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

Open Access
Stark, Michele A.
Graduate Program:
Astronomy and Astrophysics
Doctor of Philosophy
Document Type:
Date of Defense:
June 17, 2005
Committee Members:
  • Richard Alan Wade, Committee Chair
  • Robin Bruce Ciardullo, Committee Member
  • Lawrence William Ramsey, Committee Member
  • Mercedes T Richards, Committee Member
  • Richard Wallace Robinett, Committee Member
  • horizontal branch star
  • binary population synthesis
  • hot subdwarf
  • sdB
<p>For this investigation I studied hot subdwarf stars listed in the <i>Catalogue of Spectroscopically Identified Hot Subdwarfs</i> (Kilkenny, Heber, Drilling 1988, KHD). While the KHD catalog contains all varieties of hot subdwarfs, I primarily focused on the more numerous and homogeneous sdB stars.</p> <p>I used improved coordinates to collect near-IR flux measurements of KHD hot subdwarfs from the Two-Micron All Sky Survey (2MASS) All-Sky Data Release Catalog. I used these 2MASS magnitudes with visual photometry from the literature to identify those hot subdwarfs whose colors indicated the presence of a late type companion. I determined that ~40% of sdB stars are composite in a magnitude limited sample (~25% in a volume limited sample).</p> <p>I compared the distributions of hot subdwarfs in 2MASS colors and found a bimodally distributed population. The two peaks of the bimodal distribution can be understood as single hot subdwarf stars and composite systems. Based on these distributions is 2MASS colors, <i>there are no (or very few) F or dM companions of the hot subdwarfs in the KHD catalog</i>. These observed distributions of hot subdwarfs in 2MASS colors can be reproduced equally well by assuming either main sequence or subgiant companions --- photometric data alone cannot distinguish between these two possibilities.</p> <p>I obtained spectroscopy of some of the 2MASS composite-colored hot subdwarfs to break the degeneracy between main sequence and subgiant companions present in photometry alone. My analysis focused on Mg I b, Na I D, and Ca II infrared triplet equivalent widths (EWs) from the late-type companion.</p> <p>The observations (2MASS and visual photometry combined with EWs) for each composite hot subdwarf were compared with diluted models based on Hipparcos standard star observations, models of extended horizontal branch stars (Caloi 1972), and Kurucz (1998) spectral energy distributions, in order to determine the combination of sdB+late-type star that best explained all observations. With a few exceptions, I found that the late-type companions are best identified as main sequence (although some subgiant companions do exist). The majority of the well constrained main sequence companions have 0.5 < B-V<sub>c</sub> < 1.1 (spectral types ~F6-K5).</p> <p>Han et al. (2002, 20003) predict that all hot subdwarfs with main sequence companions of ~G-type or later are in short period (P < 40 days), post-common envelope binaries (anything with a companion of ~G-type or later and a long period, P > 40 days, has either a subgiant or giant companion). Yet, radial velocity studies including composite spectrum hot subdwarfs (i.e., Orosz et al. 1997; Saffer et al. 1994; Maxted et al. 2001), have said the periods for composite-spectrum binaries must be long (many months to years or more), which in the Han et al. scenario would imply that they contain the subgiant or giant companions. Yet, as I show, the majority of composite companions are consistent with main sequence stars. So, it appears something is incorrect or incomplete in the current Han et al. binary formation scenario.</p>