Metal enrichment in the atmospheres of extremely metal-deficient dwarf stars by accretion of interstellar matter
Abstract
The absence of metal-free or extremely metal-deficient halo stars is explained by the hypothesis of enrichment of stellar surface metal abundances due to accretion of metal-rich material through encounters with interstellar gas clouds. The mass accreted on a star strongly depends on the relative velocity between a star and a gas cloud as well as the path length of the orbit in the region occupied by gas clouds. The stars accrete much material when their orbits are circular and when the Galaxy contracts slowly with the gas falling onto the galactic plane. The expected degree of metal enrichment is pronounced for the extremely metal-deficient main-sequence stars and is consistent with the lower limit on observed metal abundances, because the mass of convective envelope decreases with decreasing metal abundance and is smaller than the accreted mass. The correlation between orbital eccentricities and metal abundances of dwarf stars is a consequence of the dependence of the accreted mass on orbital eccentricity. The existence of low-metal RR Lyrae variables with low eccentric orbits is explained by dilution of metal abundance due to rapid deepening of convective layer toward the end of the red-giant stage. It is suggested that the correlation between metal abundance and stellar age is violated for the stars with circular orbit by the effect of accretion.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- April 1981
- Bibcode:
- 1981A&A....97..280Y
- Keywords:
-
- Abundance;
- Dwarf Stars;
- Metals;
- Stellar Atmospheres;
- Stellar Mass Accretion;
- Interstellar Matter;
- Kinematics;
- Main Sequence Stars;
- Astrophysics