The ratio of helium- to hydrogen-atmosphere white dwarfs: Direct evidence for convective mixing

We determine the ratio of helium- to hydrogen-atmosphere white dwarf stars as a function of effective temperature from a model atmosphere analysis of the infrared photometric data from the Two Micron All Sky Survey combined with available visual magnitudes. Our study surpasses any previous analysis of this kind, both in terms of the accuracy of the T-eff determinations and the size of the sample. We observe that the ratio of helium- to hydrogen-atmosphere white dwarfs increases gradually from a constant value of similar to 0.25 between T-eff = 15,000 and 10,000 K to a value twice as large in the range 10,000 K > T-eff > 8000 K, suggesting that convective mixing, which occurs when the bottom of the hydrogen convection zone reaches the underlying convective helium envelope, is responsible for this gradual transition. The comparison of our results with an approximate model used to describe the outcome of this convective mixing process implies hydrogen mass layers in the range M-H/M-tot = 10(-10) to 10(-8) for about 15% of the DA stars that survived the DA-to-DB transition near T-eff similar to 30,000 K, the remainder having presumably more massive layers above M-H/M-tot similar to 10(-6).