accretion model for companions greater than 10MJup. Finally, we find all types of companions are ubiquitous throughout the Galactic disk with candidate planetary-mass and BD companions to distances of ∼6 and ∼16 kpc, respectively.
The VLA Nascent Disk and Multiplicity Survey of Perseus Protostars (VANDAM). II. Multiplicity of Protostars in the Perseus Molecular Cloud
Authors:
Tobin et al
Abstract:
We present a multiplicity study of all known protostars (94) in the Perseus molecular cloud from a Karl G. Jansky Very Large Array (VLA) survey at Ka-band (8 mm and 1 cm) and C-band (4 cm and 6.6 cm). The observed sample has a bolometric luminosity range between 0.1 L⊙ and ∼33 L⊙, with a median of 0.7 L⊙. This multiplicity study is based on the Ka-band data, having a best resolution of ∼0.065" (15 AU) and separations out to ∼43" (10000 AU) can be probed. The overall multiplicity fraction (MF) is found to be of 0.40±0.06 and the companion star fraction (CSF) is 0.71±0.06. The MF and CSF of the Class 0 protostars are 0.57±0.09 and 1.2±0.2, and the MF and CSF of Class I protostars are both 0.23±0.08. The distribution of companion separations appears bi-modal, with a peak at ∼75 AU and another peak at ∼3000 AU. Turbulent fragmentation is likely the dominant mechanism on greater than 1000 AU scales and disk fragmentation is likely to be the dominant mechanism on less than 200 AU scales. Toward three Class 0 sources we find companions separated by less than 30 AU. These systems have the smallest separations of currently known Class 0 protostellar binary systems. Moreover, these close systems are embedded within larger (50 AU to 400 AU) structures and may be candidates for ongoing disk fragmentation.
Near-Infrared Imaging Polarmetry of LkCa 15: A Possible Wwarped Inner Disk
Authors:
Oh et al
Abstract:
We present high-contrast H-band polarized intensity images of the transitional disk around the young solar-like star LkCa 15. By utilizing Subaru/HiCIAO for polarimetric differential imaging, both the angular resolution and the inner working angle reach 0.07" and r=0.1", respectively. We obtained a clearly resolved gap (width less than approximately 27 AU) at ~ 48 AU from the central star. This gap is consistent with images reported in previous studies. We also confirmed the existence of a bright inner disk with a misaligned position angle of 13+/-4 degree with respect to that of the outer disk, i.e., the inner disk is possibly warped. The large gap and the warped inner disk both point to the existence of a multiple planetary system with a mass of less than approximately 1Mjup.
Dust production in debris discs: constraints on the smallest grains
Authors:
Thebault et al
Abstract:
The surface energy constraint puts a limit on the smallest fragment ssurf that can be produced after a collision. Based on analytical considerations, this mechanism has been recently identified as been potentially able to prevent the production of small dust grains in debris discs and cut off their size distribution at sizes larger than the blow-out size. We numerically investigate the importance of this effect to find under which conditions it can leave a signature in the small-size end of a disc's particle size distribution (PSD). An important part of this work is to map out, in a disc at steady-state, what is the most likely collisional origin for micron-sized grains, in terms of the sizes of their collisional progenitors. We implement, for the first time, the surface energy constraint into a collisional evolution code. We consider a debris disc extending from 50 to 100AU and 2 different stellar types. We also consider two levels of stirring in the disc: dynamically "hot" (e=0.075) and "cold" (e=0.01). For all cases, we derive ssurf maps as a function of target and projectile sizes, st and sp, and compare them to equivalent maps for the dust-production rate. We then compute disc-integrated PSDs and estimate the imprint of the surface energy constraint. We find that the (sp,st) regions of high ssurf values do not coincide with those of high dust production rate. As a consequence, the surface energy constraint has generally a weak effect on the system's PSD. The maximum ssurf-induced depletion of μm-sized grains is ∼30% and is obtained for a sun-like star and a dynamically hot case. For the e=0.01 cases, the surface energy effect is negligible compared to the massive small grain depletion induced by another mechanism: the natural imbalance between dust production and destruction rates in low-stirring discs identified by Thebault&Wu(2008).
