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M. D. Reed (Iowa State University)
Subdwarf B (sdB) stars are extreme horizontal branch (EHB) stars that are thought to consist of a 0.5M\odot helium burning core with less than 10-4M\odot of hydrogen in the envelope. As such, they are extremely blue with effective temperatures between 25,000 and 40,000 kelvin. The origin of these stars is poorly understood as they require sufficient mass to undergo a degenerate helium flash as well as an enhanced mass loss mechanism to account for the thin hydrogen layer. One solution is to have the EHB star in a binary such that Roche lobe overflow occurs near the tip of the red giant branch (Green, Liebert, & Saffer, 2000, AAS, 197.4601).
To test this theory, we need to determine how often sdB stars occur in binaries. One relatively simple way to do this is to search for cooler companions. Subdwarf B stars are relatively small, with radii of approximately 0.15-0.3 R\odot, such that they do not outshine cooler companions at longer wavelengths. We have used the 2MASS survey to obtain the J, H, and Ks magnitudes for \emph{every} star classifed as sdB or sdOB from the Palomar Green (Green, Schmidt, & Liebert, 1984, ApJ, 287, 320, Edinburgh-Cape (Kilkenny et al. 1998, MNRAS, 296, 320), and Hamburg-Schmidt (Heber, 2001, private communication) surveys available in the 2MASS Second Incremental Release Point Source Catalog. We then compare colors with those obtained from models (Kurucz, 1996, IAUS, 176, 523).
The author(s) of this abstract have provided an email address for comments about the abstract: mreed@iastate.edu