Fractionated stellar wind and the H/He abundance anomalies in Bp stars
K. Hunger 1 and
D. Groote 2
Received 20 April 1999 / Accepted 10 September 1999
Radiatively driven winds occur in all main sequence stars (Babel 1995, 1996). However, due to the weak coupling between the photon absorbing metals and the inert elements H and He, the wind in the low temperature domain is fractionated: He decouples from the wind at 25 000 K, and below 17 000 K even H. The decoupled elements fall back to the surface of the star thus creating overabundances and abundance stratifications. These anomalies, however, become manifest only if atmospheric turbulence is suppressed (say by magnetic fields).
In order to prove the validity of the described scenario, all Bp stars for which reliable fundamental parameters exist, are discussed on the basis of the (augmented) ()-diagram of Babel (1996). It is shown that the fractionation process is able to explain the observed temperature sequence of He-rich and He-poor stars, additionally to classical diffusion processes. A necessary condition is that a magnetic field is present. This explains why only a fraction of B stars exhibits H/He anomalies. While classical diffusion operates in the quiet zones (no wind) of a star, fractionation takes place above the wind bases.
Key words: stars: abundances stars: chemically peculiar stars: mass-loss
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© European Southern Observatory (ESO) 1999
Online publication: November 3, 1999