Transient dynamics of perturbations in astrophysical disks
Authors:
Razdoburdin et al
Abstract:
This paper reviews some aspects of one of the major unsolved problems in understanding astrophysical (in particular, accretion) disks: whether the disk interiors may be effectively viscous in spite of the absence of marnetorotational instability? In this case a rotational homogeneous inviscid flow with a Keplerian angular velocity profile is spectrally stable, making the transient growth of perturbations a candidate mechanism for energy transfer from the regular motion to perturbations. Transient perturbations differ qualitatively from perturbation modes and can grow substantially in shear flows due to the nonnormality of their dynamical evolution operator. Since the eigenvectors of this operator, alias perturbation modes, are mutually nonorthogonal, they can mutually interfere, resulting in the transient growth of their linear combinations. Physically, a growing transient perturbation is a leading spiral whose branches are shrunk as a result of the differential rotation of the flow. This paper discusses in detail the transient growth of vortex shear harmonics in the spatially local limit as well as methods for identifying the optimal (fastest growth) perturbations. Special attention is given to obtaining such solutions variationally, by integrating the direct and adjoint equations forward and backward in time, respectively. The material is presented in a newcomer-friendly style.
Led by San Francisco State University astronomer Stephen Kane, a team of researchers has spotted an extrasolar planet about 117 light-years from earth that boasts the most eccentric orbit yet seen.
What's more, Kane and his colleagues were able to detect a signal of reflected light from the planet known as HD 20782 -- a "flash" of starlight bouncing off the eccentric planet's atmosphere as it made its closest orbital approach to its star. The discovery was announced online Feb. 28, 2016 in The Astrophysical Journal.
In this case, "eccentric" doesn't refer to a state of mind, but instead describes how elliptical a planet's orbit is around its star. While the planets in our solar system have nearly circular orbits, astronomers have discovered several extrasolar planets with highly elliptical or eccentric orbits.
HD 20782 has the most eccentric orbit known, measured at an eccentricity of .96. This means that the planet moves in a nearly flattened ellipse, traveling a long path far from its star and then making a fast and furious slingshot around the star at its closest approach.
HD 20782 offers "a particularly lucrative observing opportunity" for studying the planetary atmosphere of an eccentric-orbit planet -- a type not seen in our own solar system, the scientists say in the journal article. By studying the reflected light from HD 20782, astronomers may learn more about the structure and composition of a planetary atmosphere that can withstand a brief but blistering exposure to its star.
link.
Parallax and masses of alpha Centauri revisited
Authors:
Pourbaix et al
Abstract:
Context. Despite the thorough work of van Leeuwen (2007), the parallax of alpha Centauri is still far from being carved in stone. Any derivation of the individual masses is therefore uncertain, if not questionable. And yet, that does not prevent this system from being used for calibration purpose in several studies. Aims. Obtaining more accurate model-free parallax and individual masses of this system. Methods. With HARPS, the radial velocities are not only precise but also accurate. Ten years of HARPS data are enough to derive the complement of the visual orbit for a full 3D orbit of alpha Cen. Results. We locate alpha Cen (743 mas) right where Hipparcos (ESA 1997) had put it, i.e. slightly further away than derived by Soderhjelm (1999). The components are thus a bit more massive than previously thought (1.13 and 0.97 Msun for A and B respectively). These values are now in excellent agreement with the latest asteroseismologic results.
A lucky imaging multiplicity study of exoplanet host stars II
Authors:
Ginski et al
Abstract:
The vast majority of extrasolar planets are detected by indirect detection methods such as transit monitoring and radial velocity measurements. While these methods are very successful in detecting short-periodic planets, they are mostly blind to wide sub-stellar or even stellar companions on long orbits. In our study we present high resolution imaging observations of 63 exoplanet hosts carried out with the lucky imaging instrument AstraLux at the Calar Alto 2.2m telescope as well as with the new SPHERE high resolution adaptive optics imager at the ESO/VLT in the case of a known companion of specific interest. Our goal is to study the influence of stellar multiplicity on the planet formation process. We detected and confirmed 4 previously unknown stellar companions to the exoplanet hosts HD197037, HD217786, Kepler-21 and Kepler-68. In addition, we detected 11 new low-mass stellar companion candidates which must still be confirmed as bound companions. We also provide new astrometric and photometric data points for the recently discovered very close binary systems WASP-76 and HD2638. Furthermore, we show for the first time that the previously detected stellar companion to the HD185269 system is a very low mass binary. Finally we provide precise constraints on additional companions for all observed stars in our sample.
