Testing the Reality of Strong Magnetic Fields on T Tauri Stars: The Naked T Tauri Star Hubble 4

High resolution optical and infrared (IR) echelle spectra of the naked (diskless) T Tauri star Hubble 4 are presented. The K band IR spectra include 4 Zeeman sensitive Ti I lines along with several magnetically insensitive CO lines. Detailed spectrum synthesis combined with modern atmospheric models is used to fit the optical spectra of Hubble 4 in order to determine its key stellar parameters: Teff = 4158 +/- 56 K; log(g) = 3.61 +/- 0.50; [M/H] = -0.08 +/- 0.05; vsini = 14.6 +/- 1.7 km/s . These stellar parameters are used to synthesize K band spectra to compare with the observations. The magnetically sensitive Ti I lines are all significantly broadened relative to the lines produced in the non-magnetic model, while the magnetically insensitive CO lines are well matched by the basic non-magnetic model. Models with magnetic fields are synthesized and fit to the Ti I lines. The best fit models indicate a distribution of magnetic field strengths on the stellar surface characterized by a mean magnetic field strength of 2.51 +/- 0.18 kG. The mean field is a factor of 2.0 greater than the maximum field strength predicted by pressure equipartition arguments. To confirm the reality of such strong fields, we attempt to refit the observed profiles using a two component magnetic model in which the field strength is confined to the equipartition value representing plage-like regions in one component, and the field is allowed to vary in a cooler component representing spots. It is shown that such a model is inconsistent with the optical spectrum of the TiO bandhead at 7055 Angstroms.Comment: 31 pages, including 5 figures accepted for publication in The Astrophysical Journa

Spectral Properties of Cool Stars: Extended Abundance Analysis of 1617 Planet Search Stars

We present a catalog of uniformly determined stellar properties and abundances for 1626 F, G, and K stars using an automated spectral synthesis modeling procedure. All stars were observed using the HIRES spectrograph at Keck Observatory. Our procedure used a single line list to fit model spectra to observations of all stars to determine effective temperature, surface gravity, metallicity, projected rotational velocity, and the abundances of 15 elements (C, N, O, Na, Mg, Al, Si, Ca, Ti, V, Cr, Mn, Fe, Ni, & Y). Sixty percent of the sample had Hipparcos parallaxes and V-band photometry which we combined with the spectroscopic results to obtain mass, radius, and luminosity. Additionally, we used the luminosity, effective temperature, metallicity and alpha-element enhancement to interpolate in the Yonsei-Yale isochrones to derive mass, radius, gravity, and age ranges for those stars. Finally, we determined new relations between effective temperature and macroturbulence for dwarfs and subgiants. Our analysis achieved precisions of 25 K in Teff , 0.01 dex in [M/H], 0.028 dex for log g and 0.5 km/s in v sin ibased on multiple observations of the same stars. The abundance results were similarly precise, between 0.01 and - 0.04 dex, though trends with respect to Teff remained for which we derived empirical corrections. The trends, though small, were much larger than our uncertainties and are shared with published abundances. We show that changing our model atmosphere grid accounts for most of the trend in [M/H] between 5000 K and 5500 K indicating a possible problem with the atmosphere models or opacities.Comment: 41 pages, 23 figures, 11 tables, accepted for publication in ApJ

The Metallicity of the HD 98800 System

Pre-main sequence (PMS) binaries and multiples enable critical tests of stellar models if masses, metallicities, and luminosities of the component stars are known. We have analyzed high-resolution, high signal-to-noise echelle spectra of the quadruple-star system HD 98800 and using spectrum synthesis, computed fits to the composite spectrum for a full range of plausible stellar parameters for the components. We consistently find that sub-solar metallicity yields fits with lower $\chi^2$ values, with an overall best-fit of $[M/H] = -0.20\pm0.10$. This metallicity appears to be consistent with PMS evolutionary tracks for the measured masses and luminosities of the components of HD 98800 but additional constraints on the system and modelling are needed.Comment: 6 pages, 3 figures, 5 tables. Online-only material: color figure. Accepted in Ap

