Warwick: Two Stars Merged to Form Massive White Dwarf

[bystander]

Two Stars Merged to Form Massive White Dwarf
University of Warwick, UK | 2020 Mar 02

White dwarf that is two-thirds the size of the Earth is actually two stars merged; Scientists spotted unusual phenomena by the star's spectral signature; Star is massive enough to account for two average sized white dwarfs and may just about have failed to explode as a supernova; Provides evidence for the existence of more ultra-massive white dwarfs with hidden merger histories.

Artist's impression of two white dwarfs in the process of merging. Depending on the
combined mass, the system may explode in a thermonuclear supernova, or coalesce
into a single heavy white dwarf, as with WDJ0551+4135.
Credit: University of Warwick/Mark Garlick

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A massive white dwarf star with a bizarre carbon-rich atmosphere could be two white dwarfs merged together according to an international team led by University of Warwick astronomers, and only narrowly avoided destruction.

They have discovered an unusual ultra-massive white dwarf around 150 light years from us with an atmospheric composition never seen before, the first time that a merged white dwarf has been identified using its atmospheric composition as a clue.

The discovery ... could raise new questions about the evolution of massive white dwarf stars and on the number of supernovae in our galaxy.

This star, named WDJ0551+4135, was identified in a survey of data from the European Space Agency’s Gaia telescope. The astronomers followed up with spectroscopy taken using the William Herschel Telescope, focusing on those white dwarfs identified as particularly massive – a feat made possible by the Gaia mission. By breaking down the light emitted by the star, the astronomers were able to identify the chemical composition of its atmosphere and found that it had an unusually high level of carbon present. ...

An Ultra-Massive White Dwarf with a Mixed Hydrogen-Carbon Atmosphere as a Likely Merger Remnant ~ M.A. Hollands et al

• Nature Astronomy (online 02 Mar 2020) DOI: 10.1038/s41550-020-1028-0

[saturno2]

Why this massive white dwarf has more carbon than normal in its atmosphere?

[Ann]

Why this massive white dwarf has more carbon than normal in its atmosphere?

I wish I knew, Saturno.

I suppose the merging process may have produced a lot of carbon. But as I said, I don't know.

Ann

[bystander]

Why this massive white dwarf has more carbon than normal in its atmosphere?

Perhaps one of the white dwarf progenitors went through a shell helium flash, leaving it with a carbon rich atmosphere, prior to the binary white dwarf merger.

Just speculation.

[neufer]

Why this massive white dwarf has more carbon than normal in its atmosphere?

• 1) Massive white dwarfs should have formed from 8 to 10.5 M_☉ progenitor stars that had already fused their carbon to neon.
  
  2) Most carbon/oxygen white dwarfs should have had time for the carbon/oxygen to have settled down to the core such that it would not be detected in spectroscopy.

Hence, this massive white dwarf may be a recent merger of lighter carbon/oxygen white dwarfs.

<<Usually, white dwarfs are composed of carbon and oxygen. If the mass of the progenitor is between 8 and 10.5 solar masses (M_☉), the core temperature will be sufficient to fuse carbon but not neon, in which case an oxygen–neon–magnesium white dwarf may form. The material in a white dwarf no longer undergoes fusion reactions.

Although most white dwarfs are thought to be composed of carbon and oxygen, spectroscopy typically shows that their emitted light comes from an atmosphere which is observed to be either hydrogen or helium dominated. As explained by Schatzman in the 1940s, the high surface gravity is thought to cause this purity by gravitationally separating the atmosphere so that heavy elements are below and the lighter above.

White dwarfs whose primary spectral classification is DA have hydrogen-dominated atmospheres. They make up the majority, approximately 80%, of all observed white dwarfs. The next class in number is of DBs, approximately 16%. The hot, above 15,000 K, DQ class (roughly 0.1%) have carbon-dominated atmospheres. Those classified as DB, DC, DO, DZ, and cool DQ have helium-dominated atmospheres.>>