Stars with Unusual Compositions: Carbon and Oxygen in Cool White Dwarfs

Abstract

White dwarfs represent the end products of the evolution of all stars on the main sequence that had initial masses below ∼8M⊙. After some important mass-loss episodes in the red giant phases, followed by the end of thermonuclear activity in the interiors of such stars, they ultimately shrink to Earth-size objects with masses of about 0.4 to 1.2 M⊙. The vast majority of them are believed to be composed mainly (i.e, more than 99% by mass) of carbon and oxygen, the products of hydrogen and helium nuclear burning. Intuitively, then, one might assume that it would not be surprising to observe a significant amount of carbon and oxygen in white dwarf photospheres. However, nuclear burning and mass loss episodes do not consume 100% of the hydrogen and helium initially present in each star at birth. With a surface gravity of the order of log g ∼8, gravitational settling in white dwarf stars is quite efficient and the light elements leftover from previous evolutionary phases rapidly float to the surface, while heavier elements sink out of sight. Since there is ultimately more than enough hydrogen and helium to form an optically thick photosphere, it is not possible to directly observe the white dwarf core material. Thus, the majority of white dwarfs are thus found to have a surface composition that is completely pure in hydrogen or helium.