DR Tauri has a Planet Forming Region With Variable Brightness

DR Tau: Temporal variability of the brightness distribution in the potential planet-forming region

Authors:

Brunngräber et al

Abstract:

We investigate the variability of the brightness distribution and the changing density structure of the protoplanetary disk around DR Tau, a classical T Tauri star. DR Tau is known for its peculiar variations from the ultraviolet (UV) to the mid-infrared (MIR). Our goal is to constrain the temporal variation of the disk structure based on photometric and MIR interferometric data. We observed DR Tau with the MID-infrared Interferometric instrument (MIDI) at the Very Large Telescope Interferometer (VLTI) at three epochs separated by about nine years, two months, respectively. We fit the spectral energy distribution and the MIR visibilities with radiative transfer simulations. We are able to reproduce the spectral energy distribution as well as the MIR visibility for one of the three epochs (third epoch) with a basic disk model. We were able to reproduce the very different visibility curve obtained nine years earlier with a very similar baseline (first epoch), using the same disk model with a smaller scale height. The same density distribution also reproduces the observation made with a higher spatial resolution in the second epoch, i.e. only two months before the third epoch.

Evolution of Binary Planetesimals

Application of Gas Dynamical Friction for Planetesimals: II. Evolution of Binary Planetesimals

Authors:

Grishin et al

Abstract:

One of first the stages of planet formation is the growth of small planetesimals and their accumulation into large planetesimals and planetary embryos. This early stage occurs much before the dispersal of most of the gas from the protoplanetary disk. At this stage gas-planetesimal interactions play a key role in the dynamical evolution of \emph{single} intermediate-mass planetesimals (mp∼1021−1025g) \emph{through gas dynamical friction} (GDF). A significant fraction of all Solar system planetesimals (asteroids and Kuiper-belt objects) are known to be binary planetesimals (BPs). Here, we explore the effects of GDF on the evolution of \emph{binary} planetesimals embedded in a gaseous disk using an N-body code with a fiducial external force accounting for GDF. We find that GDF can induce binary mergers on timescales shorter than the disk lifetime for masses above mp≳1022g at 1AU, independent of the binary initial separation and eccentricity. Such mergers can affect the structure of merger-formed planetesimals, and the GDF-induced binary inspiral can play a role in the evolution of the planetesimal disk. In addition, binaries on eccentric orbits around the star may evolve in the supersonic regime, where the torque reverses and the binary expands, which would enhance the cross section for planetesimal encounters with the binary. Highly inclined binaries with small mass ratios, evolve due to the combined effects of Kozai-Lidov cycles with GDF which lead to chaotic evolution. Prograde binaries go through semi-regular Kozai-Lidov evolution, while retrograde binaries frequently flip their inclination and ∼50% of them are destroyed.

A Panoptic Model for Planetesimal Formation and Pebble Delivery

A panoptic model for planetesimal formation and pebble delivery

Authors:

Krijt et al

Abstract:

The journey from dust particle to planetesimal involves physical processes acting on scales ranging from micrometers (the sticking and restructuring of aggregates) to hundreds of astronomical units (the size of the turbulent protoplanetary nebula). Considering these processes simultaneously is essential when studying planetesimal formation. We develop a novel, global, semi-analytical model for the evolution of the mass-dominating dust particles in a turbulent protoplanetary disk that takes into account the evolution of the dust surface density while preserving the essential characteristics of the porous coagulation process. This panoptic model is used to study the growth from submicron to planetesimal sizes in disks around Sun-like stars. For highly porous ices, unaffected by collisional fragmentation and erosion, rapid growth to planetesimal sizes is possible in a zone stretching out to ∼10 AU for massive disks. When porous coagulation is limited by erosive collisions, the formation of planetesimals through direct coagulation is not possible, but the creation of a large population of aggregates with Stokes numbers close to unity might trigger the streaming instability (SI). However, we find that reaching conditions necessary for SI is difficult and limited to dust-rich disks, (very) cold disks, or disks with weak turbulence. Behind the snow-line, porosity-driven aggregation of icy grains results in rapid (∼104 yr) formation of planetesimals. If erosive collisions prevent this, SI might be triggered for specific disk conditions. The numerical approach introduced in this work is ideally suited for studying planetesimal formation and pebble delivery simultaneously and will help build a coherent picture of the start of the planet formation process.

Grain-Modified Magnetic Diffusivities in Protoplanetary Disks

On the Grain-Modified Magnetic Diffusivities in Protoplanetary Disks

Authors:

Xu et al

Abstract:

Weakly ionized protoplanetary disks (PPDs) are subject to non-ideal-magnetohydrodynamic (MHD) effects including Ohmic resistivity, the Hall effect and ambipolar diffusion (AD), and the resulting magnetic diffusivities (ηO,ηH and ηA) largely control the disk gas dynamics. The presence of grains not only strongly reduces disk ionization fraction, but also modify the scalings of ηH and ηA with magnetic field strength. We derive analytically asymptotic expressions of ηH and ηA in both strong and weak field limits and show that towards strong field, ηH can change sign (at a threshold field strength Bth), mimicking a flip of field polarity, and AD is substantially reduced. Applying to PPDs, we find that when small ∼0.1 (0.01)μm grains are sufficiently abundant [mass ratio ∼0.01 (10−4)], ηH can change sign up to ∼2−3 scale heights above midplane at modest field strength (plasma β∼100) over a wide range of disk radii. Reduction of AD is also substantial towards the AD dominated outer disk and may activate the magneto-rotational instability. We further perform local non-ideal MHD simulations of the inner disk (within 10 AU) and show that with sufficiently abundant small grains, magnetic field amplification due to the Hall-shear instability saturates at very low level near the threshold field strength Bth. Together with previous studies, we conclude by discussing the grain-abundance-dependent phenomenology of PPD gas dynamics.

Resolved gas Cavities in Transitional Disks by ALMA

Resolved gas cavities in transitional disks inferred from CO isotopologues with ALMA

Authors:

van der Marel et al

Abstract:

Transitional disks around young stars are promising candidates to look for recently formed, embedded planets. Planet-disk interaction models predict that planets clear a gap in the gas while trapping dust at larger radii. Other physical mechanisms could be responsible for cavities as well. Previous observations have revealed that gas is still present inside these cavities, but the spatial distribution of this gas remains uncertain. We present high spatial resolution observations with the Atacama Large Millimeter/submillimeter Array (ALMA) of 13CO and C18O lines of four well-studied transitional disks. The observations are used to set constraints on the gas surface density, specifically cavity size and density drop inside the cavity. The physical-chemical model DALI is used to analyze the gas images of SR21, HD135344B, DoAr44 and IRS48. The main parameters of interest are the size, depth and shape of the gas cavity. CO isotope-selective photodissociation is included to properly constrain the surface density in the outer disk from C18O emission. The gas cavities are up to 3 times smaller than those of the dust in all four disks. Model fits indicate that the surface density inside the gas cavities decreases by a factor of 100-10000 compared with the surface density profile derived from the outer disk. A comparison with an analytical model of gap depths by planet-disk interaction shows that the disk viscosities are likely low, with a less than 1E-3 for planet masses less than 10 MJup. The resolved measurements of the gas and dust in transition disk cavities support the predictions of models that describe how planet-disk interactions sculpt gas disk structures and influence the evolution of dust grains. These observed structures strongly suggest the presence of giant planetary companions in transition disk cavities, although at smaller orbital radii than is typically indicated from the dust cavity radii alone.

Magneto-thermal Disk Wind from Protoplanetary Disks

Magneto-thermal Disk Wind from Protoplanetary Disks

Authors:

Bai et al

Abstract:

Global evolution and dispersal of protoplanetary disks (PPDs) is governed by disk angular momentum transport and mass-loss processes. Recent numerical studies suggest that angular momentum transport in the inner region of PPDs is largely driven by magnetized disk wind, yet the wind mass-loss rate remains unconstrained. On the other hand, disk mass loss has conventionally been attributed to photoevaporation, where external heating on the disk surface drives a thermal wind. We unify the two scenarios by developing a 1D model of magnetized disk winds with a simple treatment of thermodynamics as a proxy for external heating. The wind properties largely depend on 1) the magnetic field strength at the wind base, characterized by the poloidal Alfv\'en speed vAp, 2) the sound speed cs near the wind base, and 3) how rapidly poloidal field lines diverge (achieve R−2 scaling). When vAp≫cs, corotation is enforced near the wind base, resulting in centrifugal acceleration. Otherwise, the wind is accelerated mainly by the pressure of the toroidal magnetic field. In both cases, the dominant role played by magnetic forces likely yields wind outflow rates that well exceed purely hydrodynamical mechanisms. For typical PPD accretion-rate and wind-launching conditions, we expect vAp to be comparable to cs at the wind base. The resulting wind is heavily loaded, with total wind mass loss rate likely reaching a considerable fraction of wind-driven accretion rate. Implications for modeling global disk evolution and planet formation are also discussed.

Flickering of Accreting White Dwarfs

Flickering of accreting white dwarfs: the remarkable amplitude - flux relation and disc viscocity

Authors:

Zamanov et al

Abstract:

We analyze optical photometric data of short term variability (flickering) of accreting white dwarfs in cataclysmic variables (KR Aur, MV Lyr, V794 Aql, TT Ari, V425 Cas), recurrent novae (RS Oph and T CrB) and jet-ejecting symbiotic stars (CH Cyg and MWC 560). We find that the amplitude-flux relationship is visible over four orders of magnitude, in the range of fluxes from 1029 to 1033 erg s−1 \AA−1, as a "statistically perfect" correlation with correlation coefficient 0.96 and p-value ∼10−28. In the above range, the amplitude of variability for any of our 9 objects is proportional to the flux level with (almost) one and the same factor of proportionality for all 9 accreting white dwarfs with ΔF=0.36(±0.05)Fav, σrms=0.086(±0.011)Fav, and σrms/ΔF=0.24±0.02. Over all, our results indicate that the viscosity in the accretion discs is practically the same for all 9 objects in our sample, in the mass accretion rate range 2×10−11−2×10−7 M⊙ yr−1.

