Binary Hypothesis for the Subdwarf B Stars
Abstract
Stellar evolution computations pertinent to the hypothesis that subdwarf B stars are formed in close binary systems are presented. Under the assumption that total mass and orbital angular momentum are conserved, we find that for systems with initial masses (0.80 M0, 0.78 Me) and composition X = 0.73, Z = 0.001, there exists a range of initial separations for which core helium-burning (horizontal-branch) stars of mass 0.5 M0 are formed. We identify these with the subdwarf B stars. For smaller separations, white dwarfs are formed. For larger separations, no mass transfer occurs, and normal horizontal-branch stars result. We suggest that the resulting mass dichotomy on the horizontal branch provides a partial explanation of the gaps discovered by Newell in the effective temperature distribution of the faint blue stars. The constraints on the production of subdwarf B stars lead to the conclusion that they are more readily produced in the old disk population than in the halo, in qualitative agreement with the suggestion of Baschek and Norris. While observational evidence exists for the composite nature of some of the subdwarf B stars, the conservation of total mass and angular momentum produces a companion whose visual magnitude is similar to that of the subdwarf in the cases we consider. Since the conservation assumptions appear to make this outcome unavoidable, we discuss briefly the possible consequences of relaxing them. Subject headings: stars: binaries - stars: horizontal-branch - stars: interiors - stars: subdwarfs
- Publication:
-
The Astrophysical Journal
- Pub Date:
- March 1976
- DOI:
- 10.1086/154193
- Bibcode:
- 1976ApJ...204..488M