Chemical separation in horizontal-branch stars

Abstract

Abundance anomalies are predicted to be produced by diffusion processes in horizontal-branch (HB) and halo Population II stars. If the outer envelope is stable enough for the gravitational settling of He to be efficient, it is here shown that overabundances of heavy elements are simultaneously produced. Abundance anomalies as large as 10/sup 3/--10/sup 4/ are expected. Detailed calculations of selective radiative accelerations and of the time evolution of abundances are presented for C, N, O, Ca, Fe, and Sr in a series of models representative of HB stars. These abundance anomalies may be important for what is called the ''second parameter problem'' in globular clusters. By detailed time dependent calculations, helium underabundances and relative overabundances of /sup 3/He are shown to be produced well within the lifetime of stars on the horizontal branch. The implications of helium underabundances on the pulsational properties of RR Lyrae variables are discussed. The role of turbulent mixing in reducing the abundance anomalies is discussed. It is shown that even with enough turbulence to prevent He from settling, heavy element abundance anomalies are possible. Even though chemical separation is very likely to be important in the blue horizontal-branch stars (T>15,000 K), it may not bemore » in the cooler stars where mass loss could eliminate any effect of chemical separation. This could be verified by observations of horizontal-branch stars in globular clusters.« less