Two White Dwarfs with Oxygen-Rich Atmospheres

Stars Going Quietly or with a Bang Stars with masses seven to ten times the mass of the Sun, which can burn carbon in their cores at their end of their lives, may end up as oxygen-neon core white dwarfs or explode as core-collapse supernovae. The defining line between these two end products of stellar evolution is not well understood. Gänsicke et al. (p. 188; published online 12 November) identified two white dwarfs whose photospheric oxygen-to-carbon abundance ratio exceeds unity. Their low carbon abundance and their large quantity of oxygen imply that they are oxygen-neon white dwarfs that lost their hydrogen envelopes. As such, they may have evolved from stars at the borderline between stars that explode as supernova and stars that form white dwarfs. Two white dwarfs may have evolved from intermediate-mass stars that avoided exploding as supernovae. Stars with masses ranging from 7 to 10 times the mass of the Sun end their lives either as massive white dwarfs or weak type II supernovae, but there are only limited observational constraints on either evolutionary channel. Here we report the detection of two white dwarfs with large photospheric oxygen abundances, implying that they are bare oxygen-neon cores and that they may have descended from the most massive progenitors that avoid core collapse.