A Correlation Between the Eclipse Depths of Kepler Gas Giant Candidates and the Metallicities of their Parent Stars

Previous studies of the interior structure of transiting exoplanets have shown that the heavy element content of gas giants increases with host star metallicity. Since metal-poor planets are less dense and have larger radii than metal-rich planets of the same mass, one might expect that metal-poor stars host a higher proportion of gas giants with large radii than metal-rich stars. Here I present evidence for a negative correlation at the 2.3-sigma level between eclipse depth and stellar metallicity in the Kepler gas giant candidates. Based on Kendall's tau statistics, the probability that eclipse depth depends on star metallicity is 0.981. The correlation is consistent with planets orbiting low-metallicity stars being, on average, larger in comparison with their host stars than planets orbiting metal-rich stars. Furthermore, since metal-rich stars have smaller radii than metal-poor stars of the same mass and age, a uniform population of planets should show a rise in median eclipse depth with [M/H]. The fact that I find the opposite trend indicates that substantial changes in gas giant interior structure must accompany increasing [M/H]. I investigate whether the known scarcity of giant planets orbiting low-mass stars could masquerade as an eclipse depth-metallicity correlation, given the degeneracy between metallicity and temperature for cool stars in the Kepler Input Catalog. While the eclise depth-metallicity correlation is not yet on firm statistical footing and will require spectroscopic [Fe/H] measurements for validation, it is an intriguing window into how the interior structure of planets and even the planet formation mechanism may be changing with Galactic chemical evolution.Comment: Accepted for publication in the Astrophysical Journal. 13 pages total, including 6 text pages, 5 figures and 2 table

Detection of Low Mass-ratio Stellar Binary Systems

O- and B-type stars are often found in binary systems, but the low binary mass-ratio regime is relatively unexplored due to observational difficulties. Binary systems with low mass-ratios may have formed through fragmentation of the circumstellar disk rather than molecular cloud core fragmen- tation. We describe a new technique sensitive to G- and K-type companions to early B stars, a mass-ratio of roughly 0.1, using high-resolution, high signal-to-noise spectra. We apply this technique to a sample of archived VLT/CRIRES observations of nearby B-stars in the CO bandhead near 2300 nm. While there are no unambiguous binary detections in our sample, we identify HIP 92855 and HIP 26713 as binary candidates warranting follow-up observations. We use our non-detections to determine upper limits to the frequency of FGK stars orbiting early B-type primaries.Comment: 13 pages, 7 figures Accepted to the Astrophysical Journa

Photophoresis boosts giant planet formation

In the core accretion model of giant planet formation, a solid protoplanetary core begins to accrete gas directly from the nebula when its mass reaches about 5 earth masses. The protoplanet has at most a few million years to reach runaway gas accretion, as young stars lose their gas disks after 10 million years at the latest. Yet gas accretion also brings small dust grains entrained in the gas into the planetary atmosphere. Dust accretion creates an optically thick protoplanetary atmosphere that cannot efficiently radiate away the kinetic energy deposited by incoming planetesimals. A dust-rich atmosphere severely slows down atmospheric cooling, contraction, and inflow of new gas, in contradiction to the observed timescales of planet formation. Here we show that photophoresis is a strong mechanism for pushing dust out of the planetary atmosphere due to the momentum exchange between gas and dust grains. The thermal radiation from the heated inner atmosphere and core is sufficient to levitate dust grains and to push them outward. Photophoresis can significantly accelerate the formation of giant planets.Comment: accepted in Astronomy and Astrophysics, 201