Using Intermediate-Luminosity Optical Transients (ILOTs) to reveal extended exo-solar Kuiper belt objects
Authors:
Bear et al
Abstract:
We suggest that in the rare case of an Intermediate-Luminosity Optical Transient (ILOTs) event, evaporation of exo-solar Kuiper belt objects (ExoKBOs) at distances of d~500 - 10000AU from the ILOT can be detected. If the ILOT lasts for 1 month to a few years, enough dust might be ejected from the ExoKBOs for the IR emission to be detected. Because of the large distance of the ExoKBOs, tens of years will pass before the ILOT wind disperses the dust. We suggest that after an ILOT outburst there is a period of months to several years during which IR excess emission might hint at the existence of a Kuiper belt analog (ExoK-Belt).
The Curious Case of Elemental Abundance Differences in the Dual Hot Jupiter Hosts WASP-94AB
Authors:
Teske et al
Abstract:
Binary stars provide an ideal laboratory for investigating the potential effects of planet formation on stellar composition. Assuming the stars formed in the same environment/from the same material, any compositional anomalies between binary components might indicate differences in how material was sequestered in planets, or accreted by the star in the process of planet formation. We present here a study of the elemental abundance differences between WASP-94AB, a pair of stars that each host a hot Jupiter exoplanet. The two stars are very similar in spectral type (F8 and F9), and their ~2700 AU separation suggests their protoplanetary disks were likely not influenced by stellar interactions, but WASP-94Ab's orbit -- misaligned with the host star spin axis and likely retrograde -- points towards a dynamically active formation mechanism, perhaps different than that of WASP-94Bb, which is not misaligned and has nearly circular orbit. Based on our high-quality spectra and strictly relative abundance analysis, we detect a depletion of volatiles (~-0.02 dex, on average) and enhancement of refractories (~0.01 dex) in WASP-94A relative to B (standard errors are ~0.005 dex). This is different than every other published case of binary host star abundances, in which either no significant abundance differences are reported, or there is some degree of enhancement in all elements, including volatiles. Several scenarios that may explain the abundance trend are discussed, but none can be definitively accepted or rejected. Additional high-contrast imaging observations to search for companions that may be dynamically affecting the system, as well as a larger sample of binary host star studies, are needed to better understand the curious abundance trends we observe in WASP-94AB.
Molecular formation along the atmospheric mass loss of HD 209458 b and similar Hot Jupiters
Authors:
Pinotti et al
Abstract:
The chemistry along the mass loss of Hot Jupiters is generally considered to be simple, consisting mainly of atoms, prevented from forming more complex species by the intense radiation field from their host stars. In order to probe the region where the temperature is low (T less than 2000 K), we developed a 1D chemical and photochemical reaction model of the atmospheric mass loss of HD 209458 b, involving 56 species, including carbon chain and oxygen bearing ones, interacting through 566 reactions. The simulation results indicate that simple molecules like OH+, H2O+ and H3O+ are formed inside the region, considering that residual H2 survives in the exosphere, a possibility indicated by recent observational work. The molecules are formed and destroyed within a radial distance of less than 10^7 km, but the estimated integrated column density of OH+, a potential tracer of H2, is high enough to allow detection, which, once achieved, would indicate a revision of chemical models of the upper atmosphere of Hot Jupiters. For low density Hot Jupiters receiving less intense XUV radiation from their host stars than HD 209458 b, molecular species could conceivably be formed with a higher total column density.
Atmospheric Circulation of Hot Jupiters: Dayside-Nightside Temperature Differences
Authors:
Komacek et al
Abstract:
The full-phase infrared light curves of low-eccentricity hot Jupiters show a trend of increasing dayside-to-nightside brightness temperature difference with increasing equilibrium temperature. Here we present a three-dimensional model that explains this relationship, in order to shed insight on the processes that control heat redistribution in tidally-locked planetary atmospheres. This three-dimensional model combines predictive analytic theory for the atmospheric circulation and dayside-nightside temperature differences over a range of equilibrium temperature, atmospheric composition, and potential frictional drag strengths with numerical solutions of the circulation that verify this analytic theory. This analytic theory shows that the longitudinal propagation of waves mediates dayside-nightside temperature differences in hot Jupiter atmospheres, analogous to the wave adjustment mechanism that regulates the thermal structure in Earth's tropics. These waves can be damped in hot Jupiter atmospheres by either radiative cooling or potential frictional drag. This frictional drag would likely be caused by Lorentz forces in a partially ionized atmosphere threaded by a background magnetic field, and would increase in strength with increasing temperature. Additionally, the amplitude of radiative heating and cooling increases with increasing temperature, and hence both radiative heating/cooling and frictional drag damp waves more efficiently with increasing equilibrium temperature. Radiative heating and cooling play the largest role in controlling day-night temperature temperature differences in both our analytic theory and numerical simulations, with frictional drag only being important if it is stronger than the Coriolis force. As a result, dayside-nightside temperature differences in hot Jupiter atmospheres increase with increasing stellar irradiation and decrease with increasing pressure.
