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

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.