SOPHIE's Hunt for Long-period Brown Dwarf Companions

The SOPHIE search for northern extrasolar planets VIII. Follow-up of ELODIE candidates: long-period brown-dwarf companions

Authors:

Bouchy et al

Abstract:

Long-period brown dwarf companions detected in radial velocity surveys are important targets for direct imaging and astrometry to calibrate the mass-luminosity relation of substellar objects. Through a 20-year radial velocity monitoring of solar-type stars that began with ELODIE and was extended with SOPHIE spectrographs, giant exoplanets and brown dwarfs with orbital periods longer than ten years are discovered. We report the detection of five new potential brown dwarfs with minimum masses between 32 and 83 Jupiter mass orbiting solar-type stars with periods longer than ten years. An upper mass limit of these companions is provided using astrometric Hipparcos data, high-angular resolution imaging made with PUEO, and a deep analysis of the cross-correlation function of the main stellar spectra to search for blend effects or faint secondary components. These objects double the number of known brown dwarf companions with orbital periods longer than ten years and reinforce the conclusion that the occurrence of such objects increases with orbital separation. With a projected separation larger than 100 mas, all these brown dwarf candidates are appropriate targets for high-contrast and high angular resolution imaging.

BD+20594b: a Neptune Sized MegaEarth

A Neptune-sized Exoplanet Consistent with a Pure Rock Composition

Authors:

Espinoza et al

Abstract:

We report the discovery of BD+20594b, a Neptune-sized exoplanet consistent with a pure rock composition, made using photometry from Campaign 4 of the two-wheeled Kepler (K2) mission. The host star is a bright (V=11.04, Ks=9.37), slightly metal poor ([Fe/H]=−0.15±0.05 dex) solar analogue located at 152.1+9.7−7.4 pc from Earth, for which we find a radius of R∗=0.928+0.055−0.040R⊙ and a mass of M∗=0.961+0.032−0.029M⊙. A joint analysis of the K2 photometry and HARPS radial velocities reveal that the planet is in a ≈42 day orbit around its host star, has a radius of 2.23+0.14−0.11R⊕, and a mass of 16.3+6.0−6.1M⊕. The data at hand are most consistent with a pure rock composition with a low volatile content, potentially making it a rare exception among Neptune-sized exoplanets discovered so far.

Modeling Exoplanet Climates in the Search for Life

Academic Bun Fight! Strong Words From Bradley Schaefer OVer Hippke's Paper Refuting the Century Long KIC 8462852 Dimming

A few hours ago, Michael Hippke posted a manuscript to arXiv (http://arxiv.org/abs/1601.07314), and submitted the same manuscript to the Astrophysical Journal Letters (ApJLett). This manuscript claims to have found that the DASCH data produces light curves with secular trends (both systematic dimmings and brightenings) over the century-long records. This same DASCH data (from the collection of archival sky photographs now at Harvard Observatory) was used to recognize a dimming of KIC 8462852 (a.k.a. ‘Tabby’s Star’ or the ‘WTF star’) at an average rate of 0.165±0.013 magnitudes per century from 1890 to 1989.

This dimming from the DASCH data is just a long-time scale version of the dimming also seen with the Kepler spacecraft, and these dimmings are still a high mystery and a perplexing problem. Hippke is taking his claimed result (that the majority of DASCH light curves have major and widespread calibration errors resulting in apparent secular trends) as then implying that KIC 8462852 does not have any secular trend. This claim is easily proved wrong.

Hippke made two major errors, both of which are beginner’s mistakes, and both of which will erroneously produce apparent dimmings and brightenings when none exist. First, Hippke explicitly includes red-sensitive and yellow-sensitive photographs together with the blue-sensitive photographs. The different colors will produce systematically different brightnesses (magnitudes). The trouble is further that the red and yellow photographs are predominantly at late times in the century-long light curve (in the 1970s and 1980s), so the inclusion of many magnitudes that are systematically high or low only at the end of the century will artificially make the star appear to brighten or dim over the century.

[...]

Perhaps from a journalistic point of view, any ‘stirring of the pot’ is good copy. But from the point of view of science and knowledge, putting up unchecked and false claims is bad all the way around. Science has a great strength of being error-correcting, with the normal procedure now for the DASCH people to put out a full formal refutation of Hippke’s claims, and such will appear in many months. But with the one-day turn-around of arXiv and with fast journalist response, there will be many months where the reputation of DASCH is maligned. So Hippke’s choice of running to reporters before the paper appeared publicly, and disdaining any experienced advice despite being a self-proclaimed “novice”, is not good science.

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Evidence for Water in the Atmosphere of hot Neptune HAT-P-26b

Evidence for Water in the Atmosphere of HAT-P-26b Using LDSS-3C

Authors:

Stevenson et al

Abstract:

The characterization of a physically-diverse set of transiting exoplanets is an important and necessary step towards establishing the physical properties linked to the production of obscuring clouds or hazes. Only planets with identifiable spectroscopic features can effectively enhance our understanding of atmospheric chemistry and metallicity. Using data acquired by the newly-commissioned LDSS-3C instrument on Magellan and the Spitzer Space Telescope, we find evidence for water in the transmission spectrum of the Neptune-mass planet HAT-P-26b. Surprisingly, we detect no trace of potassium. Our measured spectrum is best explained by either a high-metallicity, cloud-free atmosphere or a solar-metallicity atmosphere with a cloud deck at ~10 mbar. The presence of strong spectral features in our data suggests that future observations at higher precision could break this degeneracy and reveal the planet's atmospheric composition. We also update HAT-P-26b's transit ephemeris, t_0 = 2455304.65218(25) BJD_TDB, and orbital period, p = 4.2345023(7) days.

KIC 8462852 did NOT Fade During the Last 100 Years (hint: calibration is IMPORTANT!)

KIC 8462852 did likely not fade during the last 100 years

Authors:

Hippke et al

Abstract:

A recent analysis found a "completely unprecedented" dimming of 0.165±0.013 magnitudes per century in the F3 main sequence star KIC8462852. This star is interesting, as it shows episodes of day-long dips with up to 20% dimming of unknown origin. We re-analyze the same Harvard archival Johnson B photometry and find comparable dimmings, and structural breaks, for 18 of 28 checked F-dwards (64%) in the Kepler field of view. We conclude that the Harvard plates photometry suffers from imperfect long-term (1890--1989) calibration. The most likely explanation for the century-long dimming of KIC8462852 is thus a data artefact, and it is probably not of astrophysical origin.

A Robot Installed the Mirrors for the James Webb Space Telescope

Inside a massive clean room at NASA's Goddard Space Flight Center in Greenbelt, Maryland the James Webb Space Telescope team is steadily installing the largest space telescope mirror ever. Unlike other space telescope mirrors, this one must be pieced together from segments using a high-precision robotic arm.

The team uses a robotic arm called the Primary Mirror Alignment and Integration Fixture to lift and lower each of Webb's 18 primary flight mirror segments to their locations on the telescope structure. Each of the mirrors is made with beryllium, chosen for its properties to withstand the super cold temperatures of space. Each segment also has a thin gold coating to reflect infrared light. These mirror segments will function as one when the telescope is in orbit.

"In order for the combination of mirror segments to function as a single mirror they must be placed within a few millimeters of one another, to fraction-of-a-millimeter accuracy. A human operator cannot place the mirrors that accurately, so we developed a robotic system to do the assembly," said NASA's James Webb Space Telescope Program Director Eric Smith, at Headquarters in Washington.

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The Strange Kepler-454 System

The Kepler-454 System: A Small, Not-rocky Inner Planet, a Jovian World, and a Distant Companion

Authors:

Gettel et al

Abstract:

Kepler-454 (KOI-273) is a relatively bright (V = 11.69 mag), Sun-like star that hosts a transiting planet candidate in a 10.6 d orbit. From spectroscopy, we estimate the stellar temperature to be 5687 +/- 50 K, its metallicity to be [m/H] = 0.32 +/- 0.08, and the projected rotational velocity to be v sin i less than 2.4 km s-1. We combine these values with a study of the asteroseismic frequencies from short cadence Kepler data to estimate the stellar mass to be 1.028+0:04-0:03 M_Sun, the radius to be 1.066 +/- 0.012 R_Sun and the age to be 5.25+1:41-1:39 Gyr. We estimate the radius of the 10.6 d planet as 2.37 +/- 0.13 R_Earth. Using 63 radial velocity observations obtained with the HARPS-N spectrograph on the Telescopio Nazionale Galileo and 36 observations made with the HIRES spectrograph at Keck Observatory, we measure the mass of this planet to be 6.8 +/- 1.4M_Earth. We also detect two additional non-transiting companions, a planet with a minimum mass of 4.46 +/- 0.12 M_J in a nearly circular 524 d orbit and a massive companion with a period greater than 10 years and mass greater than 12.1M_J . The twelve exoplanets with radii less than 2.7 R_Earth and precise mass measurements appear to fall into two populations, with those less than 1.6 R_Earth following an Earth-like composition curve and larger planets requiring a significant fraction of volatiles. With a density of 2.76 +/- 0.73 g cm-3, Kepler-454b lies near the mass transition between these two populations and requires the presence of volatiles and/or H/He gas.

