The cool DA white dwarf G128-7 - Atmospheric parameters and evolutionary consequences

Astronomy and Astrophysics – Astrophysics

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Atmospheric Models, Stellar Atmospheres, Stellar Evolution, Stellar Spectrophotometry, White Dwarf Stars, Abundance, Line Spectra, Stellar Gravitation, Stellar Temperature

Scientific paper

Atmospheric parameters are derived for the very cool DA white dwarf G128-7 (Gr283). The best fit to the models yields Teff = 5800 °K, [M/H] ≲ -4, ɛ = nHe/ntot ≲ 0.2, log g = 8.5 ± 0.2. Thus G128-7 apparently has a hydrogen rich and extremely metal poor atmosphere, similar to hotter DA stars. The deduced high surface gravity is a consequence of the shallow Hα central depth measured with two different spectrophotometric systems. Great sensitivity of this profile for log g ≳ 8.0 is found because of the implied formation of molecular hydrogen and its effects on the temperature stratification; at higher Teff, the molecule formation is unimportant and the H-lines show little gravity dependence.
The dominance of non-DA spectral types for a sample of nearby, well observed white dwarfs with 13.0 ≦ Mυ ≦ 14.9 (i.e. 5000 ≦ Teff ≦ 8000 °K) leads us to conclude that the 3:1 ratio in favor of hydrogen atmospheres for hot white dwarfs is not preserved as the stars cool; this is evidence that convective mixing of the outer hydrogen layer has occurred for some but not all stars. Results of recent theoretical investigations suggest that the surviving cool DA stars have larger initial hydrogen layer masses (≧ 10-10 Msun) or higher than normal stellar masses. The latter possibility would be consistent with our inference for a higher than normal surface gravity in G128-7.

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