A STUDY OF METAL ABUNDANCE PATTERNS IN COOL WHITE-DWARFS .3. COMPARISON OF THE PREDICTIONS OF THE 2-PHASE ACCRETION MODEL WITH THE OBSERVATIONS

We compare the predictions of our two-phase accretion model with all the observations currently available on the metal-line phenomenon in cool white dwarfs. The model is based on a simplified picture of the interstellar medium, but makes specific predictions as to the presence of metals in the photospheres of these stars. We find that the gross observed metal abundance patterns are very well accounted for within the framework of the model. For many individual stars, it is also possible to infer directly the accretion rates necessary to maintain the photospheric metal abundances. These rates agree remarkably well with the estimates of the accretion rates from denser patches of the interstellar medium which we used in our idealized numerical simulations, thus providing an important self-consistency check. While we have assumed a priori that metals accrete in solar proportions, our final analysis suggests the possibility of mild differentiated accretion for the metals. We also compute explicitly the detection probability for metals in cool white dwarfs and find an excellent agreement with the observed frequency of calcium-line objects in the known sample of cool He-rich white dwarfs. We discuss the question of traces of hydrogen in these stars. We find that hydrogen, like the metals, must be accreted. However, hydrogen accretion must be reduced by factors of 10(2)-10(7) with respect to metal accretion to account for the observations. The idea of screening mechanisms thus surfaces again; hydrogen screening must, however, operate over the full range of effective temperature 20,000 K greater than or similar to T(eff) greater than or similar to 5000 K. We also discuss the case of carbon in cool He-rich white dwarfs. Although some carbon, along with metals and hydrogen, must also accrete onto these stars, the accretion model cannot explain the peculiar carbon abundance pattern observed in such objects. The diffusion-dredge-up pollution model is to be preferred for that element. Furthermore, there is a hint that carbon accretion is also reduced with respect to metal accretion. This suggests that further progress could be made by studying the process of fractionated accretion.