Selective Mass Loss, Abundance Anomalies, and Helium-rich Stars

Details Selective Mass Loss, Abundance Anomalies, and Helium-rich Stars Selective Mass Loss, Abundance Anomalies, and Helium-rich Stars

Nov 1987

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987apj...322..302m&link_type=abstract

Astrophysical Journal v.322, p.302

Astronomy and Astrophysics

Astronomy

39

Diffusion, Stars: Abundances, Stars: Interiors, Stars: Mass Loss, Stars: White Dwarfs

Scientific paper

In the presence of mass loss, the He abundance in the line-forming region is modified by the chemical separation that takes place not only in the atmospheric region but also in the wind and in the envelope. Since He-rich stars of the upper main sequence appear to be a hot extension of the Ap-Bp phenomenon in which mass loss is observed to be present, we here take diffusion in the wind, the atmosphere, and the envelope simultaneously into account in order to obtain constraints on the hydrodynamics of mass loss. It is shown that to explain the He enrichment by separation in the atmosphere in stars of both Teff = 20,000 K and Teff = 25,000 K requires that the mass-loss rate decrease when Teff increases. It is also shown that the mass loss allowing separation (about 3 x 10-13 Msun yr-1 at Teff = 25,000 K) is smaller than that expected from an extrapolation of the mass-loss rates observed in O stars and of that which is observed in some He-rich stars. The magnetic field seems to be required to reduce the mass-loss rate where the separation occurs. It is suggested that most of the mass loss occurs at the poles, while the chemical separation leading to He enrichment occurs at the magnetic equator where the magnetic field reduces mass loss. This explains the observed single-wave pattern of He enrichment. It also leads to an observational test of the model: the CNO abundances should be normal. The maximum mass loss for which separation can occur in the wind is determined to be 2 x 10-12 Msun yr-1 in main-sequence stars and 10 times larger in white dwarfs. However, this is obtained using models for the wind region that maximize the effect of separation, and it is argued that separation in the wind coudl actually be much smaller. It depends in particular on the temperature in the wind region, and cold winds lead to much less separation. Separation in the atmosphere is likely to be more important than separation in the wind. Separation in the envelope leads to underabundances of helium. It is, however, shown here that for mass-loss rates leading to He overabundances by chemical separation in the atmospheric region, separation in the envelope is too slow to reduce the He abundance in the atmosphere during the main-sequence life of He-rich stars. Confirming that the separation occurs in the atmospheric region has important hydrodynamical consequences. Not only does it limit the mass-loss rate and its space distribution, but it also implies that the He convection zones do not extend into the atmosphere and that there is little, if any, overshooting into the atmosphere. Evaluations of the separation in the atmosphere and wind of white dwarfs and sdB stars are also carried out to show the dependence on gravity.

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