Authors:Zhu et alAbstract:The dependence of gas giant planet occurrence rate on stellar metallicity has been firmly established. We extend this so-called planet–metallicity correlation to broader ranges of metallicities and planet masses/radii. In particular, we assume that the planet–metallicity correlation is a power law below some critical saturation threshold, and that the probability of hosting at least one planet is unity for stars with metallicity above the threshold. We then are able to explain the discrepancy between the tentative detection and null detection in previous studies regarding the planet–metallicity correlation for small planets. In particular, we find that the null detection of this correlation can be attributed to the combination of high planet occurrence rate and low detection efficiency. Therefore, a planet–metallicity correlation for small planets cannot be ruled out. We propose that stars with metallicities lower than the solar value are better targets for testing the planet–metallicity correlation for small planets.
Tuesday, February 21, 2017
DEPENDENCE OF SMALL PLANET FREQUENCY ON STELLAR METALLICITY HIDDEN BY THEIR PREVALENCE
No Sign of a Second Planet Around Proxima Centauri
Authors:Damasso et alAbstract:The detection and characterization of Earth-like planets with Doppler signals of the order of 1 m/s currently represent one of the greatest challenge for extrasolar-planet hunters. As results for such findings are often controversial, it is desirable to provide independent confirmations of the discoveries. Testing different models for the suppression of non-Keplerian stellar signals usually plaguing radial velocity data is essential to ensuring findings are robust and reproducible. Using an alternative treatment of the stellar noise to that discussed in the discovery paper, we re-analyze the radial velocity data that led to the detection of a candidate terrestrial planet orbiting the star Proxima Centauri. We aim at confirming the existence of this outstanding planet, and test the existence of a second planetary signal. Our technique jointly models Keplerian signals and residual correlated signals (the noise) in radial velocities using Gaussian Processes. We analyse only radial velocity measurements without including other ancillary data. In a second step, we compare our outputs with results coming from photometry, to provide a consistent physical interpretation. Our analysis is performed in a Bayesian framework to quantify the robustness of our findings. We show that the correlated noise can be successfully modeled as a Gaussian process regression. It contains a periodic term modulated on the stellar rotation period and characterized by an evolutionary timescale of the order of 1 year. Both findings appear to be robust when compared with results obtained from archival photometry. We confirm the existence of a coherent signal described by a Keplerian orbit equation that can be attributed to the planet Proximab, and provide an independent estimate of the planetary parameters. Our Bayesian analysis dismisses the existence of a second planetary signal in the present dataset.
The Fate of Tatooine-like Circumbinary Exoplanets
Authors:Kostov et alAbstract:Inspired by the recent Kepler discoveries of circumbinary planets orbiting nine close binary stars, we explore the fate of the former as the latter evolve off the main sequence. We combine binary star evolution models with dynamical simulations to study the orbital evolution of these planets as their hosts undergo common-envelope (CE) stages, losing in the process a tremendous amount of mass on dynamical timescales. Five of the systems experience at least one Roche-lobe overflow and CE stage (Kepler-1647 experiences three), and the binary stars either shrink to very short orbits or coalesce; two systems trigger a double-degenerate supernova explosion. Kepler's circumbinary planets predominantly remain gravitationally bound at the end of the CE phase, migrate to larger orbits, and may gain significant eccentricity; their orbital expansion can be more than an order of magnitude and can occur over the course of a single planetary orbit. The orbits these planets can reach are qualitatively consistent with those of the currently known post-CE, eclipse-time variations circumbinary candidates. Our results also show that circumbinary planets can experience both modes of orbital expansion (adiabatic and nonadiabatic) if their host binaries undergo more than one CE stage; multiplanet circumbinary systems like Kepler-47 can experience both modes during the same CE stage. Additionally, unlike Mercury orbiting the Sun, a circumbinary planet with the same semimajor axis can survive the CE evolution of a close binary star with a total mass of 1 ${M}_{\odot }$.
Monday, February 20, 2017
The Ideal Stellar Mass for Long Term Habitability of Worlds
Authors:Oishi et alAbstract:In addition to the habitable zone (HZ), the UV habitable zone (UV-HZ) is important when considering the existence of persistent life in the universe. The UV-HZ is defined as the area where the UV radiation field from a host star is moderate for persistent life existence. This is because UV is necessary for the synthesis of biochemical compounds. The UV-HZ must overlap the HZ when life appears and evolves. In this paper, following our previous study of the HZ, we examine the UV-HZ in cases with a stellar mass range from 0.08 to 4.00 M ☉ with various metallicities during the main sequence phase. This mass range was chosen because we are interested in an environment similar to that of Earth. The effect of metallicity is reflected in the spectrum of the host stars, and we reexamine it in the context of the UV-HZ. The present work shows the effect of metallicity when that in the UV-HZ is less than that in the HZ. Furthermore, we find that the chance of persistent life existence declines as the metallicity decreases, as long as the UV radiation is not protected and/or boosted by any mechanisms. This is because the overlapped region of a persistent HZ and UV-HZ decreases. We find that the most appropriate stellar mass for the persistence of life existence is from 1.0 to 1.5 M ☉ with metallicity Z = 0.02, and only about 1.2 M ☉ with Z = 0.002. When Z = 0.0002, the chance of persistent life existence is very low, assuming that the ocean does not protect the life from UV radiation.
