While white dwarf mergers have been predicted to occur, this one appears to be particularly unusual. Most of the mergers in our galaxy will be between stars with different masses, whereas this merger appears to be between two similar stars. There is also a limit to how massive the resulting white dwarf can be: at more than 1.4 solar masses it is thought that it would explode in a supernova, though it may be possible that these explosions can occur at slightly lower masses, so this star is useful in demonstrating how massive a white dwarf can get and still survive.
Because the merging process restarts the cooling of the star, it is difficult to determine how old it is. The white dwarf probably merged around 1.3 billion years ago but the two original white dwarfs may have existed for many billions of years prior to this.
It is one of only a handful of merged white dwarfs to be identified so far, and the only one established via its composition.
Dr Hollands adds: "There aren't that many white dwarfs this massive, although there are more than you would expect to see, which implies that some of them were probably formed by mergers."
"In the future we may be able to use a technique called asteroseismology to learn about the white dwarf's core composition from its stellar pulsations, which would be an independent method confirming this star formed from a merger."
"Maybe the most exciting aspect of this star is that it must have just about failed to explode as a supernova – these gargantuan explosions are really important in mapping the structure of the Universe, as they can be detected out to very large distances. However, there remains much uncertainty about which kinds of stellar systems make it to the supernova stage. Strange as it may sound, measuring the properties of this 'failed' supernova, and future look-alikes, is telling us a lot about the pathways to thermonuclear self-annihilation."
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M. A. Hollands, P.-E. Tremblay, B. T. Gänsicke, M. E. Camisassa, D. Koester, A. Aungwerojwit, P. Chote, A. H. Córsico, V. S. Dhillon, N. P. Gentile-Fusillo, M. J. Hoskin, P. Izquierdo, T. R. Marsh, & D. Steeghs, 2020, "An ultra-massive white dwarf with a mixed hydrogen–carbon atmosphere as a likely merger remnant", Nature Astronomy, DOI: 10.1038/s41550-020-1028-0.
"Two stars merged to form massive white dwarf", Univesity of Warwick press release, 2 March 2020.
"Descubren una enana blanca nacida de la fusión de dos estrellas", IAC press release, 2nd March 2020.
Based on observations made with the William Herschel Telescope operated on the island of La Palma by the Isaac Newton Group of Telescopes (ING) in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias (IAC). The ING is funded by the Science and Technology Facilities Council (STFC-UKRI) of the United Kingdom, the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO) of the Netherlands, and the IAC in Spain. IAC's contribution to ING is funded by the Spanish Ministry of Science, Innovation and Universities.