Exomoons are Removed if Exoplanets Migrate in too far

Resonant Removal of Exomoons During Planetary Migration

Authors:

Spalding et al

Abstract:

Jupiter and Saturn play host to an impressive array of satellites, making it reasonable to suspect that similar systems of moons might exist around giant extrasolar planets. Furthermore, a significant population of such planets is known to reside at distances of several Astronomical Units (AU), leading to speculation that some moons thereof might support liquid water on their surfaces. However, giant planets are thought to undergo inward migration within their natal protoplanetary disks, suggesting that gas giants currently occupying their host star's habitable zone formed further out. Here we show that when a moon-hosting planet undergoes inward migration, dynamical interactions may naturally destroy the moon through capture into a so-called "evection resonance." Within this resonance, the lunar orbit's eccentricity grows until the moon eventually collides with the planet. Our work suggests that moons orbiting within about 10 planetary radii are susceptible to this mechanism, with the exact number dependent upon the planetary mass, oblateness and physical size. Whether moons survive or not is critically related to where the planet began its inward migration as well as the character of inter-lunar perturbations. For example, a Jupiter-like planet currently residing at 1AU could lose moons if it formed beyond 5AU. Cumulatively, we suggest that an observational census of exomoons could potentially inform us on the extent of inward planetary migration, for which no reliable observational proxy currently exists.

Gas Giants HD 60532b & HD 60532c are in a 3:1 Orbital Resonance

Dynamics of the 3/1 planetary mean-motion resonance. An application to the HD60532 b-c planetary system

Authors:

Alves et al

Abstract:

In this paper, we use a semi-analytical approach to analyze the global structure of the phase space of the planar planetary 3/1 mean-motion resonance, in cases where the outer planet is more massive than its inner companion. We show that the resonant dynamics can be described using only two fundamental parameters, the total angular momentum and the scaling parameter. The topology of the Hamiltonian function describing the resonant behaviour is studied on the representative planes that allows us to investigate a large domain of the phase space of the three-body problem without time-expensive numerical integrations of the equations of motion, and without any restriction on the magnitude of the planetary eccentricities. The families of the well known Apsidal Corotation Resonances (ACR) parameterized by the planetary mass ratio are obtained and their stability is analyzed. The main dynamical features in the domains around ACR are also investigated in detail by means of spectral analysis techniques, which allow us to detect the regions of different regimes of motion of resonant systems. The construction of dynamical maps for various values of the total angular momentum shows the evolution of domains of stable motion with the eccentricities, identifying possible configurations suitable for exoplanetary systems.

James Webb Space Telescope has Only *ONE* Mirror Left to be Installed

Ten Multi-planet Systems from K2 Mission & Masses of Two Hot Super-Earths

Ten Multi-planet Systems from K2 Campaigns 1 & 2 and the Masses of Two Hot Super-Earths

Authors:

Sinukoff et al

Abstract:

We present a catalog of 10 multi-planet systems from Campaigns 1 and 2 of the K2 mission. We report the sizes and orbits of 24 planets split between six 2-planet systems and four 3-planet systems. These planets stem from a systematic search of the K2 photometry for all dwarf stars observed by K2 in these fields. We precisely characterized the host stars with adaptive optics imaging and analysis of high-resolution optical spectra from Keck/HIRES and medium-resolution spectra from IRTF/SpeX. The planets are mostly smaller than Neptune (19/24 planets) as in the Kepler mission and all have short periods (P less than 50 d) due to the duration of the K2 photometry. The host stars are relatively bright (most have Kp less than 12.5 mag) and are amenable to follow-up planet characterization. For EPIC 204221263, we measured precise radial velocities using Keck/HIRES and provide initial estimates of the planet masses. EPIC 204221263b is a short-period super-Earth with a radius of 1.55±0.16 R⊕, a mass of 12.0±2.9 M⊕, and a high density consistent with an iron-rich composition. The outer planet EPIC 204221263c is a lower density sub-Neptune-size planet with a radius of 2.42±0.29 R⊕ and a mass of 9.9±4.6 M⊕ that likely has a substantial envelope. This new planet sample demonstrates the capability of K2 to discover numerous planetary systems around bright stars.

Revising the Kepler-10 System for Exoplanet Characteristics & Adding Kepler-10d (KOI-72.X)

Revised Masses and Densities of the Planets around Kepler-10

Authors:

Weiss et al

Abstract:

Determining which small exoplanets have stony-iron compositions is necessary for quantifying the occurrence of such planets and for understanding the physics of planet formation. Kepler-10 hosts the stony-iron world Kepler-10b (K10b), and also contains what has been reported to be the largest solid silicate-ice planet, Kepler-10c (K10c). Using 220 radial velocities (RVs), including 72 precise RVs from Keck-HIRES of which 20 are new from 2014-2015, and 17 quarters of Kepler photometry, we obtain the most complete picture of the Kepler-10 system to date. We find that K10b (Rp=1.47 Re) has mass 3.72±0.42 Me and density 6.46±0.73 g/cc. Modeling the interior of K10b as an iron core overlaid with a silicate mantle, we find that the iron core constitutes 0.17±0.11 of the planet mass. For K10c (Rp=2.35 Re) we measure Mp=13.98±1.79 Me and ρ=5.94±0.76 g/cc, significantly lower than the mass computed in Dumusque et al. (2014, 17.2±1.9 Me). Internal compositional modeling reveals that at least 10% of the radius of Kepler-10c is a volatile envelope composed of hydrogen-helium (0.2% of the mass, 16% of the radius) or super-ionic water (28% of the mass, 29% of the radius). Analysis of only HIRES data yields a higher mass for K10b and a lower mass for K10c than does analysis of the HARPS-N data alone, with the mass estimates for K10c formally inconsistent by 3σ. Splitting the RVs from each instrument leads to inconsistent measurements for the mass of planet c in each data set. This suggests that time-correlated noise is present and that the uncertainties in the planet masses (especially K10c) exceed our formal estimates. Transit timing variations (TTVs) of K10c indicate the likely presence of a third planet in the system, KOI-72.X. The TTVs and RVs are consistent with KOI-72.X having an orbital period of 24, 71, or 101 days, and a mass from 1-7 Me.

EPIC 206318379b: a Close-in hot SuperEarth Around a Metal Rich M Dwarf Host Star

The K2-ESPRINT Project III: A Close-in Super-Earth around a Metal-rich Mid-M Dwarf

Authors:

Hirano et al

Abstract:

We validate a candidate super-Earth (Rp=2.38±0.25R⊕) on a close-in orbit (P=2.26 days) around EPIC 206318379, a metal-rich M4-type dwarf in the Campaign 3 field of the K2 mission. Our follow-up observations included multi-band transit observations from the optical to the near infrared, low-resolution spectroscopy, and high-resolution adaptive-optics (AO) imaging. The phase-folded K2 transit light curve has a V-shape because the transit duration around this small star is comparable to the 30-minute K2 cadence. However, the light curves from our follow-up observations exhibit a sharp ingress and/or egress and flat bottom, ruling out a grazing eclipse of a binary system. We perform a global fit to all ground-based observations using a Gaussian process-based method and show that the transit depths in all passbands (r′2,zs,2,J,H,Ks) are within 2.2σ of the K2 value. Based on a model of the background stellar population and the absence of nearby sources in our AO imaging, we estimate the probability that a background eclipsing binary could cause a false positive to be less than 2×10−5. We also show that given the almost constant transit depths in the five passbands, EPIC 206318379 cannot have a physically associated companion later than M4, and the probability that it has another M4 dwarf is low as well (≈0.0721+0.023−0.036), even in which case the size of EPIC 206318379b falls on the planetary regime. EPIC 206318379b has the same radius (within 1σ) and experiences a similar irradiation from its host star as the well-studied GJ 1214b. A comparison between the atmospheric properties of these two objects with future observations would be especially interesting.

Is Carbon monoxide Detection in an ExoPlanetary Atmosphere an 'Anti-biosignature?'

On the detection of carbon monoxide as an anti-biosignature in exoplanetary atmospheres

Authors:

Wang et al

Abstract:

Recent works suggest that oxygen can be maintained on lifeless exoplanets in the habitable zones of M dwarfs as the results of photochemical reactions. However, the same photochemical models also predict high concentrations of carbon monoxide (CO) in the corresponding atmospheres. Here we use a line-by-line radiative transfer model to investigate the observation requirements of O2 and CO in such atmospheres. The results show that photochemically produced CO can be readily detected at 1.58, 2.34, and 4.67 μm. We suggest that future missions aiming at characterization of exoplanetary atmospheres consider detections of CO as an anti-biosignature.

Greenhouse Effects From Water Vapor Can Extend the Habitable Zone

A proposal for climate stability on H2-greenhouse planets

Authors:

Abbot et al

Abstract:

A terrestrial planet in an orbit far outside of the standard habitable zone could maintain surface liquid water as a result of H2-H2 collision-induced absorption by a thick H2 atmosphere. Without a stabilizing climate feedback, however, habitability would be accidental and likely brief. In this letter I propose stabilizing climate feedbacks for such a planet that require only that biological functions have an optimal temperature and operate less efficiently at other temperatures. For example, on a planet with a net source of H2 from its interior, H2-consuming life (such as methanogens) could establish a stable climate. If a positive perturbation is added to the equilibrium temperature, H2 consumption by life will increase (cooling the planet) until the equilibrium climate is reestablished. The potential existence of such feedbacks makes H2-warmed planets more attractive astrobiological targets.

