Convective Mixing in Extended Horizontal Branch Envelope Models - the Sdb/sdo Transition

Details Convective Mixing in Extended Horizontal Branch Envelope Models - the Sdb/sdo Transition Convective Mixing in Extended Horizontal Branch Envelope Models - the Sdb/sdo Transition

Dec 1980

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1980apj...242.1166w&link_type=abstract

Astrophysical Journal, Vol.242, P.1166, 1980

Astronomy and Astrophysics

Astrophysics

13

Scientific paper

Static model envelopes are calculated from a 0.6 Msun pre-white dwarf evolutionary sequence passing through the extended horizontal branch (EHB) region of the H-R diagram. The possibility of convective mixing of compositionally stratified (H-rich layer/He layer) models is investigated. It is found that mixing does occur in the range log Teff ˜4.1-4.3. The He-enriched products of mixing do not appear at the surface until the top of the convection zone reaches the photosphere at log Teff ≍ 4.5 (independent of the Teff where mixing is initiated). This is consistent with the observed sdB/sdO transition at log Teff ≍4.5 and supports the interpretation of the EHB as an evolutionary sequence. It is found that an upper layer mass of ≤ 10-8 M* is required for mixing to occur within the EHB. The presence of some initial He in the upper H-rich layer is required for mixing to occur; simple analytical calculations suggest a lower limit on the helium mass fraction of Y˜0.1. Preliminary calculations are consistent with the interpretation of the large He deficiencies observed in the atmospheres of HBB and sdB stars as resulting from gravitational settling and not from differences in initial stellar abundances.

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