Early Terrestrial Surface UV Environment Impacts on Prebiotic Chemistry
Authors:Ranjan et alAbstract:The UV environment is a key boundary condition for the origin of life. However, considerable uncertainty exists as to planetary conditions and hence surface UV at abiogenesis. Here, we present two-stream multi-layer clear-sky calculations of the UV surface radiance on Earth at 3.9 Ga to constrain the UV surface fluence as a function of albedo, solar zenith angle (SZA), and atmospheric composition. Variation in albedo and latitude (through SZA) can affect maximum photoreaction rates by a factor of >10.4; for the same atmosphere, photoreactions can proceed an order of magnitude faster at the equator of a snowball Earth than at the poles of a warmer world. Surface conditions are important considerations when computing prebiotic UV fluences. For climatically reasonable levels of CO2, fluence shortward of 189 nm is screened out, meaning that prebiotic chemistry is robustly shielded from variations in UV fluence due to solar flares or variability. Strong shielding from CO2 also means that the UV surface fluence is insensitive to plausible levels of CH4, O2, and O3. At scattering wavelengths, UV fluence drops off comparatively slowly with increasing CO2 levels. However, if SO2 and/or H2S can build up to the 1-100 ppm level as hypothesized by some workers, then they can dramatically suppress surface fluence and hence prebiotic photoprocesses. H2O is a robust UV shield for
Tracing Carbon From the Interior of Stars to Surface of Planets
Following the Interstellar History of Carbon: From the Interiors of Stars to the Surfaces of PlanetsAuthors:Zuirys et alAbstract:The chemical history of carbon is traced from its origin in stellar nucleosynthesis to its delivery to planet surfaces. The molecular carriers of this element are examined at each stage in the cycling of interstellar organic material and their eventual incorporation into solar system bodies. The connection between the various interstellar carbon reservoirs is also examined. Carbon has two stellar sources: supernova explosions and mass loss from evolved stars. In the latter case, the carbon is dredged up from the interior and then ejected into a circumstellar envelope, where a rich and unusual C-based chemistry occurs. This molecular material is eventually released into the general interstellar medium through planetary nebulae. It is first incorporated into diffuse clouds, where carbon is found in polyatomic molecules such as H2CO, HCN, HNC, c-C3H2, and even C60+. These objects then collapse into dense clouds, the sites of star and planet formation. Such clouds foster an active organic chemistry, producing compounds with a wide range of functional groups with both gas-phase and surface mechanisms. As stars and planets form, the chemical composition is altered by increasing stellar radiation, as well as possibly by reactions in the presolar nebula. Some molecular, carbon-rich material remains pristine, however, encapsulated in comets, meteorites, and interplanetary dust particles, and is delivered to planet surfaces.
Sunday, February 19, 2017
First resolved image of the HD 114082 debris disk in the Lower Centaurus Crux with SPHERE
Authors:Wahhaj et alAbstract:We present the first resolved image of the debris disk around the 16 ± 8 Myr old star, HD 114082. The observation was made in the H-band using the SPHERE instrument. The star is at a distance of 92 ± 6 pc in the Lower Centaurus Crux association. Using a Markov chain Monte Carlo analysis, we determined that the debris is likely in the form of a dust ring with an inner edge of 27.7+2.8-3.5 au, position angle –74.3°+0.5-1.5, and an inclination with respect to the line of sight of 6.7°+3.8-0.4. The disk imaged in scattered light has a surface density that is declining with radius of ~r-4, which is steeper than expected for grain blowout by radiation pressure. We find only marginal evidence (2σ) of eccentricity and rule out planets more massive than 1.0 MJup orbiting within 1 au of the inner edge of the ring, since such a planet would have disrupted the disk. The disk has roughly the same fractional disk luminosity (Ldisk/L∗ = 3.3 × 10-3) as HR 4796 A and β Pictoris, however it was not detected by previous instrument facilities most likely because of its small angular size (radius ~0.4′′), low albedo (~0.2), and low scattering efficiency far from the star due to high scattering anisotropy. With the arrival of extreme adaptive optics systems, such as SPHERE and GPI, the morphology of smaller, fainter, and more distant debris disks are being revealed, providing clues to planet-disk interactions in young protoplanetary systems.
ALMA Measurements of Circumstellar Material in the GQ Lup System
ALMA Measurements of Circumstellar Material in the GQ Lup System
Authors:
MacGregor et al
Abstract:
We present ALMA observations of the GQ Lup system, a young Sun-like star with a substellar mass companion in a wide-separation orbit. These observations of 870μ m continuum and CO J=3-2 line emission with beam size∼0.3″ (∼45 AU) resolve the disk of dust and gas surrounding the primary star, GQ Lup A, and provide deep limits on any circumplanetary disk surrounding the companion, GQ Lup b. The circumprimary dust disk is compact with a FWHM of59±12 AU, while the gas has a larger extent with a characteristic radius of46.5±1.8 AU. By forward-modeling the velocity field of the circumprimary disk based on the CO emission, we constrain the mass of GQ Lup A to beM∗=(1.03±0.05)∗(d/156 pc) M⊙ , whered is a known distance, and determine that we view the disk at an inclination angle of60.5∘±0.5∘ and a position angle of346∘±1∘ . The3σ upper limit on the 870μ m flux density of any circumplanetary disk associated with GQ Lup b of<0.15 mJy implies an upper limit on the dust disk mass of<0.04 M⊕ for standard assumptions about optically thin emission. We discuss proposed mechanisms for the formation of wide-separation substellar companions given the non-detection of circumplanetary disks around GQ Lup b and other similar systems.