SuperEarths Around M Dwarfs Will NOT be Habitable

Habitability of Terrestrial-Mass Planets in the HZ of M Dwarfs. I. H/He-Dominated Atmospheres

Authors:

Owen et al

Abstract:

The ubiquity of M dwarfs, combined with the relative ease of detecting terrestrial-mass planets around them, has made them prime targets for finding and characterising planets in the "Habitable Zone" (HZ). However, Kepler has revealed that terrestrial-mass exoplanets are often born with voluminous H/He envelopes, comprising mass-fractions (Menv/Mcore) ≳1\%. If these planets retain such envelopes over Gyr timescales, they will not be "habitable" even within the HZ. Given the strong X-ray/UV fluxes of M dwarfs, we study whether these planets can lose sufficient envelope-mass through photoevaporation to become habitable. We improve upon previous work by using hydrodynamic models that account for radiative cooling as well as the transition from hydrodynamic to ballistic escape. Adopting the XUV spectrum of the active M dwarf AD Leo as a template, including stellar evolution, and considering both evaporation and thermal evolution, we show that: (1) the envelope-mass lost is significantly lower than previous estimates that use an "energy-limited" formalism and ignore the transition to Jeans escape, (2) at the inner edge of the HZ, planets with a core mass ≲0.9 M⊕, can lose enough H/He to be habitable at late times if their initial H/He envelope mass-fraction is ∼1\%, (3) at the outer edge of the HZ, evaporation is ineffective at removing a ∼1\% H/He envelope even from cores down to 0.8 M⊕. Thus, if planets form with bulky H/He envelopes, only those with low core masses may eventually become habitable. Cores ≳ 1 M⊕ with ≳1\% natal H/He envelopes, will not be habitable in the HZ of M dwarfs.

Enigmatic and Ephemeral M Dwarf System KOI 6705 Might be a Very Strange False Positive

The Enigmatic and Ephemeral M Dwarf System KOI 6705: Cheshire Cat or Wild Goose?

Authors:

Gaidos et al

Abstract:

We confirm a 0.995 d periodic planetary transit-like signal, KOI 6705.01, in the Kepler lightcurve of the star KIC 6423922. Optical and infrared spectra show that this star is a mid M-type dwarf with an effective temperature =3327±60K, metallicity [Fe/H] =−0.08±0.10, radius =0.31±0.03R⊙, and mass =0.28±0.05M⊙. The star is ≈70 pc away and its space motion, rotation period, and lack of Hα emission indicate it is an older member of the "thin disk" population. On the other hand, the star exhibits excess infrared emission suggesting a dust disk more typical of a very young star. If the KOI 6705.01 signal is produced by a planet, the transit depth of 60 ppm means its radius is only 0.26+0.034−0.029R⊕, or about the size of the Moon. However, the duration (≳3~hr) and time variation of KOI 6705.01 are anomalous: the signal was undetected in the first two years of the mission and increased through the latter two years. These characteristics require implausible orbits and material properties for any planet and rule out such an explanation, although a dust cloud is possible. We excluded several false positive scenarios including background stars, scattered light from stars that are nearby on the sky, and electronic cross-talk between detector readout channels. We find the most likely explanation to be that KOI 6705.01 is a false positive created by charge transfer inefficiency in a detector column on which KIC 6423922 and a 1.99 d eclipsing binary both happened to fall.

Modelling the Photosphere of HD 189733

Modelling the photosphere of active stars for planet detection and characterization

Authors:

Herrero et al

Abstract:

Stellar activity patterns are responsible for jitter effects that are observed at different timescales and amplitudes. These effects are currently in the focus of many exoplanet search projects, since the lack of a well-defined characterization and correction strategy hampers the detection of the signals associated with small exoplanets. Accurate simulations of the stellar photosphere can provide synthetic time series data. These may help to investigate the relation between activity jitter and stellar parameters when considering different active region patterns. Moreover, jitters can be analysed at different wavelength scales in order to design strategies to remove or minimize them. In this work we present the StarSim tool, which is based on a model for a spotted rotating photosphere built from the integration of the spectral contribution of a fine grid of surface elements. The model includes all significant effects affecting the flux intensities and the wavelength of spectral features produced by active regions and planets. A specific application for the characterization and modelling of the spectral signature of active regions is considered, showing that the chromatic effects of faculae are dominant for low temperature contrasts of spots. Synthetic time series are modelled for HD 189733. Our algorithm reproduces both the photometry and the RVs to good precision, generally better than the studies published to date. We evaluate the RV signature of the activity in HD 189733 by exploring a grid of solutions from the photometry. We find that the use of RV data in the inverse problem could break degeneracies and allow for a better determination of some stellar and activity parameters. In addition, the effects of spots are studied for a set of simulated transit photometry, showing that these can introduce variations which are very similar to the signal of an atmosphere dominated by dust.

GRAPHIC: Geneva Reduction and Analysis Pipeline for High-contrast Imaging of Planetary Companions

The Geneva Reduction and Analysis Pipeline for High-contrast Imaging of planetary Companions

Authors:

Hagelberg et al

Abstract:

We present graphic, a new angular differential imaging reduction pipeline where all geometric image operations are based on Fourier transforms. To achieve this goal the entire pipeline is parallelized making it possible to reduce large amounts of observation data without the need to bin the data. The specific rotation and shift algorithms based on Fourier transforms are described and performance comparison with conventional interpolation algorithm is given. Tests using fake companions injected in real science frames demonstrate the significant gain obtained by using geometric operations based on Fourier transforms compared to conventional interpolation. This also translates in a better point spread function and speckle subtraction with respect to conventional reduction pipelines, achieving detection limits comparable to current best performing pipelines. Flux conservation of the companions is also demonstrated. This pipeline is currently able to reduce science data produced by Very Large Telescope (VLT)/NACO, Gemini/NICI, VLT/SPHERE, and Subaru/SCExAO.

First Scattered-Light Image of HD 131835's Debris Disk

First Scattered-Light Image of the Debris Disk around HD 131835 with the Gemini Planet Imager

Authors:

Hung et al

Abstract:

We present the first scattered-light image of the debris disk around HD 131835 in H band using the Gemini Planet Imager. HD 131835 is a ∼15 Myr old A2IV star at a distance of ∼120 pc in the Sco-Cen OB association. We detect the disk only in polarized light and place an upper limit on the peak total intensity. No point sources resembling exoplanets were identified. Compared to its mid-infrared thermal emission, the disk in scattered light shows similar orientation but different morphology. The scattered-light disk extends from ∼75 to ∼210 AU in the disk plane with roughly flat surface density. Our Monte Carlo radiative transfer model can well describe the observations with a model disk composed of a mixture of silicates and amorphous carbon. In addition to the obvious brightness asymmetry due to stronger forward scattering, we discover a weak brightness asymmetry along the major axis with the northeast side being 1.3 times brighter than the southwest side at a 3-{\sigma} level.

Direct Imaging of the Water Snow Line int HL Tauri's Protoplanetary Disk by ALMA

Direct imaging of the water snow line at the time of planet formation using two ALMA continuum bands

Authors:

Banzatti et al

Abstract:

Molecular snow lines in protoplanetary disks have been studied theoretically for decades because of their importance in shaping planetary architectures and compositions. The water snow line lies in the planet formation region at less than 10 AU, and so far its location has been estimated only indirectly from spatially-unresolved spectroscopy. This work presents a proof-of-concept method to directly image the water snow line in protoplanetary disks through its physical and chemical imprint in the local dust properties. We adopt a physical disk model that includes dust coagulation, fragmentation, drift, and a change in fragmentation velocities of a factor 10 between dry silicates and icy grains as found by laboratory work. We find that the presence of a water snow line leads to a sharp discontinuity in the radial profile of the dust emission spectral index {\alpha}_mm, due to replenishment of small grains through fragmentation. We use the ALMA simulator to demonstrate that this effect can be observed in protoplanetary disks using spatially-resolved ALMA images in two continuum bands. We explore the model dependence on the disk viscosity and find that the spectral index reveals the water snow line for a wide range of conditions, with opposite trends when the emission is optically thin rather than thick. If the disk viscosity is low ({\alpha}_visc less than 10^-3) the snow line produces a ring-like structure with a minimum at {\alpha}_mm ~ 2 in the optically thick regime, possibly similar to what has been measured with ALMA in the innermost region of the HL Tau disk.

Physical Properties of the Inner, Gaseous Disk of the Herbig Be Star BD+65 1637

Photoionization Models of the Inner Gaseous Disk of the Herbig Be Star BD+65 1637

Authors:

Patel et al

Abstract:

We attempt to constrain the physical properties of the inner, gaseous disk of the Herbig Be star BD+65 1637 using non-LTE, circumstellar disk codes and observed spectra (3700 to 10,500 \r{A}) from the ESPaDOnS instrument on CFHT. The photoionizing radiation of the central star is assumed to be the sole source of input energy for the disk. We model optical and near-infrared emission lines that are thought to form in this region using standard techniques that have been successful in modeling the spectra of Classical Be stars. By comparing synthetic line profiles of hydrogen, helium, iron and calcium with the observed line profiles, we try to constrain the geometry, density structure, and kinematics of the gaseous disk. Reasonable matches have been found for all line profiles individually; however, no disk density model based on a single power-law for the equatorial density was able to simultaneously fit all of the observed emission lines. Amongst the emission lines, the metal lines, especially the Ca II IR triplet, seem to require higher disk densities than the other lines. Excluding the Ca II lines, a model in which the equatorial disk density falls as 10−10(R∗/R)3gcm−3 seen at an inclination of 45{\deg} for a 50R∗ disk provides reasonable matches to the overall line shapes and strengths. The Ca II lines seem to require a shallower drop off as 10−10(R∗/R)2gcm−3 to match their strength. More complex disk density models are likely required to refine the match to the BD+65 1637 spectrum.

Radial velocity information content of M dwarf spectra in the near-infrared

Radial velocity information content of M dwarf spectra in the near-infrared

Authors:

Figuera et al

Abstract:

Aims:

We evaluate the radial velocity (RV) information content and achievable precision on M0-M9 spectra covering the ZYJHK bands. We do so while considering both a perfect atmospheric transmission correction and discarding areas polluted by deep telluric features, as done in previous works.


Methods:

To simulate the M-dwarf spectra, PHOENIX-ACES model spectra were employed; they were convolved with rotational kernels and instrumental profiles to reproduce stars with a v.sini of 1.0, 5.0, and 10.0 km/s when observed at resolutions of 60 000, 80 000, and 100 000. We considered the RV precision as calculated on the whole spectra, after discarding strongly polluted areas, and after applying a perfect telluric correction. In our simulations we paid particular attention to the details of the convolution and sampling of the spectra, and we discuss their impact on the final spectra.

