Friday, November 17, 2017

Kronos & Krios: Evidence for accretion of a massive, rocky planetary system in a comoving pair of solar-type stars

Kronos & Krios: Evidence for accretion of a massive, rocky planetary system in a comoving pair of solar-type stars 

Authors:


Oh et al

Abstract: 
We report and discuss the discovery of a comoving pair of bright solar-type stars, HD 240430 and HD 240429, with a significant difference in their chemical abundances. The two stars have an estimated 3D separation of ≈0.6 pc (≈0.01 pc projected) at a distance of r≈100 pc with nearly identical three-dimensional velocities, as inferred from Gaia TGAS parallaxes and proper motions, and high-precision radial velocity measurements. Stellar parameters determined from high-resolution Keck HIRES spectra indicate that both stars are ∼4 Gyr old. The more metal-rich of the two, HD 240430, shows an enhancement of refractory (TC greater than 1200 K) elements by ≈0.2 dex and a marginal enhancement of (moderately) volatile elements (TC less than 1200 K, C, N, O, Na, and Mn). This is the largest metallicity difference found in a wide binary pair yet. Additionally, HD 240430 shows an anomalously high surface lithium abundance (A(Li)=2.75), higher than its companion by 0.5 dex. The proximity in phase-space and ages between the two stars suggests that they formed together with the same composition, at odds with the observed differences in metallicity and abundance patterns. We therefore suggest that the star HD~240430, "Kronos", accreted 15 M⊕ of rocky material after birth, selectively enhancing the refractory elements as well as lithium in its surface and convective envelope.

The Effect of Atmospheric Cooling on Vertical Velocity Dispersion and Density Distribution of Brown Dwarfs

The Effect of Atmospheric Cooling on Vertical Velocity Dispersion and Density Distribution of Brown Dwarfs

Authors:


Ryan et al

Abstract:
We present a Monte Carlo simulation designed to predict the vertical velocity dispersion of brown dwarfs in the Milky Way. We show that since these stars are constantly cooling, the velocity dispersion has a noticeable trend with the spectral type. With realistic assumptions for the initial mass function, star formation history, and the cooling models, we show that the velocity dispersion is roughly consistent with what is observed for M dwarfs, decreases to cooler spectral types, and increases again for the coolest types in our study (~T9). We predict a minimum in the velocity dispersions for L/T transition objects, however, the detailed properties of the minimum predominately depend on the star formation history. Since this trend is due to brown dwarf cooling, we expect that the velocity dispersion as a function of spectral type should deviate from the constancy around the hydrogen-burning limit. We convert from velocity dispersion to vertical scale height using standard disk models and present similar trends in disk thickness as a function of spectral type. We suggest that future, wide-field photometric and/or spectroscopic missions may collect sizable samples of distant ($\sim 1$ kpc) dwarfs that span the hydrogen-burning limit. As such, we speculate that such observations may provide a unique way of constraining the average spectral type of hydrogen burning.

The Young Substellar Companion ROXs 12 B: Near-infrared Spectrum, System Architecture, and Spin–Orbit Misalignment

The Young Substellar Companion ROXs 12 B: Near-infrared Spectrum, System Architecture, and Spin–Orbit Misalignment

Authors:


Bowler et al

Abstract:

ROXs 12 (2MASS J16262803–2526477) is a young star hosting a directly imaged companion near the deuterium-burning limit. We present a suite of spectroscopic, imaging, and time-series observations to characterize the physical and environmental properties of this system. Moderate-resolution near-infrared spectroscopy of ROXs 12 B from Gemini-North/NIFS and Keck/OSIRIS reveals signatures of low surface gravity including weak alkali absorption lines and a triangular H-band pseudocontinuum shape. No signs of Paβ emission are evident. As a population, however, we find that about half (46% ± 14%) of young (lesssim15 Myr) companions with masses lesssim20 M Jup possess actively accreting subdisks detected via Paβ line emission, which represents a lower limit on the prevalence of circumplanetary disks in general, as some are expected to be in a quiescent phase of accretion. The bolometric luminosity of the companion and age of the host star (${6}_{-2}^{+4}$ Myr) imply a mass of 17.5 ± 1.5 M Jup for ROXs 12 B based on hot-start evolutionary models. We identify a wide (5100 au) tertiary companion to this system, 2MASS J16262774–2527247, that is heavily accreting and exhibits stochastic variability in its K2 light curve. By combining v sin i * measurements with rotation periods from K2, we constrain the line-of-sight inclinations of ROXs 12 A and 2MASS J16262774–2527247 and find that they are misaligned by ${{60}_{-11}^{+7}}^\circ $. In addition, the orbital axis of ROXs 12 B is likely misaligned from the spin axis of its host star, ROXs 12 A, suggesting that ROXs 12 B formed akin to fragmenting binary stars or in an equatorial disk that was torqued by the wide stellar tertiary.

