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

被引:69
作者
BERGERON, P [1 ]
WESEMAEL, F [1 ]
FONTAINE, G [1 ]
LIEBERT, J [1 ]
机构
[1] UNIV ARIZONA,STEWARD OBSERV,TUCSON,AZ 85724
关键词
Convection; Stars: abundances; Stars: white dwarfs;
D O I
10.1086/185670
中图分类号
P1 [天文学];
学科分类号
0704 ;
摘要
A determination of the helium 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 ∼ 11,500 K are contaminated by significant amounts of helium, with abundances sometimes as high as N(He)/N(H) ∼ 20. This result is interpreted as the result of convective mixing between the thin superficial hydrogen layer with the more massive underlying helium envelope. The lack of hydrogen-rich objects in our sample in the range 7500 < Te < 11,500 K indicates that most DA white dwarfs mix near Te ∼ 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 helium abundances is also observed. These results are briefly discussed in terms of current models of convective mixing.
引用
收藏
页码:L21 / L24
页数:4
相关论文
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