The Far-Ultraviolet Spectra of TW Hya. II. Models of H2 Fluorescence in a Disk

We measure the temperature of warm gas at planet-forming radii in the disk around the classical T Tauri star (CTTS) TW Hya by modelling the H2 fluorescence observed in HST/STIS and FUSE spectra. Strong Ly-alpha emission irradiates a warm disk surface within 2 AU of the central star and pumps certain excited levels of H2. We simulate a 1D plane-parallel atmosphere to estimate fluxes for the 140 observed H2 emission lines and to reconstruct the Ly-alpha emission profile incident upon the warm H2. The excitation of H2 can be determined from relative line strengths by measuring self-absorption in lines with low-energy lower levels, or by reconstructing the Ly-alpha profile incident upon the warm H2 using the total flux from a single upper level and the opacity in the pumping transition. Based on those diagnostics, we estimate that the warm disk surface has a column density of log N(H2)=18.5^{+1.2}_{-0.8}, a temperature T=2500^{+700}_{-500} K, and a filling factor of H2, as seen by the source of Ly-alpha emission, of 0.25\pm0.08 (all 2-sigma error bars). TW Hya produces approximately 10^{-3} L_\odot in the FUV, about 85% of which is in the Ly-alpha emission line. From the H I absorption observed in the Ly-alpha emission, we infer that dust extinction in our line of sight to TW Hya is negligible.Comment: Accepted by ApJ. 26 pages, 17 figures, 6 table

A Gas Giant Circumbinary Planet Transiting the F Star Primary of the Eclipsing Binary Star KIC 4862625 and the Independent Discovery and Characterization of the two transiting planets in the Kepler-47 System

We report the discovery of a transiting, gas giant circumbinary planet orbiting the eclipsing binary KIC 4862625 and describe our independent discovery of the two transiting planets orbiting Kepler-47 (Orosz et al. 2012). We describe a simple and semi-automated procedure for identifying individual transits in light curves and present our follow-up measurements of the two circumbinary systems. For the KIC 4862625 system, the 0.52+/-0.018 RJup radius planet revolves every ~138 days and occults the 1.47+/-0.08 MSun, 1.7 +/-0.06 RSun F8 IV primary star producing aperiodic transits of variable durations commensurate with the configuration of the eclipsing binary star. Our best-fit model indicates the orbit has a semi-major axis of 0.64 AU and is slightly eccentric, e=0.1. For the Kepler-47 system, we confirm the results of Orosz et al. (2012). Modulations in the radial velocity of KIC 4862625A are measured both spectroscopically and photometrically, i.e. via Doppler boosting, and produce similar results.Comment: 40 pages, 17 figure

The NASA-UC Eta-Earth Program: III. A Super-Earth orbiting HD 97658 and a Neptune-mass planet orbiting Gl 785

We report the discovery of planets orbiting two bright, nearby early K dwarf stars, HD 97658 and Gl 785. These planets were detected by Keplerian modelling of radial velocities measured with Keck-HIRES for the NASA-UC Eta-Earth Survey. HD 97658 b is a close-in super-Earth with minimum mass Msini = 8.2 +/- 1.2 M_Earth, orbital period P = 9.494 +/- 0.005 d, and an orbit that is consistent with circular. Gl 785 b is a Neptune-mass planet with Msini = 21.6 +/- 2.0 M_Earth, P = 74.39 +/- 0.12 d, and orbital eccentricity 0.30 +/- 0.09. Photometric observations with the T12 0.8 m automatic photometric telescope at Fairborn Observatory show that HD 97658 is photometrically constant at the radial velocity period to 0.09 mmag, supporting the existence of the planet.Comment: Submitted to ApJ, 7 pages, 6 figures, 5 table

The WFC3 Galactic Bulge Treasury Program: Metallicity Estimates for the Stellar Population and Exoplanet Hosts

We present new UV-to-IR stellar photometry of four low-extinction windows in the Galactic bulge, obtained with the Wide Field Camera 3 on the Hubble Space Telescope (HST). Using our five bandpasses, we have defined reddening-free photometric indices sensitive to stellar effective temperature and metallicity. We find that the bulge populations resemble those formed via classical dissipative collapse: each field is dominated by an old (~10 Gyr) population exhibiting a wide metallicity range (-1.5 < [Fe/H] < 0.5). We detect a metallicity gradient in the bulge population, with the fraction of stars at super-solar metallicities dropping from 41% to 35% over distances from the Galactic center ranging from 0.3 to 1.2 kpc. One field includes candidate exoplanet hosts discovered in the SWEEPS HST transit survey. Our measurements for 11 of these hosts demonstrate that exoplanets in the distinct bulge environment are preferentially found around high-metallicity stars, as in the solar neighborhood, supporting the view that planets form more readily in metal-rich environments.Comment: Accepted for publication in The Astrophysical Journal Letters. Latex, 5 pages, ApJ forma