Astronomy and Astrophysics – Astrophysics
Scientific paper
Feb 1979
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1979a%26a....72..104w&link_type=abstract
Astronomy and Astrophysics, vol. 72, no. 1-2, Feb. 1979, p. 104-110. Research supported by the Ministere de l'Education du Queb
Astronomy and Astrophysics
Astrophysics
34
Interstellar Matter, Stellar Evolution, Stellar Mass Accretion, Stellar Spectra, White Dwarf Stars, Abundance, Convective Flow, Helium, Hydrogen Clouds, Stellar Envelopes, Stellar Luminosity
Scientific paper
A semiquantitative investigation of the influence of encounters between interstellar clouds and white dwarfs with helium-rich envelopes on the spectrum of the degenerate star is presented. The use of a fluid description of the accretion process is justified, and Bondi's (1952) formula is used to estimate the amount of hydrogen-rich material accreted over a typical cloud crossing time and over a cooling time (down to a luminosity of about 100 millionths of a sun). Maximum dilution of the accreted material is assumed in both cases in order to minimize the resulting hydrogen abundances. It is found that only low-mass and cool helium-rich white dwarfs can preserve their hydrogen-poor spectrum after one single encounter. For low-luminosity white dwarfs the resulting theoretical lower limits on the hydrogen abundance in the spectrum are shown to exceed the observed upper limits by factors ranging from about 10 to 10 million, the smallest discrepancy being at low stellar masses. This suggests that, within the framework of Bondi's formula, most cool helium-rich degenerate dwarfs could be low-mass objects.
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