The influence of gravitational settling and selective radiative forces in PG 1159 stars. II.

Details The influence of gravitational settling and selective radiative forces in PG 1159 stars. II. The influence of gravitational settling and selective radiative forces in PG 1159 stars. II.

May 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997a%26a...321..485u&link_type=abstract

Astronomy and Astrophysics, v.321, p.485-491

Astronomy and Astrophysics

Astrophysics

6

Stars: Abundances, Stars: Evolution, Diffusion, White Dwarfs

Scientific paper

Previous investigations have shown that time-independent diffusion calculations cannot explain the large abundances of carbon and oxygen in the helium-dominated atmospheres of hot pre-white dwarfs. Therefore we investigate in this paper the time evolution of the chemical composition in the outer regions with surface layer masses <=10^-8^M_*_. The elements He, C, N, O and Ne are taken into account. It is shown that for logg=7.0 and T_eff_=140000K and 65000K, respectively, gravitational settling should lead to a strong depletion of both elements within a time-scale of 1000y. According to theoretical evolutionary tracks for post-AGB stars, PG 1159 stars with M_*_~0.6Msun_ should have passed through regions in the HRD with T_eff_=140000K and logg~6.0. For these model parameters the radiative forces are strong enough to transform a helium-rich atmosphere with only traces of heavy elements into a metal-rich one. However, this transformation takes about 800000y, long compared to the corresponding time-scales of stellar evolution. For more massive objects, the observational results cannot be explained by diffusion processes alone.

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