Results:

Our simulations show that the most important parameter ruling the difference in attainable precision between the considered bands is the spectral type. For M0-M3 stars, the bands that deliver the most precise RV measurements are the Z, Y, and H band, with relative merits depending on the parameters of the simulation. For M6-M9 stars, the bands show a difference in precision that is within a factor of ∼2 and does not clearly depend on the band; this difference is reduced to a factor smaller than ∼1.5 if we consider a non-rotating star seen at high resolution. We also show that an M6-M9 spectrum will deliver a precision about two times better as an M0-M3 spectra with the same signal-to-noise ratio. Finally, we note that the details of modelling the Earth atmosphere and interpreting the results have a significant impact on which wavelength regions are discarded when setting a limit threshold at 2-3%.

Asteroseismology of Exoplanet Host Stars

Asteroseismology of Exoplanet Host Stars

Authors:

Huber et al

Abstract:

Asteroseismology is among the most powerful observational tools to determine fundamental properties of stars. Space-based photometry has recently enabled the systematic detection of oscillations in exoplanet host stars, allowing a combination of asteroseismology with transit and radial-velocity measurements to characterize planetary systems. In this contribution I will review the key synergies between asteroseismology and exoplanet science such as the precise determination of radii and ages of exoplanet host stars, as well as applications of asteroseismology to measure spin-orbit inclinations in multiplanet systems and orbital eccentricities of small planets. Finally I will give a brief outlook on asteroseismic studies of exoplanet hosts with current and future space-based missions such as K2 and TESS.

Variable WZ Sagittae is Eating an L Class Brown Dwarf

Direct Detection of the L-Dwarf Donor in WZ Sagittae

Author:

Harrison

Abstract:

Analysis of a large set of phase-resolved K-band spectra of the cataclysmic variable WZ Sge shows that the secondary star of this system appears to be an L-dwarf. Previous K-band spectra of WZ Sge found that the CO overtone bandheads were in emission. We show that absorption from the 12CO(2,0) bandhead of the donor star creates a dip in the 12CO(2,0) emission feature. Measuring the motion of this feature over the orbital period, we construct a radial velocity curve that gives a velocity amplitude of Kabs = 520 ± 35 km s−1, consistent with the previously published values for this parameter.

The Transmission Spectra of HD 209458b

A new approach to analysing HST spatial scans: the transmission spectrum of HD 209458b

Authors:

Tsiaras et al

Abstract:

The Wide Field Camera 3 (WFC3) on Hubble Space Telescope (HST) is currently one of the most popular instruments for observing exoplanetary atmospheres, especially with the use of the spatial scanning technique. An increasing number of exoplanets have been studied using this technique as it enables the observation of bright targets without saturating the sensitive detectors. In this work we present a new pipeline for analysing the data obtained with the spatial scanning technique, starting from the raw data provided by the instrument. In addition to commonly used correction techniques, we take into account the geometric distortions of the instrument, whose impact may become important when combined to the scanning process. Our approach can improve the photometric precision for existing data and also push further the limits of the spatial scanning technique, as it allows the analysis of even longer spatial scans. As an application of our method and pipeline, we present the results from a reanalysis of the spatially scanned transit spectrum of HD 209458b. We calculate the transit depth per wavelength channel with an average relative error of 40 ppm. We interpret the final spectrum with T-REx, our line-by-line fully bayesian spectral retrieval code, which confirms the presence of water vapour and investigates the additional presence of NH3, HCN and clouds in the atmosphere of HD 209458b. The narrow wavelength range limits our ability to disentangle the degeneracy between a cloudy atmosphere or a water-poor atmosphere. Additional data over a broader spectral range are needed to address this issue.

DEdicated MONitor of EXotransits (DEMONEX) Observes Seven Transits of Hot Jupiter XO-4b

The DEdicated MONitor of EXotransits (DEMONEX): Seven Transits of XO-4b

Authors:

Villaneuva et al

Abstract:

The DEdicated MONitor of EXotransits (DEMONEX) was a 20 inch robotic and automated telescope to monitor bright stars hosting transiting exoplanets to discover new planets and improve constraints on the properties of known transiting planetary systems. We present results for the misaligned hot Jupiter XO-4b containing 7 new transits from the DEMONEX telescope, including 3 full and 4 partial transits. We combine these data with archival light curves and archival radial velocity measurements to derive the host star mass M∗=1.293+0.030−0.029M⊙ and radius R∗=1.554+0.042−0.030R⊙ as well as the planet mass MP=1.615+0.10−0.099MJ and radius RP=1.317+0.040−0.029RJ and a refined ephemeris of P=4.1250687±0.0000024 days and T0=2454758.18978±0.00024BJDTDB. We include archival Rossiter-McLaughlin measurements of XO-4 to infer the stellar spin-planetary orbit alignment λ=−40.0+8.8−7.5 degrees.

We test the effects of including various detrend parameters, theoretical and empirical mass-radius relations, and Rossiter-McLaughlin models. We infer that detrending against CCD position and time or airmass can improve data quality, but can have significant effects on the inferred values of many parameters --- most significantly RP/R∗ and the observed central transit times TC. In the case of RP/R∗ we find that the systematic uncertainty due to detrending can be three times that of the quoted statistical uncertainties. The choice of mass-radius relation has little effect on our inferred values of the system parameters. The choice of Rossiter-McLaughlin models can have significant effects of the inferred values of vsinI∗ and the stellar spin-planet orbit angle λ.

HATS-15b & HATS-16b: Two hot Jupiters Transiting old G dwarf stars

HATS-15 b and HATS-16 b: Two massive planets transiting old G dwarf stars

Authors:

Ciceri et al

Abstract:

We report the discovery of HATS-15 b and HATS-16 b, two massive transiting extrasolar planets orbiting evolved (∼10 Gyr) main-sequence stars. The planet HATS-15 b, which is hosted by a G9V star (V=14.8 mag), is a hot Jupiter with mass of 2.17±0.15MJ and radius of 1.105±0.0.040RJ, and completes its orbit in nearly 1.7 days. HATS-16 b is a very massive hot Jupiter with mass of 3.27±0.19MJ and radius of 1.30±0.15RJ; it orbits around its G3 V parent star (V=13.8 mag) in ∼2.7 days. HATS-16 is slightly active and shows a periodic photometric modulation, implying a rotational period of 12 days which is unexpectedly short given its isochronal age. This fast rotation might be the result of the tidal interaction between the star and its planet.

Evidence for Reflected Light from Gas Giant HD 20872b, the Most Eccentric Exoplanet Known

Evidence for Reflected Light from the Most Eccentric Exoplanet Known

Authors:

Kane et al

Abstract:

Planets in highly eccentric orbits form a class of objects not seen within our Solar System. The most extreme case known amongst these objects is the planet orbiting HD 20782, with an orbital period of 597 days and an eccentricity of 0.96. Here we present new data and analysis for this system as part of the Transit Ephemeris Refinement and Monitoring Survey (TERMS). We obtained CHIRON spectra to perform an independent estimation of the fundamental stellar parameters. New radial velocities from AAT and PARAS observations during periastron passage greatly improve the our knowledge of the eccentric nature of the orbit. The combined analysis of our Keplerian orbital and Hipparcos astrometry show that the inclination of the planetary orbit is greater than 1.25 degrees, ruling out stellar masses for the companion. Our long-term robotic photometry show that the star is extremely stable over long timescales. Photometric monitoring of the star during predicted transit and periastron times using MOST rule out a transit of the planet and reveal evidence of phase variations during periastron. These possible photometric phase variations are likely caused by reflected light from the planet's atmosphere and the dramatic change in star--planet separation surrounding the periastron passage.

Nemesis Class Gas Giant HD 106906b is Warping the System's Debris Disk

Direct imaging of an asymmetric debris disk in the HD 106906 planetary system

Authors:

Kalas et al

Abstract:

We present the first scattered light detections of the HD 106906 debris disk using Gemini/GPI in the infrared and HST/ACS in the optical. HD 106906 is a 13 Myr old F5V star in the Sco-Cen association, with a previously detected planet-mass candidate HD 106906b projected 650 AU from the host star. Our observations reveal a near edge-on debris disk that has a central cleared region with radius ∼50 AU, and an outer extent >500 AU. The HST data show the outer regions are highly asymmetric, resembling the ''needle'' morphology seen for the HD 15115 debris disk. The planet candidate is oriented ∼21deg away from the position angle of the primary's debris disk, strongly suggesting non-coplanarity with the system. We hypothesize that HD 106906b could be dynamically involved in the perturbation of the primary's disk, and investigate whether or not there is evidence for a circumplanetary dust disk or cloud that is either primordial or captured from the primary. We show that both the existing optical properties and near-infrared colors of HD 106906b are weakly consistent with this possibility, motivating future work to test for the observational signatures of dust surrounding the planet.

Gas Giants HD 47366b & HD 47366c Have Potentially Weird Orbits Around Their K Giant Star

A Pair of Giant Planets around the Evolved Intermediate-Mass Star HD 47366: Multiple Circular Orbits or a Mutually Retrograde Configuration

Authors:

Sato et al

Abstract:

We report the detection of a double planetary system around the evolved intermediate-mass star HD 47366 from precise radial-velocity measurements at Okayama Astrophysical Observatory, Xinglong Station, and Australian Astronomical Observatory. The star is a K1 giant with a mass of 1.81+-0.13M_sun, a radius of 7.30+-0.33R_sun, and solar metallicity. The planetary system is composed of two giant planets with minimum mass of 1.75^{+0.20}_{-0.17}Mjup and 1.86^{+0.16}_{-0.15}Mjup, orbital period of 363.3^{+2.5}_{-2.4} d and 684.7^{+5.0}_{-4.9} d, and eccentricity of 0.089^{+0.079}_{-0.060} and 0.278^{+0.067}_{-0.094}, respectively, which are derived by a double Keplerian orbital fit to the radial-velocity data. The system adds to the population of multi-giant-planet systems with relatively small orbital separations, which are preferentially found around evolved intermediate-mass stars. Dynamical stability analysis for the system revealed, however, that the best-fit orbits are unstable in the case of a prograde configuration. The system could be stable if the planets were in 2:1 mean-motion resonance, but this is less likely considering the observed period ratio and eccentricity. A present possible scenario for the system is that both of the planets have nearly circular orbits, namely the eccentricity of the outer planet is less than ~0.15, which is just within 1.4sigma of the best-fit value, or the planets are in a mutually retrograde configuration with a mutual orbital inclination larger than 160 degree.

