SDSS J060851.44-005950.3 Ultramassive White Dwarf Favors Oxygen-Neon Core with Bayes Factor 2.7

Astronomers extracted gravitational redshifts from high-resolution spectra of the ultramassive white dwarf SDSS J060851.44-005950.3 and combined them with photometric constraints to determine precise mass and radius. These values were compared against state-of-the-art mass-radius relations calculated separately for carbon-oxygen and oxygen-neon core compositions. The analysis isolates core composition as the dominant variable while holding envelope parameters fixed.

The measured parameters align more closely with oxygen-neon models, producing a Bayes factor of 2.7 in their favor. At 1.226 solar masses this object sits above the threshold where carbon-oxygen cores can ignite as Type Ia supernovae under current single-degenerate or double-degenerate channels. Its location on the Q-branch without cooling delay further supports the structural interpretation of an oxygen-neon interior.

This result tightens the mapping between the Q-branch and core composition, implying that many ultramassive white dwarfs traversing this region are structurally barred from thermonuclear explosion. It also highlights the need for improved three-dimensional evolutionary models that track the transition from super-asymptotic-giant-branch progenitors to oxygen-neon cores.

Targeted high-resolution spectroscopy of additional Q-branch candidates above 1.2 solar masses will test whether the oxygen-neon fraction rises sharply with mass, refining both supernova rate predictions and the initial-final mass relation at its upper end.