Two white dwarfs merge into a single unusual star

When the European Space Agency’s Gaia mission released its second catalog of stars in 2019, astronomers took notice of a particular white dwarf named WD J0551+4135. At 1.14 solar masses, it’s larger than most other white dwarfs. (The theoretical limit before gravity collapses one into a neutron star or black hole is 1.4 solar masses.) To learn more about it, Mark Hollands from the University of Warwick in Coventry, UK, and his colleagues acquired spectral measurements (gray solid line in the figure below) using the William Herschel Telescope in the Canary Islands. The data show that the star has another peculiar property: Its surface contains a high ratio of carbon to hydrogen. Hollands and his colleagues now think they know why. The remarkable mass and rare composition of WD J0551+4135 suggests that it formed from the merging of two similarly sized white dwarfs.

Many previously studied white dwarfs contain surface carbon, and that subset usually has a substantial amount of helium, which convectively carries carbon from the core to the surface (see Physics Today, March 2019, page 14 ). But helium was undetectable in WD J0551+4135. A model (red solid line) fitted to the data of the upper layers of the star shows spectrum features that correspond to trace levels of helium (dashed green vertical bar) and a hydrogen-dominated composition (solid blue vertical bars), the latter of which is consistent with many white dwarfs.

The unusually large mass and helium-deficient composition of WD J0551+4135 would be difficult to form from a single parent star, so Hollands and his colleagues suspected that it originated from the merger of two older ones. The radial velocity of white dwarfs tends to increase with age, and the researchers found that WD J0551+4135 is faster than 99% of other nearby white dwarfs. Using the inferred surface temperature, Hollands and his colleagues calculated that the merger probably occurred 1.3 billion years ago. The cores of white dwarfs contain neon, which astronomers are keen to measure using asteroseismology methods. The amount of neon would tell them more conclusively whether WD J0551+4135 originated from a single progenitor star or a merger of two. (M. A. Hollands et al., Nat. Astron., 2020, doi:10.1038/s41550-020-1028-0 .)