Geodynamics of Mars-sized Terrestrial Planets

The early geodynamic evolution of Mars-type planets

Authors:

Zhang et al

Abstract:

It is not clear whether Mars once possessed active tectonics, yet the question is critical for understanding the thermal evolution of Mars, and the origin and longevity of its early dynamo. To address these issues, we have coupled mantle flow simulations, together with parameterized core evolution models, to simulate the early evolution of Mars-like planets, and constrain the influence of early mobile-lid tectonics on core evolution. We have explored a wide parameter suite, encapsulating a range of uncertainties in initial conditions, rheological parameters, and surface strength. We present successful models that experience early mobile-lid behaviour, with a later transition into a stagnant-lid mode, which reproduce core dynamo histories similar to the magnetic history of early Mars.

How Kepler-36 Formed SuperEarth Kepler-36b & Mini Neptune Kepler-36c

The initial physical conditions of Kepler-36 b & c

Authors:

Owen et al

Abstract:

The Kepler-36 planetary system consists of two exoplanets at similar separations (0.115 & 0.128 AU), which have dramatically different densities. The inner planet has a density consistent with an Earth-like composition, while the outer planet is extremely low-density, such that it must contain a voluminous H/He envelope. Such a density difference would pose a problem for any formation mechanism if their current densities were representative of their composition at formation. However, both planets are at close enough separations to have undergone significant evaporation in the past. We constrain the core mass, core composition, initial envelope mass, and initial cooling time of each planet using evaporation models conditioned on their present-day masses and radii, as inferred from Kepler photometry and transit timing analysis. The inner planet is consistent with being an evaporatively stripped core, while the outer planet has retained some of its initial envelope due to its higher core mass. Therefore, both planets could have had a similar formation pathway, with the inner planet having an initial envelope mass fraction of ≲10% and core mass of ∼4.4 M⊕, while the outer had an initial envelope mass fraction of order 15−30% and core mass ∼7.3 M⊕. Finally, our results indicate that the outer planet had a long (≳30 Myr) initial cooling time, much longer than would naively be predicted from simple time-scale arguments. The long initial cooling time could be evidence for a dramatic early cooling episode such as the recently proposed "boil-off" process.

Exoplanetary Systems Without Gas Giants Will be More Water Rich

Fossilized condensation lines in the Solar System protoplanetary disk

Authors:

Morbidelli et al

Abstract:

The terrestrial planets and the asteroids dominant in the inner asteroid belt are water poor. However, in the protoplanetary disk the temperature should have decreased below water condensation level well before the disk was photoevaporated. Thus, the global water depletion of the inner Solar System is puzling. We show that, even if the inner disk becomes cold, there cannot be direct condensation of water. This is because the snowline moves towards the Sun more slowly than the gas itself. The appearance of ice in a range of heliocentric distances swept by the snowline can only be due to the radial drift of icy particles from the outer disk. However, if a sufficiently massive planet is present, the radial drift of particles is interrupted, because the disk acquires a superKeplerian rotation just outside of the planetary orbit. From this result, we propose that the precursor of Jupiter achieved about 20 Earth masses when the snowline was still around 3 AU. This effectively fossilized the snowline at that location. Although cooling, the disk inside of the Jovian orbit remained ice-depleted because the flow of icy particles from the outer system was intercepted by the planet. This scenario predicts that planetary systems without giant planets should be much more rich in water in their inner regions than our system. We also show that the inner edge of the planetesimal disk at 0.7AU, required in terrestrial planet formation models to explain the small mass of Mercury and the absence of planets inside of its orbit, could be due to the silicate condensation line, fossilized at the end of the phase of streaming instability that generated the planetesimal seeds. Thus, when the disk cooled, silicate particles started to drift inwards of 0.7AU without being sublimated, but they could not be accreted by any pre-existing planetesimals.

Way Premature: How Changing Fundamental Constants in Alternate Universes Effect Habitability

Constraints on Alternate Universes: Stars and habitable planets with different fundamental constants

Author:

Adams

Abstract:

This paper develops constraints on the values of the fundamental constants that allow universes to be habitable. We focus on the fine structure constant α and the gravitational structure constant αG, and find the region in the α-αG plane that supports working stars and habitable planets. This work is motivated, in part, by the possibility that different versions of the laws of physics could be realized within other universes. The following constraints are enforced: [A] long-lived stable nuclear burning stars exist, [B] planetary surface temperatures are hot enough to support chemical reactions, [C] stellar lifetimes are long enough to allow biological evolution, [D] planets are massive enough to maintain atmospheres, [E] planets are small enough in mass to remain non-degenerate, [F] planets are massive enough to support sufficiently complex biospheres, [G] planets are smaller in mass than their host stars, and [H] stars are smaller in mass than their host galaxies. This paper delineates the portion of the α-αG plane that satisfies all of these constraints. The results indicate that viable universes --- with working stars and habitable planets --- can exist within a parameter space where the structure constants α and αG vary by several orders of magnitude. These constraints also provide upper bounds on the structure constants (α,αG) and their ratio. We find the limit αG/α less than 10^−34, which shows that habitable universes must have a large hierarchy between the strengths of the gravitational force and the electromagnetic force.

Exploring the Inner Edge of the Habitable Zone with Fully Coupled Oceans

Exploring the Inner Edge of the Habitable Zone with Fully Coupled Oceans

Authors:

Way et al

Abstract:

Rotation in planetary atmospheres plays an important role in regulating atmospheric and oceanic heat flow, cloud formation and precipitation. Using the Goddard Institute for Space Studies (GISS) three dimension General Circulation Model (3D-GCM) we investigate how the effects of varying rotation rate and increasing the incident stellar flux on a planet set bounds on a planet's habitable zone with its parent star. From ensemble climate simulations we identify which factors are the primary controllers of uncertainty in setting these bounds. This is shown in particular for fully coupled ocean (FCO) runs -- some of the first that have been utilized in this context. Results with a Slab Ocean (SO) of 100m mixed layer depth are compared with a similar study by Yang et al. 2014, which demonstrates consistency across models. However, there are clear differences for rotations rates of 1-16x present Earth sidereal day lengths between the 100m SO and FCO models, which points to the necessity of using FCOs whenever possible. The latter was recently demonstrated quite clearly by Hu & Yang 2014 in their aquaworld study with a FCO when compared with similar mixed layer ocean studies and by Cullum et al. 2014.

We also show how these results have implications for Venus in the early history of our Solar System since even at this time Venus received more solar flux than Earth does today while it may still have had a slow retrograde rotation. The Venus runs utilize a 2.9Gya solar spectrum generated with the code of Claire et al. 2012, a modern Venus topography with an ocean filling the lowlands (giving an equivalent depth of 310 meters if spread across the entire surface), atmosphere of 1 bar N2, CO2=0.4mb, CH4=0.001mb and present day orbital parameters, radius, & gravity. We demonstrate that ancient Venus could have had quite moderate surface temperatures given these assumptions.

The Jupiter's Driving Role in Earth's Orbital Evolution

The role of Jupiter in driving Earth's orbital evolution: an update

Authors:

Horner et al

Abstract:

In the coming decades, the discovery of the first truly Earth-like exoplanets is anticipated. The characterisation of those planets will play a vital role in determining which are chosen as targets for the search for life beyond the Solar system. One of the many variables that will be considered in that characterisation and selection process is the nature of the potential climatic variability of the exoEarths in question.

In our own Solar system, the Earth's long-term climate is driven by several factors - including the modifying influence of life on our atmosphere, and the temporal evolution of Solar luminosity. The gravitational influence of the other planets in our Solar system add an extra complication - driving the Milankovitch cycles that are thought to have caused the on-going series of glacial and interglacial periods that have dominated Earth's climate for the past few million years.

Here, we present the results of a large suite of dynamical simulations that investigate the influence of the giant planet Jupiter on the Earth's Milankovitch cycles. If Jupiter was located on a different orbit, we find that the long-term variability of Earth's orbit would be significantly different. Our results illustrate how small differences in the architecture of planetary systems can result in marked changes in the potential habitability of the planets therein, and are an important first step in developing a means to characterise the nature of climate variability on planets beyond our Solar system.

How do Monster-sized 100+ Solar Mass Stars Form?

Supermassive star formation via episodic accretion: protostellar disc instability and radiative feedback efficiency

Authors:

Sakurai et al

Abstract:

The formation of SMSs is a potential pathway to seed SMBHs in the early universe. A critical issue for forming SMSs is stellar UV feedback, which may limit the stellar mass growth via accretion. In this paper we study the evolution of an accreting SMS and its UV emissivity under conditions of realistic variable accretion from a self-gravitating circumstellar disc. First we conduct a 2D hydrodynamical simulation to follow the long-term protostellar accretion until the stellar mass exceeds 104 M⊙. The disc fragments due to gravitational instability, creating a number of small clumps that rapidly migrate inward to fall onto the star. The resulting accretion history is thus highly time-dependent: short episodic accretion bursts are followed by longer, relative quiescent phases. We show that the circumstellar disc for the so-called direct collapse model is more unstable and generates greater variability over shorter timescales than normal Pop III cases. We conduct a post-process stellar evolution calculation using the obtained accretion history. Our results show that, regardless of the strong variability of the accretion rates, the stellar radius monotonically increases with almost constant effective temperature at Teff≃5000 K as the stellar mass increases. The resulting UV feedback is too weak to hinder mass accretion due to the low flux of stellar UV photons, thus verifying our implicit assumption of no stellar feedback during the hydrodynamic simulations. The insensitivity of stellar evolution to variable accretion is attributed to the fact that typical timescales of variability, ≲103 years, are too short to affect the stellar structure. We argue that this evolution will continue until the SMS eventually collapses to produce a massive black hole by the general relativistic instability after the stellar mass reaches ≳105 M⊙.