A WISE-based search for debris discs amongst M-dwarfs in nearby, young, moving groups
A WISE-based search for debris discs amongst M-dwarfs in nearby, young, moving groups
Authors:
Binks et al
Abstract:
We present a search for debris discs amongst M-dwarf members of nearby, young (5-150 Myr) moving groups (MGs) using infrared (IR) photometry, primarily from the Wide Infrared Survey Explorer (WISE). A catalogue of 100 MG M-dwarfs that have suitable WISE data is compiled and 19 of these are found to have significant IR excess emission at 22μ m. Our search is likely to be complete for discs where the ratio of flux from the disc to flux from the starfd/f∗>10−3 . The spectral energy distributions are supplemented with 2MASS photometry and data at longer wavelengths and fitted with simple disc models to characterise the IR excesses. There is a bimodal distribution -- twelve targets haveW1−W4>3 , corresponding tofd/f∗>0.02 and are likely to be gas-rich, primordial discs. The remaining seven targets haveW1−W4<1 (fd/f∗≲10−3 ) and include three objects with previously known or suspected debris discs and four new debris disc candidates that are all members of the Beta Pic MG. All of the IR excesses are identified in stars that are likely members of MGs with age<30 Myr. The detected debris disc frequency falls from 13 to 5 per cent to<7 per cent (at 95 per cent confidence) for objects younger or older than 30 Myr respectively. This provides evidence for the evolution of debris discs on this timescale and does not support models where the maximum of debris disc emission occurs much later in lower-mass stars.
Saturday, February 18, 2017
Grand Design Spiral Arms in A Young Forming Circumstellar Disk
Grand Design Spiral Arms in A Young Forming Circumstellar DiskAuthors:Tomida et alAbstract:We study formation and long-term evolution of a circumstellar disk in a collapsing molecular cloud core using a resistive magnetohydrodynamic simulation. While the formed circumstellar disk is initially small, it grows as accretion continues and its radius becomes as large as 200 AUs toward the end of the Class-I phase. A pair of grand-design spiral arms form due to gravitational instability in the disk, and they transfer angular momentum in the highly resistive disk. Although the spiral arms disappear in a few rotations as expected in a classical theory, new spiral arms form recurrently as the disk soon becomes unstable again by gas accretion. Such recurrent spiral arms persist throughout the Class-0 and I phase. We then perform synthetic observations and compare our model with a recent high-resolution observation of a young stellar object Elias 2-27, whose circumstellar disk has grand design spiral arms. We find an excellent agreement between our theoretical model and the observation. Our model suggests that the grand design spiral arms around Elias 2-27 are consistent with material arms formed by gravitational instability. It also implies that the age of Elias 2-27 can be younger than the previous estimate.
Zoom-Simulations of Protoplanetary Disks starting from GMC scales
Authors:Kuffmeier et alAbstract:We investigate the formation of protoplanetary disks around nine solar mass stars formed in the context of a 40 pc Giant Molecular Cloud model, using \ramses \ adaptive-mesh resolution simulations extending over a scale range of about 4 million, from an outer scale of 40 pc to a smallest cell size of 2 AU. Our most important qualitative result is that the accretion process is heterogeneous in multiple ways; in time, in space, and among protostars of otherwise similar mass. Accretion is heterogeneous in time, in the sense that accretion rates vary in time, with generally decreasing profiles, whose slopes can vary over a wide range, and where accretion can increase again if a protostar enters a region with increased density and low speed. Accretion is heterogeneous in space, because of the distribution of mass in space, with mass approaching the accreting star and disk in filaments and sheets. Finally, accretion is heterogeneous among stars, since the detailed conditions and dynamics in the neighborhood of each star can vary widely. We investigate in particular the sensitivity of disk formation to physical conditions, such as mass-to-flux ratio, and to sink particle recipe parameters. We find that disks frequently form, even when choosing the least favorable sink particle parameters, and that turbulence carried along from larger scales is a main factor in allowing disks to form even when the magnetic field is comparatively strong.
First Detection of Hydrogen in the β Pictoris Gas Disk
Authors:Wilson et alAbstract:The young and nearby star \beta\ Pictoris (\beta\ Pic) is surrounded by a debris disk composed of dust and gas known to host a myriad evaporating exocomets, planetesimals and at least one planet. At an edge-on inclination, as seen from Earth, this system is ideal for debris disk studies providing an excellent opportunity to use absorption spectroscopy to study the planet forming environment. Using the Cosmic Origins Spectrograph (COS) instrument on the Hubble Space Telescope (HST) we observe the most abundant element in the disk, hydrogen, through the HI Lyman \alpha\ (Ly-\alpha\) line. We present a new technique to decrease the contamination of the Ly-\alpha\ line by geocoronal airglow in COS spectra. This Airglow Virtual Motion (AVM) technique allows us to shift the Ly-\alpha\ line of the astrophysical target away from the contaminating airglow emission revealing more of the astrophysical line profile. The column density of hydrogen in the \beta\ Pic stable gas disk at the stellar radial velocity is measured to be log(NH/1cm2)≪18.5. The Ly-\alpha\ emission line profile is found to be asymmetric and we propose that this is caused by HI falling in towards the star with a bulk radial velocity of 41±6 km/s relative to \beta\ Pic and a column density of log(NH/1cm2)=18.6±0.1. The high column density of hydrogen relative to the hydrogen content of CI chondrite meteorites indicates that the bulk of the hydrogen gas does not come from the dust in the disk. This column density reveals a hydrogen abundance much lower than solar, which excludes the possibility that the detected hydrogen could be a remnant of the protoplanetary disk or gas expelled by the star. We hypothesise that the hydrogen gas observed falling towards the star arises from the dissociation of water originating from evaporating exocomets.
