Search for Oxygen in Cool DQ White Dwarf Atmospheres

The existence of carbon in cool He white dwarf (WD) atmospheres has been known for a relatively long time[5]. The presence of carbon in these atmospheres is explained by convective dredge up of interior carbon [6]. Pelletier et al. (1986) presented the first detailed calculations of this process and showed that carbon diffuses upwards from the core into the base of the He-rich envelope where it can be dredged up by a surface convection zone. As the temperature of the star decreases, more carbon diffuses upward, and the base of convection zone moves deeper into the star, further enriching the surface layers with carbon [2]. The diffusion time scales of C and O do not differ by large amounts, and are found to be essentially the same for some models [3]. We expect all non-interacting WDs with 0.45 < M wd /M ⊙ < 1.1 to have C/O cores. Since there is oxygen in these cores, there is a priori no reason to prevent the dredge up of oxygen as well as that of carbon. This paper reports new infrared spectroscopic observations of DQ WDs searching for oxygen and describes model atmospheres for these stars.