Thursday, November 16, 2017

Evidence for Atmospheric Cold-trap Processes in the Noninverted Emission Spectrum of Kepler-13Ab Using HST/WFC3

Evidence for Atmospheric Cold-trap Processes in the Noninverted Emission Spectrum of Kepler-13Ab Using HST/WFC3 

Authors: 
Beatty et al 
Abstract: 
We observed two eclipses of the Kepler-13A planetary system, on UT 2014 April 28 and UT 2014 October 13, in the near-infrared using Wide Field Camera 3 on the Hubble Space Telescope. By using the nearby binary stars Kepler-13BC as a reference, we were able to create a differential light curve for Kepler-13A that had little of the systematics typically present in HST/WFC3 spectrophotometry. We measure a broadband (1.1–1.65 μm) eclipse depth of 734 ± 28 ppm and are able to measure the emission spectrum of the planet at R ≈ 50 with an average precision of 70 ppm. We find that Kepler-13Ab possesses a noninverted, monotonically decreasing vertical temperature profile. We exclude an isothermal profile and an inverted profile at more than 3σ. We also find that the dayside emission of Kepler-13Ab appears generally similar to an isolated M7 brown dwarf at a similar effective temperature. Due to the relatively high mass and surface gravity of Kepler-13Ab, we suggest that the apparent lack of an inversion is due to cold-trap processes in the planet's atmosphere. Using a toy model for where cold traps should inhibit inversions, as well as observations of other planets in this temperature range with measured emission spectra, we argue that with more detailed modeling and more observations we may be able to place useful constraints on the size of condensates on the daysides of hot Jupiters.

Diffusive Tidal Evolution for Migrating hot Jupiters

Diffusive Tidal Evolution for Migrating hot Jupiters

Author:


Wu

Abstract:

I consider a Jovian planet on a highly eccentric orbit around its host star, a situation possibly produced by secular interactions with its planetary or stellar companions. At every periastron passage, tidal interactions lead to an energy exchange between the orbit and the planet's internal oscillations (predominantly an l=2 f-mode). Starting from zero energy, this f-mode can be diffusively excited if the one-kick energy gain is greater than (ωPorb)−1 of the orbital energy. This occurs at a pericentre distance of 4 tidal radii (or 1.6 Roche radius). Furthermore, when the f-mode has a non-negligible initial energy, this diffusive evolution can set in at a much reduced threshold. The first finding is important for stalling the secular migration. The f-mode can absorb orbital energy and decouple the planet from its secular perturbers, parking all migrating jupiters safely outside the zone of tidal disruption. The second finding is important for circularizing the planet's orbit. It allows an excited f-mode to continuously absorb orbital energy even when the one-kick energy is weakening along the path of circularization (due to increasing pericentre distance). So without any explicit dissipation, other than the fact that the f-mode will damp nonlinearly when its amplitude reaches unity, the planet can be transported from a few AU to 0.2 AU in 10^4 yrs. Such a rapid circularization corresponds to an equivalent tidal dissipation factor Q ~ 1, and it explains the observed deficit of super-eccentric Jovian planets. Lastly, the repeated f-mode breaking deposits energy and angular momentum in the outer shells of the planet. This likely alters the planet's thermal structure, but should fall short of ablating it. Overall, this work boosts the case for forming hot Jupiters through high-eccentricity secular migration.

Aerosol Constraints on the Atmosphere of the Hot Saturn-mass planet WASP-49b

Aerosol Constraints on the Atmosphere of the Hot Saturn-mass planet WASP-49b 
Authors:
Cubillos et al

Abstract:
The strong, nearly wavelength-independent absorption cross section of aerosols produces featureless exoplanet transmission spectra, limiting our ability to characterize their atmospheres. Here we show that even in the presence of featureless spectra, we can still characterize certain atmospheric properties. Specifically, we constrain the upper and lower pressure boundaries of aerosol layers, and present plausible composition candidates. We study the case of the bloated Saturn-mass planet WASP-49b, where near-infrared observations reveal a flat transmission spectrum between 0.7 and 1.0 {\microns}. First, we use a hydrodynamic upper-atmosphere code to estimate the pressure reached by the ionizing stellar high-energy photons at 10−8 bar, setting the upper pressure boundary where aerosols could exist. Then, we combine HELIOS and Pyrat Bay radiative-transfer models to constrain the temperature and photospheric pressure of atmospheric aerosols, in a Bayesian framework. For WASP-49b, we constrain the transmission photosphere (hence, the aerosol deck boundaries) to pressures above 10−5 bar (100× solar metallicity), 10−4 bar (solar), and 10−3 bar (0.1× solar) as lower boundary, and below 10−7 bar as upper boundary. Lastly, we compare condensation curves of aerosol compounds with the planet's pressure-temperature profile to identify plausible condensates responsible for the absorption. Under these circumstances, we find as candidates: Na2S (at 100× solar metallicity); Cr and MnS (at solar and 0.1× solar); and forsterite, enstatite, and alabandite (at 0.1× solar).

Wednesday, November 15, 2017

A Direct Imaging Survey of Spitzer detected debris disks: Occurrence of giant planets in dusty systems

A Direct Imaging Survey of Spitzer detected debris disks: Occurrence of giant planets in dusty systems

Authors:


Meshkat et al

Abstract:

We describe a joint high contrast imaging survey for planets at Keck and VLT of the last large sample of debris disks identified by the Spitzer Space Telescope. No new substellar companions were discovered in our survey of 30 Spitzer-selected targets. We combine our observations with data from four published surveys to place constraints on the frequency of planets around 130 debris disk single stars, the largest sample to date. For a control sample, we assembled contrast curves from several published surveys targeting 277 stars which do not show infrared excesses. We assumed a double power law distribution in mass and semi-major axis of the form f(m,a) = Cmαaβ, where we adopted power law values and logarithmically flat values for the mass and semi-major axis of planets. We find that the frequency of giant planets with masses 5-20 MJup and separations 10-1000 AU around stars with debris disks is 6.27% (68% confidence interval 3.68 - 9.76%), compared to 0.73% (68% confidence interval 0.20 - 1.80%) for the control sample of stars without disks. These distributions differ at the 88% confidence level, tentatively suggesting distinctness of these samples.