An ultramassive white dwarf half Earth's size may hold a rare oxygen-neon core

Scientists have discovered compelling evidence suggesting an ultramassive white dwarf, approximately half the size of Earth, may possess an oxygen-neon core. This discovery, published in The Astrophysical Journal, is significant as the core composition dictates a white dwarf's evolution. Typically, white dwarfs, with masses of 0.5–0.7 times the sun's, possess carbon-oxygen cores, but ultramassive white dwarfs, exceeding 1.05–1.1 solar masses, present a different scenario.

These larger white dwarfs, born from progenitor stars weighing between 8–10 times the sun, are theorized to have oxygen-neon cores due to higher temperatures and densities during their core fusion process. The team led by Stefan M. Arseneau of Boston University studied the white dwarf SDSS J060851.44-005950.3, which resides in a binary system with a distant main-sequence companion star.

Due to the white dwarf's small visible photosphere, its core composition could only be inferred indirectly. The team utilized gravitational redshift, the stretching of light wavelengths as photons escape the intense gravity of the white dwarf, and its motion through space to estimate the mass-to-radius ratio. By combining this data with brightness measurements from various surveys and running statistical simulations, they concluded the white dwarf likely has an oxygen-neon core rather than the more common carbon-oxygen core.

This finding also rules out the possibility of the high mass resulting from a white dwarf merger, as the star's characteristics align more with normal single-star evolution. The researchers note that further data, such as improved Gaia observations or additional ultraviolet data, could refine this conclusion.