Is the Galactic bulge devoid of planets?
Authors:
Penny et al
Abstract:
Using a sample of 31 microlensing exoplanet hosts, we investigate whether or not the distances to these systems conform to the Galactic distribution of planets expected from a model. We derive the expected distribution of distances from a simulated microlensing survey, correcting (roughly) for the dominant selection effects that affect the detection sensitivity to planets as a function of their distance, and compare with the observed distribution using Anderson-Darling (AD) hypothesis testing. We find that the AD test rejects the hypothesis that the observed sample is drawn from our model distribution of distances with p-value 5.0×10−4. Interestingly, we find that an AD test can not reject (p=0.18) the hypothesis that the observed sample is drawn from a model distribution where only disk stars host planets (i.e., a model where there are no bulge planets), though AD tests of the relative proper motion distributions indicate tension with this extreme hypothesis. Allowing the relative abundance of planets in the bulge to that in the disk, fb, to be a free parameter, we find that AD tests do not reject models where fb less than 0.54 for p greater than 0.01. We find that allowing for a dependence of planet occurrence rate on host mass and metallicity does not significantly change our results, and if we fix these dependencies to their respective trends as inferred from RV surveys, the limit only weakens to fb less than 0.7. We suspect that some of the distance estimates for nearby (Dl less than 2 kpc) lenses could be significantly in error. If this is indeed the case, removal of two of the potentially problematic hosts we identify would allow for fb less than 0.96. While we cannot conclusively infer fb given current observations, more homogeneous samples from on-going surveys that are easier to model will allow the measurement of fb using similar methods to those developed in this work. (abridged)
KIC 7177553: a quadruple system of two close binaries
Authors:
Lehmann et al
Abstract:
KIC 7177553 was observed by the Kepler satellite to be an eclipsing eccentric binary star system with an 18-day orbital period. Recently, an eclipse timing study of the Kepler binaries has revealed eclipse timing variations in this object with an amplitude of about 100 sec, and an outer period of 529 days. The implied mass of the third body is that of a superJupiter, but below the mass of a brown dwarf. We therefore embarked on a radial velocity study of this binary to determine its system configuration and to check the hypothesis that it hosts a giant planet. From the radial velocity measurements, it became immediately obvious that the same Kepler target contains another eccentric binary, this one with a 16.5-day orbital period. Direct imaging using adaptive optics reveals that the two binaries are separated by 0.4 arcsec (about 167 AU), and have nearly the same magnitude (to within 2%). The close angular proximity of the two binaries, and very similar Gamma velocities, strongly suggest that KIC 7177553 is one of the rare SB4 systems consisting of two eccentric binaries where at least one system is eclipsing. Both systems consist of slowly rotating, non-evolved, solar-like stars of comparable masses. From the orbital separation and the small difference in Gamma velocity, we infer that the period of the outer orbit most likely lies in the range 1000 to 3000 years. New images taken over the next few years, as well as the high-precision astrometry of the Gaia satellite mission, will allow us to set much narrower constraints on the system geometry. Finally, we note that the observed eclipse timing variations in the Kepler data cannot be produced by the second binary. Further spectroscopic observations on a longer time scale will be required to prove the existence of the massive planet.
