CHEMICAL ABUNDANCES OF LUMINOUS COOL STARS IN THE GALACTIC CENTER FROM HIGH-RESOLUTION INFRARED SPECTROSCOPY
We present chemical abundances in a sample of luminous cool stars located within 30 pc of the Galactic center. Abundances of carbon, nitrogen, oxygen, calcium, and iron were derived from high-resolution infrared spectra in the H and K bands. The abundance results indicate that both [O/Fe] and [Ca/Fe] are enhanced, respectively, by averages of +0.2 and +0.3 dex, relative to either the Sun or the Milky Way disk at near-solar Fe abundances. The Galactic center stars show a nearly uniform and nearly solar iron abundance. The mean value of A(Fe) = 7.59 ± 0.06 agrees well with previous work. The total range in Fe abundance among Galactic center stars, 0.16 dex, is significantly narrower than the iron abundance distributions found in the literature for the older bulge population. Our snapshot of the current-day Fe abundance within 30 pc of the Galactic center samples stars with an age less than 1 Gyr; a larger sample in time (or space) may find a wider spread in abundances.

title: Keck Spectra of Brown Dwarf Candidates and a Precise Determination of the Lithium Depletion Boundary in the α Persei Open Cluster
abstract: We have identified 27 candidate very low mass members of the relatively young α Persei open cluster from a 6 deg2 CCD imaging survey. Based on their I magnitudes and the nominal age and distance to the cluster, these objects should have masses less than 0.1 ☉ if they are cluster members. We subsequently obtained intermediate-resolution spectra of 17 of these objects using the Keck II telescope and Low-Resolution Imaging Spectrograph (LRIS). We also obtained near-IR photometry for many of the stars. Our primary goal was to determine the location of the "lithium depletion boundary" and, hence, to derive a precise age for the cluster. Most of our program objects have radial velocities consistent with cluster membership, moderately strong Hα emission, and spectral types of M5.5 to M8, as expected from their (R-I)C colors. We detected lithium with equivalent widths greater than or equal to 0.4 A in five of the program objects. We have constructed a color-magnitude diagram for the faint end of the α Persei main sequence, including stars for which high signal-to-noise spectra in the region of the lithium λ6708 absorption line have been obtained. These data allow us to accurately determine the α Persei single-star lithium depletion boundary at M(IC) = 11.47, Mbol = 11.42, (R-I)C0 = 2.12, spectral type M6.0. By reference to theoretical evolutionary models, this converts fairly directly into an age for the α Persei cluster of 90 ± 10 Myr. That age is considerably older than most previously quoted ages for the cluster but consistent with ages estimated from the upper main-sequence turnoff using recent models that include a moderate amount of convective core overshoot. At this age, the two faintest of our spectroscopically confirmed members should be substellar (i.e., brown dwarfs) according to theoretical models.

title: Time-series spectroscopy and photometry of the pulsating subdwarf B star PG 1219+534 (KY UMa)
abstract: We present observations and analysis of time-series spectroscopy and photometry of the pulsating subdwarf B star PG 1219+534 (KY UMa). Subdwarf B stars are blue horizontal branch stars that have shed most of their hydrogen envelopes. Pulsating subdwarf B stars allow a probe into this interesting phase of evolution.  Low-resolution spectra were obtained at the Nordic Optical Telescope and Kitt Peak National Observatory, and photometric observations were obtained at MDM and Baker observatories in 2006. We extracted radial velocity and equivalent width variations from several Balmer and He I lines in individual spectra. The pulsation frequencies were separated via phase binning to detect lineprofile variations in Balmer and helium lines, which were subsequently matched to atmospheric models to infer effective temperature and gravity changes throughout the pulsation cycle.  From the photometry we recovered the four previously observed frequencies and detected a new fifth frequency. From the spectra we directly measured radial velocity and equivalent width variations for the four main frequencies and from atmospheric models. We successfully inferred temperature and gravity changes for these four frequencies. We compared amplitude ratios and phase differences of these quantities and searched for outliers that could be identified as high-degree modes. These are the first such measurements for a “normal” amplitude pulsating subdwarf B star, indicating that spectroscopic studies can benefit the majority of pulsating subdwarf B stars.

title: The Distances to Open Clusters as Derived from Main-sequence Fitting. II. Construction of Empirically Calibrated Isochrones
abstract: We continue our series of papers on open cluster distances by comparing multicolor photometry of single stars in the Hyades to theoretical isochrones constructed with various color-temperature relations. After verifying that the isochrone effective temperatures agree well with spectroscopically determined values, we argue that mismatches between the photometry and the theoretical colors likely arise from systematic errors in the color-temperature relations. We then describe a method for empirically correcting the isochrones to match the photometry, and discuss the dependence of the isochrone luminosity on metallicity.