Friday, February 17, 2017
DOES A DIFFERENTIATED, CARBONATE-RICH, ROCKY OBJECT POLLUTE THE WHITE DWARF SDSS J104341.53+085558.2?
Authors:Melis et alAbstract:We present spectroscopic observations of the dust- and gas-enshrouded, polluted, single white dwarf star SDSS J104341.53+085558.2 (hereafter SDSS J1043+0855). Hubble Space Telescope Cosmic Origins Spectrograph far-ultraviolet spectra combined with deep Keck HIRES optical spectroscopy reveal the elements C, O, Mg, Al, Si, P, S, Ca, Fe, and Ni and enable useful limits for Sc, Ti, V, Cr, and Mn in the photosphere of SDSS J1043+0855. From this suite of elements we determine that the parent body being accreted by SDSS J1043+0855 is similar to the silicate Moon or the outer layers of Earth in that it is rocky and iron-poor. Combining this with comparison to other heavily polluted white dwarf stars, we are able to identify the material being accreted by SDSS J1043+0855 as likely to have come from the outermost layers of a differentiated object. Furthermore, we present evidence that some polluted white dwarfs (including SDSS J1043+0855) allow us to examine the structure of differentiated extrasolar rocky bodies. Enhanced levels of carbon in the body polluting SDSS J1043+0855 relative to the Earth–Moon system can be explained with a model where a significant amount of the accreted rocky minerals took the form of carbonates; specifically, through this model the accreted material could be up to 9% calcium-carbonate by mass.
HIP 67537B: A Brown Dwarf in an Eccentric Orbit
Authors:Jones et alAbstract:We report the discovery of a substellar companion around the giant star HIP67537. Based on precision radial velocity measurements from CHIRON and FEROS high-resolution spectroscopic data, we derived the following orbital elements for HIP67537 b: mbsini = 11.1 ± 0.6 MJ, a = 5.0 ± 0.2 AU and e = 0.59 ± 0.04. Considering random inclination angles, this object has ≳ 65% probability to be above the theoretical deuterium-burning limit, thus it is one of the few known objects in the planet to brown-dwarf transition region. In addition, we analyzed the Hipparcos astrometric data of this star, from which we derived a minimum inclination angle for the companion of 3 degrees. This value corresponds to an upper mass limit of 0.22 M⊙, therefore the probability that HIP67537 b is stellar in nature is only ∼ 6%. The large mass of the host star and the high orbital eccentricity makes HIP67537 b a very interesting and rare substellar objects. This is the second companion in the "brown dwarf desert" among our sample of EXPRESS intermediate-mass stars, corresponding to a detection fraction of f = 1.6+2.0−0.5%. This value is larger than the fraction observed in solar-type stars, providing further observational evidence of the enhanced formation efficiency of very massive planets and low-mass brown dwarfs in massive disks. Finally, we speculate about different formation channels for objects like HIP67537 b and HIP97233 b
Properties of 11 T Class Brown Dwarfs
Authors:Line et alAbstract:Brown dwarf spectra are rich in information revealing of the chemical and physical processes operating in their atmospheres. We apply a recently developed atmospheric retrieval tool to an ensemble of late T-dwarf (600-800K) near infrared spectra. With these spectra we are able to place direct constraints the molecular abundances of H2O, CH4, CO, CO2, NH3, H2S, and Na+K, gravity, thermal structure (and effective temperature), photometric radius, and cloud optical depths. We find that ammonia, water, methane, and the alkali metals are present and well constrained in all 11 objects. From the abundance constraints we find no significant trend in the water, methane, or ammonia abundances with temperature, but find a very strong (>25σ) increasing trend in the alkali metal abundances with effective temperature, indicative of alkali rainout. We also find little evidence for optically thick clouds. With the methane and water abundances, we derive the intrinsic atmospheric metallicity and carbon-to-oxygen ratios. We find in our sample, that metallicities are typically sub solar and carbon-to-oxygen ratios are somewhat super solar, different than expectations from the local stellar population. We also find that the retrieved vertical thermal profiles are consistent with radiative equilibrium over the photospheric regions. Finally, we find that our retrieved effective temperatures are lower than previous inferences for some objects and that our radii are larger than expectations from evolutionary models, possibly indicative of un-resolved binaries. This investigation and methodology represents a paradigm in linking spectra to the determination of the fundamental chemical and physical processes governing cool brown dwarf atmospheres.