How Common are Circumstellar Disks Around Solitary Stars in the Chamaeleon I Region?

The frequency of accretion disks around single stars: Chamaeleon I

Authors:

Daemgen et al

Abstract:

It is well known that stellar companions can influence the evolution of a protoplanetary disk. Nevertheless, previous disk surveys did not - and could not - consistently exclude binaries from their samples. We present a study dedicated to investigating the frequency of ongoing disk accretion around single stars in a star-forming region. We obtained near-infrared spectroscopy of 54 low-mass stars selected from a high-angular resolution survey in the 2-3 Myr-old Chamaeleon I region to determine the presence of Brackett-γ emission, taking the residual chance of undetected multiplicity into account, which we estimate to be on the order of 30%. The result is compared with previous surveys of the same feature in binary stars of the same region to provide a robust estimate of the difference between the accretor fractions of single stars and individual components of binary systems. We find Brγ emission among 39.5+14.0−9.9% of single stars, which is a significantly higher fraction than for binary stars in Chamaeleon I. In particular, close binary systems with separations less than 100 AU show emission in only 6.5+16.5−3.0% of the cases according to the same analysis. The emitter frequency of wider binaries appears consistent with the single star value. Interpreting Brγ emission as a sign of ongoing accretion and correcting for sensitivity bias, we infer an accretor fraction of single stars of F_acc=47.8+14.0−9.9%. This is slightly higher but consistent with previous estimates that do not clearly exclude binaries from their samples. Through our robust and consistent analysis, we confirm that the fraction of young single stars harboring accretion disks is much larger than that of close binaries at the same age. Our findings have important implications for the timescales of disk evolution and planet formation.

GG Tauri A triple System: A System With a Circumtrinary (?) Protoplanetary Disk Around Stars With Disks

CO Gas orbiting around the GG Tauri A triple System: rings beyond the ring ?

Authors:

Tang et al

Abstract:

We aim at unveiling the observational imprint of physical mechanisms that govern planetary formation in the young, multiple system GG Tau A. We present ALMA observations of 12CO and 13CO 3-2 and continuum at 0.9 mm at 0.35" resolution. The 12CO gas, found in the cavity of the dust ring where no 13CO gas is detected, confirms the existence of a CO accretion shock near the circumstellar disk of GG Tau Aa. The outer disk and the hot spot lying at the outer edge of the dust ring are observed both in 12CO and 13CO. The gas emission in the outer disk can be radially decomposed in a series of slightly overlapping gaussian rings, suggesting the presence of unresolved gaps. The dip closest to the disk center lies at a radius very close to the CO hot spot location (∼250−260~au). Studies of the CO excitation conditions reveal that the outer disk remains in the shadow of the ring. The hot spot probably results from local heating processes. The two latter points strongly support the hypothesis making the hot spot an embedded proto-planet shepherding the outer disk and accreting surrounding material which may be traced by the the redshifted component observed in the spectra around the hot spot.

The Differences Between the Water, Carbon monoxide and Carbon dioxide Snowlines in Protoplanetary Disks

C/O and Snowline Locations in Protoplanetary Disks: The Effect of Radial Drift and Viscous Gas Accretion

Authors:

Piso et al

Abstract:

The C/O ratio is a defining feature of both gas giant atmospheric and protoplanetary disk chemistry. In disks, the C/O ratio is regulated by the presence of snowlines of major volatiles at different distances from the central star. We explore the effect of radial drift of solids and viscous gas accretion onto the central star on the snowline locations of the main C and O carriers in a protoplanetary disk, H2O, CO2 and CO, and their consequences for the C/O ratio in gas and dust throughout the disk. We determine the snowline locations for a range of fixed initial particle sizes and disk types. For our fiducial disk model, we find that grains with sizes ~0.5 cm less than s less than 7 m for an irradiated disk, and ~0.001 cm less than s less than 7 m for an evolving and viscous disk, desorb at a size-dependent location in the disk, which is independent of the particle's initial position. The snowline radius decreases for larger particles, up to sizes of ~7 m. Compared to a static disk, we find that radial drift and gas accretion in a viscous disk move the H2O snowline inwards by up to 40%, the CO2 snowline by up to 60%, and the CO snowline by up to 50%. We thus determine an inner limit on the snowline locations when radial drift and gas accretion are accounted for.

Nearby Lambda Boo Stars Have Debris Disks

IR-excesses around nearby Lambda Boo stars are caused by debris disks rather than ISM bow waves

Authors:

Draper et al

Abstract:

Lambda Boo stars are predominately A-type stars with solar abundant C, N, O, and S, but up to 2 dex underabundances of refractory elements. The stars' unusual surface abundances could be due to a selective accretion of volatile gas over dust. It has been proposed that there is a correlation between the Lambda Boo phenomenon and IR-excesses which are the result of a debris disk or interstellar medium (ISM) interaction providing the accreting material. We observe 70 or 100 and 160 μm excess emission around 9 confirmed Lambda Boo stars with the Herschel Space Observatory, to differentiate whether the dust emission is from a debris disk or an ISM bow wave. We find that 3/9 stars observed host well resolved debris disks. While the remaining 6/9 are not resolved, they are inconsistent with an ISM bow wave based on the dust emission being more compact for its temperature and predicted bow wave models produce hotter emission than what is observed. We find the incidence of bright IR-excesses around Lambda Boo stars is higher than normal A-stars. To explain this given our observations, we explore Poynting-Robertson (PR) drag as a mechanism of accretion from a debris disk but find it insufficient. As an alternative, we propose the correlation is due to higher dynamical activity in the disks currently underway. Large impacts of planetesimals or a higher influx of comets could provide enough volatile gas for accretion. Further study on the transport of circumstellar material in relation to the abundance anomalies are required to explain the phenomenon through external accretion.

Models of the Eta Corvi Debris Disk

Models of the Eta Corvi debris disk from the Keck Interferometer, Spitzer and Herschel

Authors:

Lebreton et al

Abstract:

Debris disks are signposts of analogues to small body populations of the Solar System, often however with much higher masses and dust production rates. The disk associated with the nearby star Eta Corvi is especially striking as it shows strong mid- and far-infrared excesses despite an age of ~1.4 Gyr. We undertake to construct a consistent model of the system able to explain a diverse collection of spatial and spectral data. We analyze Keck Interferometer Nuller measurements and revisit Spitzer and additional spectro-photometric data, as well as resolved Herschel images to determine the dust spatial distribution in the inner exozodi and in the outer belt. We model in detail the two-component disk and the dust properties from the sub-AU scale to the outermost regions by fitting simultaneously all measurements against a large parameter space. The properties of the cold belt are consistent with a collisional cascade in a reservoir of ice-free planetesimals at 133 AU. It shows marginal evidence for asymmetries along the major axis. KIN enables us to establish that the warm dust consists in a ring that peaks between 0.2 and 0.8 AU. To reconcile this location with the ~400 K dust temperature, very high albedo dust must be invoked and a distribution of forsterite grains starting from micron sizes satisfies this criterion while providing an excellent fit to the spectrum. We discuss additional constraints from the LBTI and near-infrared spectra, and we present predictions of what JWST can unveil about this unusual object and whether it can detect unseen planets.

Spin Casting a Segment for the Giant Magellan Telescope

VHS 1256-1257 is the Only Known Trinary Brown Dwarf System

Adaptive Optics imaging of VHS 1256-1257: A Low Mass Companion to a Brown Dwarf Binary System

Authors:

Stone et al

Abstract:

Recently, Gauza et al. (2015) reported the discovery of a companion to the late M-dwarf, VHS J125601.92-125723.9 (VHS 1256-1257). The companion's absolute photometry suggests its mass and atmosphere are similar to the HR 8799 planets. However, as a wide companion to a late-type star, it is more accessible to spectroscopic characterization. We discovered that the primary of this system is an equal-magnitude binary. For an age ∼300 Myr the A and B components each have a mass of 64.6+0.8−2.0 MJup, and the b component has a mass of 11.2+9.7−1.8, making VHS 1256-1257 only the third brown dwarf triple system. There exists some tension between the spectrophotometric distance of 17.2±2.6 pc and the parallax distance of 12.7±1.0 pc. At 12.7 pc VHS1256-1257 A and B would be the faintest known M7.5 objects, and are even faint outliers among M8 types. If the larger spectrophotmetric distance is more accurate than the parallax, then the mass of each component increases. In particular, the mass of the b component increases well above the deuterium burning limit to ∼35 MJup and the mass of each binary component increases to 73+20−17 MJup. At 17.1 pc, the UVW kinematics of the system are consistent with membership in the AB~Dor moving group. The architecture of the system resembles a hierarchical stellar multiple suggesting it formed via an extension of the star-formation process to low masses. Continued astrometric monitoring will resolve this distance uncertainty and will provide dynamical masses for a new benchmark system.

