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M. A. Stark (Univ. of Wyoming), R. A. Wade (Penn State Univ.), R. F. Green (LBTO), P. R. Durrell (Youngstown State Univ.)
Fashionable scenarios for the formation of extreme horizontal branch (EHB, or hot subdwarf) stars involve envelope mass loss via either Roche-lobe overflow (RLOF) or a common-envelope (CE) process. There has been considerable success at finding short-period binaries that include EHB stars; this is because the companions in these short-period (post-CE) systems are either white dwarfs or dM stars, so that the more luminous component of the binary is the hot subdwarf, and these systems are catalogued as such. The postulated RLOF systems are another matter. Proponents of the binary model for EHB stars have suggested that many RLOF-binaries containing EHB stars were excluded from the PG catalog, which includes about 1/3 of the known hot subdwarf stars; this hypothesized GK selection effect against PG binaries is based on the appearance in the spectrum of the Ca II K-line from the companion star. To test this hypothesis, we have assembled accurate coordinates for these 1125 ``PG-rejected'' (PGR) stars, and we have examined both SDSS and Galex archival data (in combination with 2MASS) for substantial numbers of these, to see whether they are part of this postulated missing group. From these samples we find that most PGR stars are consistent with being single stars (likely metal poor sdF and sdG types), and only about 3 percent have composite energy distributions. Supported by NASA.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.