Thursday, February 16, 2017
Near-IR Emission Spectrum of hot Jupiter WASP-103b
Authors:Cartier et alAbstract:We present here our observations and analysis of the dayside emission spectrum of the hot Jupiter WASP-103b. We observed WASP-103b during secondary eclipse using two visits of the Hubble Space Telescope with the G141 grism on Wide Field Camera 3 in spatial scan mode. We generated secondary eclipse light curves of the planet in both blended white-light and spectrally binned wavechannels from 1.1-1.7 micron and corrected the light curves for flux contamination from a nearby companion star. We modeled the detector systematics and secondary eclipse spectrum using Gaussian process regression and found that the near-IR emission spectrum of WASP-103b is featureless across the observed near-IR region to down to a sensitivity of 175 ppm, and shows a shallow slope towards the red. The atmosphere has a single brightness temperature of T_B = 2890 K across this wavelength range. This region of the spectrum is indistinguishable from isothermal, but may not manifest from a physically isothermal system, i.e. pseudo-isothermal. A Solar-metallicity profile with a thermal inversion layer at 10^-2 bar fits WASP-103b's spectrum with high confidence, as do an isothermal profile with Solar metallicity and a monotonically decreasing atmosphere with C/O>1. The data rule out a monotonically decreasing atmospheric profile with Solar composition, and we rule out a low-metallicity decreasing profile as non-physical for this system. The pseudo-isothermal profile could be explained by a thermal inversion layer just above the layer probed by our observations, or by clouds or haze in the upper atmosphere. Transmission spectra at optical wavelengths would allow us to better differentiate between potential atmospheric models.
Refined architecture of the WASP-8 system: a cautionary tale for traditional Rossiter-McLaughlin analysis
Authors:Bourrier et alAbstract:Probing the trajectory of a transiting planet across the disk of its star through the analysis of its Rossiter-McLaughlin effect can be used to measure the differential rotation of the host star and the true obliquity of the system. Highly misaligned systems could be particularly conducive to these mesurements, which is why we reanalysed the HARPS transit spectra of WASP-8b using the 'Rossiter-McLaughlin effect reloaded' (reloaded RM) technique. This approach allows us to isolate the local stellar CCF emitted by the planet-occulted regions. As a result we identified a ∼35% variation in the local CCF contrast along the transit chord, which might trace a deepening of the stellar lines from the equator to the poles. Whatever its origin, such an effect cannot be detected when analyzing the RV centroids of the disk-integrated CCFs through a traditional velocimetric analysis of the RM effect. Consequently it injected a significant bias into the results obtained by Queloz et al. (2010) for the projected rotational velocity veqsini⋆ (1.59−0.09+0.08 km/s) and the sky-projected obliquity λ (-123.0−4.4+3.4∘). Using our technique, we measured these values to be veqsini⋆ = 1.90±0.05 km/s and λ = -143.0−1.5+1.6∘. We found no compelling evidence for differential rotation of the star, although there are hints that WASP-8 is pointing away from us with the stellar poles rotating about 25% slower than the equator. Measurements at higher accuracy during ingress/egress will be required to confirm this result. In contrast to the traditional analysis of the RM effect, the reloaded RM technique directly extracts the local stellar CCFs, allowing us to analyze their shape and to measure their RV centroids, unbiased by variations in their contrast or FWHM.
Hot-Jupiter Core Mass from Roche-lobe Overflow
Authors:Ginzburg et alAbstract:The orbits of many observed hot Jupiters are decaying rapidly due to tidal interaction, eventually reaching the Roche limit. We analytically study the ensuing coupled mass loss and orbital evolution during the Roche-lobe overflow and find two possible scenarios. Planets with light cores Mc≲6M⊕ (assuming a nominal tidal dissipation factor Q∼106 for the host star) are transformed into Neptune-mass gas planets, orbiting at a separation (relative to the stellar radius) a/R⋆≈3.5. Planets with heavier cores Mc≳6M⊕ plunge rapidly until they are destroyed at the stellar surface. Remnant gas-Neptunes, which are stable to photo-evaporation, are absent from the observations, despite their unique transit radius (5−10R⊕). This result suggests that Mc≳6M⊕, providing a useful constraint on the poorly-known core mass that may distinguish between different formation theories of gas giants. Alternatively, given a prior estimate of Mc≈6M⊕ from the core-accretion theory, our results exclude the range 106≲Q≲107.
Wednesday, February 15, 2017
Kepler-223 System is a Chain of 4 Sub Neptunes
Authors:Mills et alAbstract:Surveys have revealed many multi-planet systems containing super-Earths and Neptunes in orbits of a few days to a few months. There is debate whether in situ assembly or inward migration is the dominant mechanism of the formation of such planetary systems. Simulations suggest that migration creates tightly packed systems with planets whose orbital periods may be expressed as ratios of small integers (resonances), often in a many-planet series (chain). In the hundreds of multi-planet systems of sub-Neptunes, more planet pairs are observed near resonances than would generally be expected, but no individual system has hitherto been identified that must have been formed by migration. Proximity to resonance enables the detection of planets perturbing each other. Here we report transit timing variations of the four planets in the Kepler-223 system, model these variations as resonant-angle librations, and compute the long-term stability of the resonant chain. The architecture of Kepler-223 is too finely tuned to have been formed by scattering, and our numerical simulations demonstrate that its properties are natural outcomes of the migration hypothesis. Similar systems could be destabilized by any of several mechanisms, contributing to the observed orbital-period distribution, where many planets are not in resonances. Planetesimal interactions in particular are thought to be responsible for establishing the current orbits of the four giant planets in the Solar System by disrupting a theoretical initial resonant chain similar to that observed in Kepler-223.