Revisiting the microlensing event OGLE 2012-BLG-0026: A solar mass star with two cold giant planets
Authors:
Beaulieu et al
Abstract:
Two cold, gas giant planets orbiting a G-type main sequence star in the galactic disk have previously been discovered in the high magnification microlensing event OGLE-2012-BLG-0026 (Han et al. 2013). Here we present revised host star flux measurements and a refined model for the two-planet system using additional light curve data. We performed high angular resolution adaptive optics imaging with the Keck and Subaru telescopes at two epochs while the source star was still amplified. We detected the lens flux, H=16.39±0.08. The lens, a disk star, is brighter than predicted from the modeling in the original study. We revisited the light curve modeling using additional photometric data from the B\&C telescope in New Zealand and CTIO 1.3m H band light curve. We then include the Keck and Subaru adaptive optic observation constraints. The system is composed of a ∼4−9 Gyr lens star of Mlens=1.06±0.05 M⊙ at a distance of Dlens=4.0±0.3 kpc, orbited by two giant planets of 0.145±0.008 MJup and 0.86±0.06 MJup with projected separations of 4.0±0.5 AU and 4.8±0.7 AU respectively. Since the lens is brighter than the source star by 16±8% in H, with no other blend within one arcsec, it will be possible to estimate its metallicity by subsequent IR spectroscopy with 8--10~m class telescopes. By adding a constraint on the metallicity it will be possible to refine the age of the system.
Dust photophoretic transport around a T Tauri star: Implications for comets composition
Authors:
Cordier et al
Abstract:
There is a growing body of evidences for the presence of crystalline material in comets. These crystals are believed to have been annealed in the inner part of the proto-solar nebula, while comets should have been formed in the outer regions. Several transport processes have been proposed to reconcile these two facts; among them a migration driven by photophoresis. The primarily goal of this work is to assess whether disk irradiation by a Pre-Main Sequence star would influence the photophoretic transport. To do so, we have implemented an evolving 1+1D model of an accretion disk, including advanced numerical techniques, undergoing a time-dependent irradiation, consistent with the evolution of the proto-Sun along the Pre-Main Sequence. The photophoresis is described using a formalism introduced in several previous works. Adopting the opacity prescription used in these former studies, we find that the disk irradiation enhances the photophoretic transport: the assumption of a disk central hole of several astronomical units in radius is no longer strictly required, whereas the need for an ad hoc introduction of photoevaporation is reduced. However, we show that a residual trail of small particles could annihilate the photophoretic driven transport via their effect on the opacity. We have also confirmed that the thermal conductivity of transported aggregates is a crucial parameter which could limit or even suppress the photophoretic migration and generate several segregation effects.
Numerical predictions for planets in the debris discs of HD 202628 and HD 207129
Authors:
Thilliez et al
Abstract:
Resolved debris disc images can exhibit a range of radial and azimuthal structures, including gaps and rings, which can result from planetary companions shaping the disc by their gravitational influence. Currently there are no tools available to determine the architecture of potential companions from disc observations. Recent work by Rodigas et al. (2014) presents how one can estimate the maximum mass and minimum semi major axis of a hidden planet empirically from the width of the disc in scattered light. In this work, we use the predictions of Rodigas et al. applied to two debris discs HD 202628 and HD 207129. We aim to test if the predicted orbits of the planets can explain the features of their debris disc, such as eccentricity and sharp inner edge. We first run dynamical simulations using the predicted planetary parameters of Rodigas et al., and then numerically search for better parameters. Using a modified N-body code including radiation forces, we perform simulations over a broad range of planet parameters and compare synthetics images from our simulations to the observations. We find that the observational features of HD 202628 can be reproduced with a planet five times smaller than expected, located 30 AU beyond the predicted value, while the best match for HD 207129 is for a planet located 5-10 AU beyond the predicted location with a smaller eccentricity. We conclude that the predictions of Rodigas et al. provide a good starting point but should be complemented by numerical simulations.
Orbital Dynamics of Exoplanetary Systems Kepler-62, HD 200964 and Kepler-11
Authors:
Mia et al
Abstract:
The presence of mean-motion resonances (MMR) in exoplanetary systems is a new exciting field of celestial mechanics which motivate us to consider the present work to study the dynamical behaviour of exoplanetary systems by time evolution of the orbital elements of the planets. Mainly we study the influence of planetary perturbations on semi-major axis and eccentricity. We identify (r+1):r mean-motion resonance terms in the expression of disturbing function and obtain the perturbations from the truncated disturbing function. Using the expansion of the disturbing function of three body problem and an analytical approach, we solve the equations of motion. The solution which is obtained analytically is compared with that of obtained by numerical method to validate our analytical result. In the present work we consider three exoplanetary systems namely Kepler-62, HD 200964 and Kepler-11. We have plotted the evolution of the resonant angles and found that they librate around constant value. In view of this, our opinion is that two planets of each system Kepler-62, HD 200964 and Kepler-11 are in 2:1, 4:3 and 5:4 mean motion resonances respectively.
