A hot white dwarf merger remnant revealed by an ultraviolet detection of carbon

Abstract

Atmospheric carbon has been detected in the optical spectra of six hydrogen-rich ultra-massive white dwarfs, revealing large carbon abundances (log C/H > $-$0.5) attributable to the convective dredge-up of internal carbon into thin hydrogen surface layers. These rare white dwarfs likely originate from stellar mergers, making them "smoking guns" for one of the binary evolution channels leading to thermonuclear supernovae. However, optical spectroscopy can uncover only the most carbon-enriched objects, suggesting that many more merger remnants may masquerade as normal pure-hydrogen atmosphere white dwarfs. Here, we report the discovery of atmospheric carbon in a Hubble Space Telescope far-ultraviolet spectrum of WD0525+526, a long-known hydrogen-rich ultra-massive white dwarf. The carbon abundance (log C/H = $-$4.62) is 4$-$5 dex lower than in the six counterparts and thus detectable only at ultraviolet wavelengths. We find that the total masses of hydrogen and helium in the envelope ($10^{-13.8}$ and $10^{-12.6}$ of the total white dwarf mass) are substantially lower than those expected from single-star evolution, implying that WD0525+526 is a merger remnant. Our modelling indicates that the low surface carbon abundance arises from an envelope structure in which a thin hydrogen-rich layer floats atop a semi-convection zone$-$a process that has been largely overlooked in white dwarfs. Our study highlights the importance of ultraviolet spectroscopy in identifying and characterising merger remnants.

Published in Nature Astronomy on 6th August 2025, see https://www.nature.com/articles/s41550-025-02590-y

References in corpus (30)

• Gaia Data Release 3: Summary of the content and survey properties
• The Transiting Exoplanet Survey Satellite
• The Zwicky Transient Facility: System Overview, Performance, and First Results
• The frequency of planetary debris around young white dwarfs
• The Structure of the Local Interstellar Medium V: Electron Densities
• A catalogue of white dwarfs in Gaia EDR3
• On the Spectral Evolution of Hot White Dwarf Stars. I. A Detailed Model-Atmosphere Analysis of Hot White Dwarfs from SDSS DR12
• Current challenges in the physics of white dwarf stars
• Binospec: A Wide-field Imaging Spectrograph for the MMT
• $Gaia$ white dwarfs within 40 pc II: the volume-limited northern hemisphere sample
• The 40 pc sample of white dwarfs from Gaia
• ${}^{22}$Ne Phase Separation As A Solution To The Ultramassive White Dwarf Cooling Anomaly
• Calibration of the Mixing-Length Theory for Convective White Dwarf Envelopes
• Convective Overshoot and Macroscopic Diffusion in Pure-Hydrogen Atmosphere White Dwarfs
• Multi-Gigayear White Dwarf Cooling Delays from Clustering-Enhanced Gravitational Sedimentation
• An ultra-massive white dwarf with a mixed hydrogen-carbon atmosphere as a likely merger remnant
• Binospec Software System
• The formation of ultra-massive carbon-oxygen core white dwarfs and their evolutionary and pulsational properties
• Buoyant crystals halt the cooling of white dwarf stars
• On the Spectral Evolution of Hot White Dwarf Stars. III. The PG 1159$-$DO$-$DB$-$DQ Evolutionary Channel Revisited
• On the Spectral Evolution of Hot White Dwarf Stars. II. Time-dependent Simulations of Element Transport in Evolving White Dwarfs with STELUM
• Calibration of the mixing length theory for structures of helium-dominated atmosphere white dwarfs
• Kinematic properties of white dwarfs. Galactic orbital parameters and age-velocity dispersion relation
• The spectral evolution of white dwarfs: where do we stand?
• Formation of ultra-massive carbon-oxygen white dwarfs from the merger of carbon-oxygen and helium white dwarf pairs
• Carbon dredge-up required to explain the Gaia white dwarf colour-magnitude bifurcation
• On the formation of hot DQ white dwarfs
• On the Spectral Evolution of Hot White Dwarf Stars. IV. The Diffusion and Mixing of Residual Hydrogen in Helium-rich White Dwarfs
• Atmospheric parameters and carbon abundance for hot DB white dwarfs
• Discovery of a young pre-intermediate polar