A Candidate Massive Saturn Analog Orbiting HD 30177 Authors:
Authors:Wittenmyer et alAbstract:We report the discovery of a second long-period giant planet orbiting HD 30177, a star previously known to host a massive Jupiter analog (HD 30177b: a=3.8±0.1 au, m sin i=9.7±0.5 Mjup). HD 30177c can be regarded as a massive Saturn analog in this system, with a=9.9±1.0 au and m sin i=7.6±3.1 Mjup. The formal best fit solution slightly favours a closer-in planet at a∼7 au, but detailed n-body dynamical simulations show that configuration to be unstable. A shallow local minimum of longer-period, lower-eccentricity solutions was found to be dynamically stable, and hence we adopt the longer period in this work. The proposed ∼32 year orbit remains incomplete; further monitoring of this and other stars is necessary to reveal the population of distant gas giant planets with orbital separations a∼10 au, analogous to that of Saturn.
Tuesday, February 14, 2017
No Civilizations are Within a few Thousand Light Years
Author:WandelAbstract:The Kepler mission has shown that a significant fraction of all stars may have an Earth-size habitable planet. A dramatic support was the recent detection of Proxima Centauri b. Using a Drake-equation like formalism I derive an equation for the abundance of biotic planets as a function of the relatively modest uncertainty in the astronomical data and of the (yet unknown) probability for the evolution of biotic life, Fb. I suggest that Fb may be estimated by future spectral observations of exoplanet biomarkers. It follows that if Fb is not very small, then a biotic planet may be expected within about 10 light years from Earth. Extending this analyses to advanced life, I derive expressions for the distance to putative civilizations in terms of two additional Drake parameters - the probability for evolution of a civilization, Fc, and its average longevity. Assuming "optimistic" values for the Drake parameters, (Fb~Fc~1), and a broadcasting duration of a few thousand years, the likely distance to the nearest civilizations detectable by SETI is of the order of a few thousand light years. Finally I calculate the distance and probability of detecting intelligent signals with present and future radio telescopes such as Arecibo and SKA and how it could constrain the Drake parameters.
Upper Limits to the Exoplanets Around Proxima Centauri
Authors:Mesa et alAbstract:The recent discovery of an earth-like planet around Proxima Centauri has drawn much attention to this star and its environment. We performed a series of observations of Proxima Centauri using SPHERE, the planet finder instrument installed at the ESO Very Large Telescope UT3, using its near infrared modules, IRDIS and IFS. No planet was directly detected but we set upper limits on the mass up to 7 au exploiting the AMES-COND models. Our IFS observations reveal that no planet more massive than ~6-7 M Jup can be present within 1 au. The dual band imaging camera IRDIS also enables us to probe larger separations than the other techniques like the radial velocity or astrometry. We obtained mass limits of the order of 4 M Jup at separations of 2 au or larger representing the most stringent mass limits at separations larger than 5 au available at the moment. We also did an attempt to estimate the radius of possible planets around Proxima using the reflected light. Since the residual noise for this observations are dominated by photon noise and thermal background, longer exposures in good observing conditions could further improve the achievable contrast limit.
On the Orbital Inclination of Proxima Centauri b
Authors:Kane et alAbstract:The field of exoplanetary science has seen discovery rates increase dramatically over recent years, due largely to the data from the Kepler mission. Even so, individual discoveries of planets orbiting nearby stars are very important for studies of characterization and near-term follow-up prospects. The recent discovery of a terrestrial planet candidate orbiting Proxima Centauri presents numerous opportunities for studying a Super-Earth within our own stellar backyard. One of the remaining ambiguities of the discovery is the true mass of the planet since the discovery signature was obtained via radial velocities. Here we describe the effect of orbital inclination on the Proxima Centauri planet, in terms of mass, radius, atmosphere, and albedo. We calculate the astrometric, angular separation, and reflected light properties of the planet including the effects of orbital eccentricity. We further provide dynamical simulations that show how the presence of additional terrestrial planets within the Habitable Zone varies as a function of inclination. Finally, we discuss these effects in the context of future space-based photometry and imaging missions that could potentially detect the planetary signature and resolve the inclination and mass ambiguity of the planet.
Monday, February 13, 2017
SuperEarth Gliese 1132b's Atmosphere has been Detected
Authors:Southworth et alAbstract:Detecting the atmospheres of low-mass low-temperature exoplanets is a high-priority goal on the path to ultimately detect biosignatures in the atmospheres of habitable exoplanets. High-precision HST observations of several super-Earths with equilibrium temperatures below 1000 K have to date all resulted in featureless transmission spectra, which have been suggested to be due to high-altitude clouds. We report the detection of an atmospheric feature in the atmosphere of a 1.6 Mearth transiting exoplanet, GJ 1132b, with an equilibrium temperature of ~600 K and orbiting a nearby M dwarf. We present observations of nine transits of the planet obtained simultaneously in the griz and JHK passbands. We find an average radius of 1.44 +/- 0.21 Rearth for the planet, averaged over all the passbands, which can be decomposed into a "surface radius" at ~1.35 Rearth, and higher contributions in the z and K bands. The z-band radius is 4 sigma higher than the continuum, suggesting a strong detection of an atmosphere. We deploy a suite of tests to verify the reliability of the transmission spectrum, which are greatly helped by the existence of repeat observations. The large z-band transit depth indicates strong opacity from H2O and/or CH4 or an hitherto unconsidered opacity. A surface radius of 1.35 +/- 0.21 Rearth allows for a wide range of interior compositions ranging from a nearly Earth-like rocky interior, with ~70% silicate and ~30% Fe, to a substantially H2O-rich water world. New observations with HST and existing ground-based facilities would be able to confirm the present detection and further constrain the atmospheric composition of the planet.