Hot Stars With Cool Companions

Young intermediate-mass stars have become high-priority targets for direct-imaging planet searches following the recent discoveries of planets orbiting e.g. HR 8799 and Beta Pictoris. Close stellar companions to these stars can affect the formation and orbital evolution of any planets, and so a census of the multiplicity properties of nearby intermediate mass stars is needed. Additionally, the multiplicity can help constrain the important binary star formation physics. We report initial results from a spectroscopic survey of 400 nearby A- and B-type stars. We search for companions by cross-correlating high resolution and high signal-to-noise ratio echelle spectra of the targets stars against model spectra for F- to M-type stars. We have so far found 18 new candidate companions, and have detected the spectral lines of the secondary in 4 known spectroscopic binary systems. We present the distribution of mass-ratios for close companions, and find that it differs from the distribution for wide ($a < 100$ AU) intermediate-mass binaries, which may indicate a different formation mechanism for the two populations.Comment: Submitted as part of the 18th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun Proceedings of Lowell Observatory (9-13 June 2014

Correcting For Telluric Absorption: Methods, Case Studies, And Release Of The TelFit Code

Ground-based astronomical spectra are contaminated by the Earth's atmosphere to varying degrees in all spectral regions. We present a Python code that can accurately fit a model to the telluric absorption spectrum present in astronomical data, with residuals of similar to 3%-5% of the continuum for moderately strong lines. We demonstrate the quality of the correction by fitting the telluric spectrum in a nearly featureless A0V star, HIP 20264, as well as to a series of dwarf M star spectra near the 819 nm sodium doublet. We directly compare the results to an empirical telluric correction of HIP 20264 and find that our model-fitting procedure is at least as good and sometimes more accurate. The telluric correction code, which we make freely available to the astronomical community, can be used as a replacement for telluric standard star observations for many purposes.UT Austin Hutchinson fellowshipUniversity of TexasAstronom

Saturn Forms by Core Accretion in 3.4 Myr

We present two new in situ core accretion simulations of Saturn with planet formation timescales of 3.37 Myr (model S0) and 3.48 Myr (model S1), consistent with observed protostellar disk lifetimes. In model S0, we assume rapid grain settling reduces opacity due to grains from full interstellar values (Podolak 2003). In model S1, we do not invoke grain settling, instead assigning full interstellar opacities to grains in the envelope. Surprisingly, the two models produce nearly identical formation timescales and core/atmosphere mass ratios. We therefore observe a new manifestation of core accretion theory: at large heliocentric distances, the solid core growth rate (limited by Keplerian orbital velocity) controls the planet formation timescale. We argue that this paradigm should apply to Uranus and Neptune as well.Comment: 4 pages, including 1 figure, submitted to ApJ Letter

The Formation of Uranus and Neptune in Solid-Rich Feeding Zones: Connecting Chemistry and Dynamics

The core accretion theory of planet formation has at least two fundamental problems explaining the origins of Uranus and Neptune: (1) dynamical times in the trans-Saturnian solar nebula are so long that core growth can take > 15 Myr, and (2) the onset of runaway gas accretion that begins when cores reach 10 Earth masses necessitates a sudden gas accretion cutoff just as the ice giant cores reach critical mass. Both problems may be resolved by allowing the ice giants to migrate outward after their formation in solid-rich feeding zones with planetesimal surface densities well above the minimum-mass solar nebula. We present new simulations of the formation of Uranus and Neptune in the solid-rich disk of Dodson-Robinson et al. (2009) using the initial semimajor axis distribution of the Nice model (Gomes et al. 2005; Morbidelli et al. 2005; Tsiganis et al. 2005), with one ice giant forming at 12 AU and the other at 15 AU. The innermost ice giant reaches its present mass after 3.8-4.0 Myr and the outermost after 5.3-6 Myr, a considerable time decrease from previous one-dimensional simulations (e.g. Pollack et al. 1996). The core masses stay subcritical, eliminating the need for a sudden gas accretion cutoff. Our calculated carbon mass fractions of 22% are in excellent agreement with the ice giant interior models of Podolak et al. (1995) and Marley et al. (1995). Based on the requirement that the ice giant-forming planetesimals contain >10% mass fractions of methane ice, we can reject any solar system formation model that initially places Uranus and Neptune inside the orbit of Saturn. We also demonstrate that a large population of planetesimals must be present in both ice giant feeding zones throughout the lifetime of the gaseous nebula.Comment: Accepted for publication in Icarus. 9 pages, including 3 figure