title: STELLAR IRON ABUNDANCES AT THE GALACTIC CENTER
abstract: We present measurements of (Fe/H) for six M supergiant stars and three giant stars within 2.5 pc of the Galactic center (GC) and one M supergiant star within 30 pc of the GC. The results are based on high-resolution (j/*j \ 40,000) K-band spectra, taken with CSHELL at the NASA Infrared Telescope Facility. We determine the iron abundance by detailed abundance analysis, performed with the spectral synthesis program MOOG. The mean (Fe/H) of the GC stars is determined to be near solar, (Fe/H) \ ) 0.12 ^ 0.22. Our analysis is a diUerential analysis, as we have observed and applied the same analysis technique to 11 cool, luminous stars in the solar neighborhood with similar temperatures and luminosities as the GC stars. The mean (Fe/H) of the solar neighborhood comparison stars, (Fe/H) \) 0.03 ^ 0.16, is similar to that of the GC stars. The width of the GC (Fe/H) distribution is found to be narrower than the width of the (Fe/H) distribution of Baades window in the bulge but consistent with the width of the (Fe/H) distribution of giant and supergiant stars in the solar neighbor- hood.  stellar content ¨ stars: abundancesstars: late-type

title: Resolving the pulsations of subdwarf B stars: PG 0154+182, HS 1824+5745 and HS 2151+0857
abstract: We continue our programme of extended single-site observations of pulsating subdwarf B (sdB) stars and present the results of extensive time-series photometry to resolve the pulsation spectra for use in asteroseismological analyses. PG 0154+182, HS 1824+5745 and HS2151+0857 were observed at the MDM Observatory during 2004 and 2005. Our observations are sufficient to resolve the pulsations of all three target stars. We extend the number of known frequencies for PG 0154+182 from one to six, confirm that HS 1824+5745 is a monoperiodic pulsator and extend the number of known frequencies to five for HS 2151+0857. We perform standard tests to search for multiplet structure, measure amplitude variations as pertains to stochastic excitation and examine the mode density to constrain the mode degree l.

title: Identifying Very Metal-Rich Stars with Low-Resolution Spectra: Finding Planet-Search Targets
abstract: We present empirical calibrations that estimate stellar metallicity, effective temperature and surface gravity as a function of Lick/IDS indices. These calibrations have been derived from a training set of 261 stars for which (1) high-precision measurements of [Fe/H], T_eff and log g have been made using spectral-synthesis analysis of HIRES spectra, and (2) Lick indices have also been measured. Our [Fe/H] calibration, which has precision 0.07 dex, has identified a number of bright (V<9) metal-rich stars which are now being screened for hot Jupiter-type planets. Using the Yonsei-Yale stellar models, we show that the calibrations provide distance estimates accurate to 20% for nearby stars. This paper outlines the second tier of the screening of planet-search targets by the N2K Consortium, a project designed to identify the stars most likely to harbor extrasolar planets. Discoveries by the N2K Consortium include the transiting hot Saturn HD 149026 b (Sato et al. 2005, astro-) and HD 88133 b (Fischer et al. 2005). See Ammons et al. (2005, In Press) for a description of the first tier of N2K metallicity screening, calibrations using broadband photometry.

title: Sw 1644+57/GRB 110328A: The Physical Origin and the Composition of the Relativistic Outflow
abstract: Sw 1644+57/GRB 110328A is a remarkable cosmological X-ray outburst detected by the Swift satellite. Its early-time (t less than or similar to 0.1 days since the trigger) X-ray emission resembles some gamma-ray bursts (GRBs), e. g., GRB 090417B. But the late-time flaring X-ray plateau lasting >40 days renders it unique. We examine the possibilities that the outburst is a super-long GRB powered either by the fallback accretion onto a nascent black hole or by a millisecond pulsar, and find out that these two scenarios can address some but not all of the main observational features. We then focus on the model of tidal disruption of a (giant) star by a massive black hole. The mass of the tidal-disrupted star is estimated to be greater than or similar to a few solar masses. A simple/straightforward argument for a magnetic origin of the relativistic outflow is presented.

title: Silicon and Nickel Enrichment in Planet Host Stars: Observations and Implications for the Core Accretion Theory of Planet Formation
abstract: We present evidence that stars with planets exhibit statistically significant silicon and nickel enrichment over the general metal-rich population. We also present simulations that predict silicon enhancement of planet hosts within the context of the core accretion hypothesis for giant planet formation. Because silicon and oxygen are both α-elements, [Si/Fe] traces [O/Fe], so the silicon enhancement in planet hosts predicts that these stars are oxygen-rich as well. We present new numerical simulations of planet formation by core accretion that establish the timescale on which a Jovian planet reaches rapid gas accretion, trga, as a function of solid surface density σsolid: (trga/1 Myr) = (σsolid/25.0 g cm-2)-1.44. This relation enables us to construct Monte Carlo simulations that predict the fraction of star-disk systems that form planets as a function of [Fe/H], [Si/Fe], disk mass, outer disk radius, and disk lifetime. Our simulations reproduce both the known planet-metallicity correlation and the planet-silicon correlation reported in this paper. The simulations predict that 15% of solar-type stars form Jupiter-mass planets, in agreement with 12% predicted from extrapolation of the observed planet frequency-semimajor axis distribution. Although a simple interpretation of core accretion predicts that the planet-silicon correlation should be much stronger than the planet-nickel correlation, we observe the same degree of silicon and nickel enhancement in planet hosts. If this result persists once more planets have been discovered, it might indicate a complexity in the chemistry of planet formation beyond the simple accumulation of solids in the core accretion theory.