Is KIC 8462852 (Tabby's Star) Being Eclipsed by Four Dusty Exoplanets
Authors:Neslusan et alAbstract:Apart from thousands of `regular' exoplanet candidates, Kepler satellite has discovered a few stars exhibiting peculiar eclipse-like events. They are most probably caused by disintegrating bodies transiting in front of the star. However, the nature of the bodies and obscuration events, such as those observed in KIC8462852, remain mysterious. A swarm of comets or artificial alien mega-structures have been proposed as an explanation for the latter object.We explore the possibility that such eclipses are caused by the dust clouds associated with massive parent bodies orbiting the host star.We assumed a massive object and a simple model of the dust cloud surrounding the object. Then, we used the numerical integration to simulate the evolution of the cloud, its parent body, and resulting light-curves as they orbit and transit the star.We found that it is possible to reproduce the basic features in the light-curve of KIC8462852 with only four objects enshrouded in dust clouds. The fact that they are all on similar orbits and that such models require only a handful of free parameters provides additional support for this hypothesis.This model provides an alternative to the comet scenario. With such physical models at hand, at present, there is no need to invoke alien mega-structures for an explanation of these light-curves.
Did KIC 8462852 (Tabby's Star) Dim Because it Consumed an Exoplanet?
Authors:Metzger et alAbstract:The Kepler-field star KIC 8462852, an otherwise apparently ordinary F3 main-sequence star, showed several highly unusual dimming events of variable depth and duration. Adding to the mystery was the discovery that KIC 8462852 faded by 14% from 1890 to 1989, as well as by another 3% over the 4 year Kepler mission. Following an initial suggestion by Wright & Sigurdsson, we propose that the secular dimming behavior is the result of the inspiral of a planetary body or bodies into KIC 8462852, which took place ~10 to 1e4 years ago (depending on the planet mass). Gravitational energy released as the body inspirals into the outer layers of the star caused a temporary and unobserved brightening, from which the stellar flux is now returning to the quiescent state. The transient dimming events could then be due to obscuration by planetary debris from an earlier partial disruption of the same inspiraling bodies, or due to evaporation and out-gassing from a tidally detached moon system. Alternatively, the dimming events could arise from a large number of bodies comet- or planetesimal-mass bodies placed onto high eccentricity orbits by the same mechanism (e.g. Lidov-Kozai oscillations due to the outer M-dwarf companion) responsible for driving the more massive planets into KIC 8462852. The required high occurrence rate of KIC 8462852-like systems which have undergone recent major planet inspiral event(s) is the greatest challenge to the model, placing large lower limits on the mass of planetary systems surrounding F stars and/or requiring an unlikely probability to catch KIC 8462852 in its current state.
Sunday, February 12, 2017
How Bright are Planet-Induced Spiral Arms in Scattered Light?
Authors:Dong et alAbstract:Recently, high angular resolution imaging instruments such as SPHERE and GPI have discovered many spiral-arm-like features in near-infrared scattered light images of protoplanetary disks. Theory and simulations have suggested that these arms are most likely excited by planets forming in the disks; however, a quantitative relation between the arm-to-disk brightness contrast and planet mass is still missing. Using 3D hydrodynamics and radiative transfer simulations, we examine the morphology and contrast of planet-induced arms in disks. We find a power-law relation for the face-on arm contrast (δmax) as a function of planet mass (Mp) and disk aspect ratio (h/r): δmax≈((Mp/MJ)/(h/r)1.38)0.22. With current observational capability, at a 30 AU separation, the minimum planet mass for driving detectable arms in a disk around a 1 Myr 1M⊙ star at 140 pc at low inclinations is around Saturn mass. For planets more massive than Neptune masses, they typically drive multiple arms. Therefore in observed disks with spirals, it is unlikely that each spiral arm originates from a different planet. We also find only massive perturbers with at least multi-Jupiter masses are capable of driving bright arms with δmax≳2 as found in SAO 206462, MWC 758, and LkHα~330, and these arms do not follow linear wave propagation theory. Additionally, we find the morphology and contrast of the primary and secondary arms are largely unaffected by a modest level of viscosity with α≲0.01. Finally, the contrast of the arms in the SAO 206462 disk suggests that the perturber SAO 206462 b at ∼100 AU is about 5−10MJ in mass.\end{abstract}
Multiple rings in the transition disk and companion candidates around RX J1615.3-3255
High contrast imaging with VLT/SPHEREAuthors:de Boer et alAbstract:Context.The effects of a planet sculpting the disk from which it formed are most likely to be found in disks that are in transition between being classical protoplanetary and debris disks. Recent direct imaging of transition disks has revealed structures such as dust rings, gaps, and spiral arms, but an unambiguous link between these structures and sculpting planets is yet to be found.Aims.We aim to find signs of ongoing planet-disk interaction and study the distribution of small grains at the surface of the transition disk around RX J1615.3-3255 (RX J1615).Methods.We observed RX J1615 with VLT/SPHERE. From these observations, we obtained polarimetric imaging with ZIMPOL (R′-band) and IRDIS (J), and IRDIS (H2H3) dual-band imaging with simultaneous spatially resolved spectra with the IFS (YJ).Results.We image the disk for the first time in scattered light and detect two arcs, two rings, a gap and an inner disk with marginal evidence