On the surface composition of cool, hydrogen-line white dwarfs - Discovery of helium in the atmospheres of cool DA stars and evidence for convective mixing

Details On the surface composition of cool, hydrogen-line white dwarfs - Discovery of helium in the atmospheres of cool DA stars and evidence for convective mixing On the surface composition of cool, hydrogen-line white dwarfs - Discovery of helium in the atmospheres of cool DA stars and evidence for convective mixing

Mar 1990

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990apj...351l..21b&link_type=abstract

Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X), vol. 351, March 1, 1990, p. L21-L24. Research supported by NSERC and M

Astronomy and Astrophysics

Astrophysics

64

Convection, Cool Stars, Helium, Stellar Atmospheres, Stellar Composition, White Dwarf Stars, Balmer Series, H Lines, Stellar Spectra

Scientific paper

A determination of the He abundance in 37 cool DA white dwarfs is presented. A spectroscopic analysis of the high Balmer lines reveals that the atmospheres of most objects below Te of about 11,500 K are contaminated by significant amounts of He, with abundances sometimes as high as N(He)/N(H) of about 20. This result is interpreted as the result of convective mixing between the thin superficial H layer with the more massive underlying He envelope. The lack of H-rich objects in the sample in the range Te = 7500-11,500 K indicates that most DA white-dwarfs mix near Te of about 11,500 K, a result which is consistent with the location of the red edge of the ZZ Ceti instability strip. A definite trend for cooler objects to have lower He abundances is also observed.

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