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Title:
Hot Subdwarf Stars Among the Objects Rejected from the PG Catalog: A First Assessment Using GALEX Photometry
Authors:
Wade, Richard A.; Stark, M. A.; Green, Richard F.; Durrell, Patrick R.
Affiliation:
AA(Department of Astronomy and Astrophysics, The Pennsylvania State University, 525 Davey Lab, University Park, PA 16802, USA ), AB(Department of Astronomy and Astrophysics, The Pennsylvania State University, 525 Davey Lab, University Park, PA 16802, USA ), AC(Large Binocular Telescope Observatory, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721-0065, USA ), AD(Department of Physics and Astronomy, Youngstown State University, Youngstown, OH, USA )
Publication:
The Astronomical Journal, Volume 138, Issue 2, pp. 606-614 (2009). (AJ Homepage)
Publication Date:
08/2009
Origin:
IOP
Astronomy Keywords:
binaries: close, stars: horizontal-branch, subdwarfs, ultraviolet: stars
DOI:
10.1088/0004-6256/138/2/606
Bibliographic Code:
2009AJ....138..606W

Abstract

The hot subdwarf (sd) stars in the Palomar Green (PG) catalog of ultraviolet excess (UVX) objects play a key role in investigations of the frequency and types of binary companions and the distribution of orbital periods. These are important for establishing whether and by which channels the sd stars arise from interactions in close binary systems. It has been suggested that the list of PG sd stars is biased by the exclusion of many stars in binaries, whose spectra show the Ca II K line in absorption. A total of 1125 objects that were photometrically selected as candidates were ultimately rejected from the final PG catalog using this K-line criterion. We study 88 of these "PG-Rejects" (PGRs), to assess whether there are significant numbers of unrecognized sd stars in binaries among the PGR objects. The presence of an sd should cause a large UVX, compared with the cool K-line star. We assemble GALEX, Johnson V, and Two Micron All Sky Survey photometry and compare the colors of these PGR objects with those of known sd stars, cool single stars, and hot+cool binaries. Sixteen PGRs were detected in both the far-ultraviolet (FUV) and near-ultraviolet GALEX passbands. Eleven of these, plus the 72 cases with only an upper limit in the FUV band, are interpreted as single cool stars, appropriately rejected by the PG spectroscopy. Of the remaining five stars, three are consistent with being sd stars paired with a cool main-sequence companion, while two may be single stars or composite systems of another type. We discuss the implications of these findings for the 1125 PGR objects as a whole. An enlarged study is desirable to increase confidence in these first results and to identify individual sd+cool binaries or other composites for follow-up study. The GALEX All-sky Imaging Survey data have sufficient sensitivity to carry out this larger study.
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