AAS 205th Meeting, 9-13 January 2005
Session 170 White Dwarfs and Hot Subdwarfs
Oral, Thursday, January 13, 2005, 2:00-3:30pm, Sunrise

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[170.03] The Origins of Hot Subdwarf Stars as Illuminated by Composite-Spectrum Binaries

M. A. Stark, R. A. Wade (Penn State)

Hot subdwarf stars are the Galactic field equivalent of the extended horizontal branch seen in some globular clusters, and they are the primary contributors to the ultraviolet excess observed in old stellar populations. The current leading theory for the formation of hot subdwarfs predict that they were formed through binary interactions (either Roche-lobe overflow or common envelope processes).

Utilizing the Two Micron All Sky Survey (2MASS) All-Sky Data Release Catalog, we retrieved useful near-IR J, H, and KS magnitudes for more than 800 hot subdwarfs (sdO and sdB stars) drawn from the {\em Catalogue of Spectroscopically Identified Hot Subdwarfs} (Kilkenny, Heber, &\ Drilling 1988, 1992). Of these subdwarfs, ~40%\ in a magnitude limited sample have colors that are consistent with the presence of an unresolved late type (FGK) companion. Binary stars are over-represented in a magnitude limited sample. In an approximately volume limited sample the fraction of composite-color binaries is ~25%.

The exact nature of these late-type companions has been disputed in the literature --- some argue that they are main sequence stars, and others claim they are subgiants. To determine the properties of the late-type companions, we conducted a spectroscopic study of a sub-sample of the 2MASS composite-colored hot subdwarfs. The sample consists of photometrically and spectroscopically single and composite hot subdwarfs. We also obtained spectra of single late-type stars with {\em Hipparcos} parallaxes for calibration. Preliminary results from combining the spectroscopic data with model energy distributions indicate that the late-type companions in composite-spectrum systems are best described by main sequence companions overall. Understanding the frequency and present status of composite spectrum hot subdwarfs will lead to a refinement of the tools used for binary population synthesis.

Significant support from NASA is acknowledged.

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