The Nature of the Late-Type Companions in Hot Subdwarf Composite-Spectrum Binaries

We present the results of a study of the late-type companions in hot subdwarf composite spectrum binaries. The exact nature of these late-type companions has been disputed in the literature -- some argue that they are main sequence stars, and others have claimed they are subgiants. To determine the properties of the late-type companions, we first conducted a survey utilizing the Two Micron All Sky Survey (2MASS) All-Sky Data Release Catalog to identify composite-colored binaries in the "Catalogue of Spectroscopically Identified Hot Subdwarfs" (Kilkenny, Heber, & Drilling 1988, 1992). We then 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 (14 single and 51 composite). We also obtained spectra of 59 single late-type stars with Hipparcos parallaxes for calibration. We used measured equivalent width (EW) indices from the composite systems to estimate the temperature and gravity of the late-type star, taking into account the dilution of its spectral features by light from the hot subdwarf. 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.