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2000AJ....120.1014P - Astron. J., 120, 1014-1055 (2000/August-0)
What are these blue metal-poor stars?
PRESTON G.W. and SNEDEN C.
Abstract (from CDS):
The high binary fraction fBMP∼0.6 of BMP stars compared with that found for the F- and G-type stars near the Sun, the systematically low mass functions of these binaries, and the paucity of double-lined binaries among them lead us to suggest that at least half of the BMP binaries are blue stragglers and that these blue stragglers are formed by McCrea mass transfer rather than by the various merger processes that are currently believed to produce most blue stragglers in globular clusters. This conclusion is supported by the abnormally high proportion of BMP binaries with long periods and small orbital eccentricities, properties these binaries share with McClure's carbon star binaries. The great majority of field blue stragglers (BSs) probably are created by Roche-lobe overflow during red giant branch evolution. Primaries of more widely separated binaries that survive this phase of stellar evolution may engage in mass transfer during subsequent asymptotic giant branch evolution to form s-process abundance enhanced carbon stars.
Our result requires a major downward revision of the fraction of BMP stars attributed to a captured intermediate-age population of metal-poor field stars. The high original estimate of the size of this component probably arose from improper use of the globular cluster BS specific frequency, SBS=n(BS)/n(HB)∼1, to estimate the halo BS space density. We use a simple model to calculate the specific frequency of BSs produced by McCrea mass transfer in an old metal-poor population with a given primordial binary fraction fB. Our model calculations return values of SBS∼5 for fB=0.15, much more like our value for the field blue stragglers. We suggest that globular clusters either destroy the primordial binaries that produce long period BS binaries like those in the Galactic field reported here, or they never possessed them.
Abstract Copyright: ∼
Journal keyword(s): Stars: Binaries: General
Simbad objects: 94
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