for an inner cavity. The shapes of the arcs suggest that they are probably segments of full rings. Ellipse fitting for the two rings and inner disk yield a disk inclination i = 47 ± 2° and find semi-major axes of 1.50 ± 0.01′′ (278 au), 1.06 ± 0.01′′ (196 au) and 0.30 ± 0.01′′ (56 au), respectively. We determine the scattering surface height above the midplane, based on the projected ring center offsets. Nine point sources are detected between 2.1′′ and 8.0′′ separation and considered as companion candidates. With NACO data we recover four of the nine point sources, which we determine to be not co-moving, and therefore unbound to the system.Conclusions.We present the first detection of the transition disk of RX J1615 in scattered light. The height of the rings indicate limited flaring of the disk surface, which enables partial self-shadowing in the disk. The outermost arc either traces the bottom of the disk or it is another ring with semi-major axis ≳ 2.35′′ (435 au). We explore both scenarios, extrapolating the complete shape of the feature, which will allow us to distinguish between the two in future observations. The most attractive scenario, where the arc traces the bottom of the outer ring, requires the disk to be truncated at r ≈ 360 au. If the closest companion candidate is indeed orbiting the disk at 540 au, then it would be the most likely cause for such truncation. This companion candidate, as well as the remaining four, all require follow up observations to determine if they are bound to the system.
Shadows cast on the transition disk of HD 135344B
Shadows cast on the transition disk of HD 135344B
Multiwavelength VLT/SPHERE polarimetric differential imaging
Author:
Stolker et al
Abstract:
Context.
The protoplanetary disk around the F-type star HD 135344B (SAO 206462) is in a transition stage and shows many intriguing structures both in scattered light and thermal (sub-)millimeter emission which are possibly related to planet formation processes.
Aims.
We aim to study the morphology and surface brightness of the disk in scattered light to gain insight into the innermost disk regions, the formation of protoplanets, planet-disk interactions traced in the surface and midplane layers, and the dust grain properties of the disk surface.
Methods.
We have carried out high-contrast polarimetric differential imaging (PDI) observations with VLT/SPHERE and obtained polarized scattered light images with ZIMPOL in the R and I-bands and with IRDIS in the Y and J-bands. The scattered light images and surface brightness profiles are used to study in detail structures in the disk surface and brightness variations. We have constructed a 3D radiative transfer model to support the interpretation of several detected shadow features.
Results.
The scattered light images reveal with unprecedented angular resolution and sensitivity the spiral arms as well as the 25 au cavity of the disk. Multiple shadow features are discovered on the outer disk with one shadow only being present during the second observation epoch. A positive surface brightness gradient is observed in the stellar irradiation corrected (r2-scaled) images in southwest direction possibly due to an azimuthally asymmetric perturbation of the temperature and/or surface density by the passing spiral arms. The disk integrated polarized flux, normalized to the stellar flux, shows a positive trend towards longer wavelengths which we attribute to large (2πa ≳ λ) aggregate dust grains in the disk surface. Part of the non-azimuthal polarization signal in the Uφ image of the J-band observation can be attributed to multiple scattering in the disk.
Conclusions.
The detected shadow features and their possible variability have the potential to provide insight into the structure of and processes occurring in the innermost disk regions. Possible explanations for the presence of the shadows include a 22° misaligned inner disk, a warped disk region that connects the inner disk with the outer disk, and variable or transient phenomena such as a perturbation of the inner disk or an asymmetric accretion flow. The spiral arms are best explained by one or multiple protoplanets in the exterior of the disk although no gap is detected beyond the spiral arms up to 1.′′0.
Saturday, February 11, 2017
Transient dynamics of large scale vortices in Keplerian disk
Author:RazdoburdinAbstract:The mechanism of transition from laminar state to turbulent state in Keplerian disks is still unknown. The most popular version today is generation of turbulence due to magnetorotational instability (MRI). However magnetohydrodynamic simulations give the value of Shakura-Sunyaev parameter more then an order of magnitude smaller rather than that found from observations. One way to solve this problem is the existence of an alternative or additional mechanism for generating turbulence. It can be the bypass mechanism, which is responsible for transition to turbulence in Couette and Poiseuille flows. This mechanism is based on the transient growth of linear perturbations in the flow with the subsequent transition to the nonlinear stage. In order to clarify the role of this mechanism in astrophysical disks first of all it is necessary to calculate the maximal possible growth factor of linear perturbations in the flow. In this paper the results of such calculations are presented for perturbations on different scales compared with the disk thickness. Qualitative description of mechanisms responsible for the growth will also be presented. It was found that the most rapidly growing shear harmonics have azimuthal wavelength of the order of the disk thickness. In addition, their initial form is always similar to the vortex perturbations with the same potential vorticity. It was shown that the vortices with azimuthal wavelength more than an order of magnitude in excess of the disc thickness, are still able to grow by dozens of times.
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