Using PAN-STARRS1 and WISE to Hunt for Brown Dwarfs at the L/T Transition

A SEARCH FOR L/T TRANSITION DWARFS WITH PAN-STARRS1 AND WISE. II. L/T TRANSITION ATMOSPHERES AND YOUNG DISCOVERIES

Authors:

Best et al

Abstract:

The evolution of brown dwarfs from L to T spectral types is one of the least understood aspects of the ultracool population, partly for lack of a large, well-defined, and well-characterized sample in the L/T transition. To improve the existing census, we have searched ≈28,000 deg2 using the Pan-STARRS1 and Wide-field Infrared Survey Explorer surveys for L/T transition dwarfs within 25 pc. We present 130 ultracool dwarf discoveries with estimated distances ≈9–130 pc, including 21 that were independently discovered by other authors and 3 that were previously identified as photometric candidates. Seventy-nine of our objects have near-IR spectral types of L6–T4.5, the most L/T transition dwarfs from any search to date, and we have increased the census of L9–T1.5 objects within 25 pc by over 50%. The color distribution of our discoveries provides further evidence for the "L/T gap," a deficit of objects with (J − K)MKO ≈ 0.0–0.5 mag in the L/T transition, and thus reinforces the idea that the transition from cloudy to clear photospheres occurs rapidly. Among our discoveries are 31 candidate binaries based on their low-resolution spectral features. Two of these candidates are common proper motion companions to nearby main sequence stars; if confirmed as binaries, these would be rare benchmark systems with the potential to stringently test ultracool evolutionary models. Our search also serendipitously identified 23 late-M and L dwarfs with spectroscopic signs of low gravity implying youth, including 10 with vl-g or int-g gravity classifications and another 13 with indications of low gravity whose spectral types or modest spectral signal-to-noise ratio do not allow us to assign formal classifications. Finally, we identify 10 candidate members of nearby young moving groups (YMG) with spectral types L7–T4.5, including three showing spectroscopic signs of low gravity. If confirmed, any of these would be among the coolest known YMG members and would help to determine the effective temperature at which young brown dwarfs cross the L/T transition.

UScoJ155150.2−213457: A new Free-floating ExoPlanet (Brown Dwarf?) in the Upper Scorpius association

A new free-floating planet in the Upper Scorpius association

Authors:

Ramírez et al

Abstract:

We report on a deep photometric survey covering an area of 1.17 deg2 in the young Upper Scorpius stellar association using VIMOS Iz and UKIDSS ZJHK data taking several years apart. The search for the least massive population of Upper Scorpius (∼5-10 Myr, 145 pc) is performed on the basis of various optical and infrared color-color and color-magnitude diagrams, including WISE photometry, in the magnitude interval J=14.5-19 mag (completeness), which corresponds to substellar masses from 0.028 through 0.004 M⊙ at the age and distance of Upper Scorpius. We also present the proper motion analysis of the photometric candidates, finding that two objects successfully pass all photometric and astrometric criteria for membership in the young stellar association. One of them, UScoJ155150.2−213457, is a new discovery. We obtained low resolution, near-infrared spectroscopy (R∼450, 0.85--2.35 μm) of this new finding using the FIRE instrument. We confirmed its low-gravity atmosphere expected for an Upper Scorpius member (weak alkaline lines, strong VO absorption, peaked H-band pseudocontinuum). By comparison with spectroscopic standards, we derive a spectral type of L6±1, and estimate a mass of ≈0.008-0.010 M⊙ for UScoJ155150.2−213457. The colors and spectral slope of this object resemble those of other young, cool members of Upper Scorpius and σ Orionis (∼3 Myr) and field, high gravity dwarfs of related classification in contrast with the very red indices of field, low gravity, L-type dwarfs of intermediate age. UScoJ155150.2−213457, which does not show infrared flux excesses up to 4.5 μm, becomes one of the least massive and latest type objects known in the entire Upper Scorpius stellar association.

WASP-135b: an Inflated hot Jupiter in a 1.4 day Orbit Around a G5 Dwarf Star

WASP-135b: a highly irradiated, inflated hot Jupiter orbiting a G5V star

Authors:

Spake et al

Abstract:

We report the discovery of a new transiting planet from the WASP survey. WASP-135b is a hot Jupiter with a radius of 1.30 pm 0.09 Rjup, a mass of 1.90 pm 0.08 Mjup and an orbital period of 1.401 days. Its host is a Sun-like star, with a G5 spectral type and a mass and radius of 0.98 pm 0.06 Msun and 0.96 pm 0.05 Rsun respectively. The proximity of the planet to its host means that WASP-135b receives high levels of insolation, which may be the cause of its inflated radius. Additionally, we find weak evidence of a transfer of angular momentum from the planet to its star.

KIC 8462852's Circumstellar Dust Matches the Recent Cometary Breakup Scenario

Constraints on the circumstellar dust around KIC 8462852

Authors:

Thompson

Abstract:

We present millimetre (SMA) and sub-millimetre (SCUBA-2) continuum observations of the peculiar star KIC 8462852 which displayed several deep and aperiodic dips in brightness during the Kepler mission. Our observations are approximately confusion-limited at 850 μm and are the deepest millimetre and sub-millimetre photometry of the star that has yet been carried out. No significant emission is detected towards KIC 8462852. We determine upper limits for dust between a few 10−6 M⊕ and 10−3 M⊕ for regions identified as the most likely to host occluding dust clumps and a total overall dust budget of less than 7.7 M⊕ within a radius of 200 AU. Such low limits for the inner system make the catastrophic planetary disruption hypothesis unlikely. Integrating over the Kepler lightcurve we determine that at least 10−9 M⊕ of dust is required to cause the observed Q16 dip. This is consistent with the currently most favoured cometary breakup hypothesis, but nevertheless implies the complete breakup of ∼ 30 Comet 1/P Halley type objects. Finally, in the wide SCUBA-2 field-of-view we identify another candidate debris disc system that is potentially the largest yet discovered.

KIC 8462852 Needs 648,000 Two Hundred km Comets to Fade as Much as Observed From 1890 to 1989

KIC 8462852 Faded at an Average Rate of 0.165+-0.013 Magnitudes Per Century From 1890 To 1989

Author:

Schafer

Abstract:

The star KIC 8462852 is a completely-ordinary F3 main sequence star, except that the light curve from the Kepler spacecraft shows episodes of unique and inexplicable day-long dips with up to 20% dimming. Here, I provide a light curve of 1232 Johnson B-band magnitudes from 1890 to 1989 taken from archival photographic plates at Harvard. KIC 8462852 displays a highly significant and highly confident secular dimming at an average rate of 0.165+-0.013 magnitudes per century. From the early 1890s to the late 1980s, KIC 8462852 has faded by 0.193+-0.030 mag. This century-long dimming is completely unprecedented for any F-type main sequence star. So the Harvard light curve provides the first confirmation (past the several dips seen in the Kepler light curve alone) that KIC 8462852 has anything unusual going on. The century-long dimming and the day-long dips are both just extreme ends of a spectrum of timescales for unique dimming events, so by Ockham's Razor, all this is produced by one physical mechanism. This one mechanism does not appear as any isolated catastrophic event in the last century, but rather must be some ongoing process with continuous effects. Within the context of dust-occultation models, the century-long dimming trend requires 10^4 to 10^7 times as much dust as for the one deepest Kepler dip. Within the context of the comet-family idea, the century-long dimming trend requires an estimated 648,000 giant comets (each with 200 km diameter) all orchestrated to pass in front of the star within the last century.

The Mass-Metallicity Relation for Giant Planets

The Mass-Metallicity Relation for Giant Planets

Authors:

Thorngren et al

Abstract:

Exoplanet discoveries of recent years have provided a great deal of new data for studying the bulk compositions of giant planets. Here we identify 38 transiting giant planets (20M⊕less than M less than 20MJ) whose stellar insolation is low enough (F∗ less than 2×108ergs−1cm−2, or roughly Teff less than 1000) that they are not affected by the hot Jupiter radius inflation mechanism(s). We compute a set of new thermal and structural evolution models and use these models in comparison with properties of the 38 transiting planets (mass, radius, age) to determine their heavy element masses. A clear correlation emerges between the planetary heavy element mass Mz and the total planet mass, approximately of the form Mz∝M‾‾√. This finding is consistent with the core accretion model of planet formation. We also study how stellar metallicity [Fe/H] affects planetary metal-enrichment and find a weaker correlation than has been previously reported from studies with smaller sample sizes. Our results suggest that planets with large heavy element masses are more common around stars with a high iron abundance, but are not found there exclusively. We confirm a strong relationship between the planetary metal-enrichment relative to the parent star Zplanet/Zstar and the planetary mass, but see no relation in Zplanet/Zstar with planet orbital properties or stellar mass. Suggestively, circumbinary planets are more enriched in heavy elements than similar mass single-star planets, but with only four such planets the effect is not yet significant. The large heavy element masses of many planets (greater than 50M⊕) suggest significant amounts of heavy elements in H/He envelopes, rather than cores, such that metal-enriched giant planet atmospheres should be the rule.

Rayleigh Scattering Detected in Warm Neptune GJ 3470b's Atmosphere

Rayleigh Scattering in the Atmosphere of the Warm Exo-Neptune GJ 3470b

Authors:

Dragomir et al

Abstract:

GJ 3470b is a warm Neptune-size planet transiting a M dwarf star. Like the handful of other small exoplanets for which transmission spectroscopy has been obtained, GJ 3470b exhibits a flat spectrum in the near- and mid-infrared. Recently, a tentative detection of Rayleigh scattering in its atmosphere has been reported. This signal manifests itself as an observed increase of the planetary radius as a function of decreasing wavelength in the visible. We set out to verify this detection and observed several transits of this planet with the LCOGT network and the Kuiper telescope in four different bands (Sloan g', Sloan i', Harris B and Harris V). Our analysis reveals a strong Rayleigh scattering slope, thus confirming previous results. This makes GJ 3470b the smallest known exoplanet with a detection of Rayleigh scattering. We find that the most plausible scenario is a hydrogen/helium-dominated atmosphere covered by clouds which obscure absorption features in the infrared and hazes which give rise to scattering in the visible. Our results demonstrate the feasibility of exoplanet atmospheric characterization from the ground, even with meter-class telescopes.