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UK astronomers have contributed to new research that has identified for the first time ever a merged white dwarf star.

This massive white dwarf star, with a bizarre carbon-rich atmosphere that is around 150 light years from earth, has been discovered by an international team led by STFC funded astronomers from the University of Warwick. The team spotted unusual phenomena in the star’s spectral signature and realised that the white dwarf was actually two stars that had merged. This is 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. It also provides evidence for the existence of more ultra-massive white dwarfs with hidden merger histories. The star is two-thirds the size of the Earth, is one of only a handful of merged white dwarfs to be identified so far, and the only one via its composition.

This star, named WDJ0551+4135, was identified in a survey of data from the European Space Agency’s Gaia telescope. The UK, through STFC, is one of the teams across Europe that are contributing to the processing of the vast amounts of data coming from the Gaia mission. The astronomers followed up with spectroscopy taken using the STFC funded and run William Herschel Telescope, focusing on those white dwarfs identified as particularly massive – a feat made possible by the Gaia space 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. Lead author Dr Mark Hollands, from the University of Warwick Department of Physics, recently said:

“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 what kind 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.”

The research was funded by the Science and Technologies Facilities Council (STFC) part of UK Research and Innovation (UKRI) and the European Research Council.

Further information:

• 'An ultra-massive white dwarf with a mixed hydrogen–carbon atmosphere as a likely merger remnant' is published in Nature Astronomy, DOI: 10.1038/s41550-020-1028-0
• The research involved the use of the STFC funded William Herschel Telescope.
• STFC and the ESA GAIA mission https://stfc.ukri.org/news/lift-off-for-billion-star-surveyor/ and https://stfc.ukri.org/news/gaia-results-revealed/

The UK has two major roles in the Gaia mission: it built the spacecraft, and it helps deliver the science.

UK industry and science institutes won some €80 million of industrial contracts to build Gaia, with leadership roles in building the heart of Gaia, the array of 106 CCDs, the control avionics and the critical micro-propulsion system, as well as playing a critical role in the development of the Gaia spectrometer.

Gaia data is now processed and analysed ready for release to the scientific community and public at six data centres, including one in the UK, operating software developed and tested by a consortium of 400 people across Europe, including some 50 people at 6 Institutes in the UK (Cambridge, UCL-MSSL, Leicester, Edinburgh, The Open University, STFC RAL Space and Bristol).

The William Herschel Telescope is a 4.2 metre optical infrared telescope, part of the Isaac Newton Group of Telescopes (ING) on La Palma in the Canary Islands. The ING is owned by STFC and is operated in collaboration with the Netherlands and Spain.

The William Herschel Telescope is the largest optical telescope of its kind in Europe, with a 4.2-metre primary mirror. It is a general-purpose facility, instrumented to allow a wide range of astronomical observations, from the optical wavelengths to the infrared. Through continued development of instrumentation, the William Herschel Telescope remains at the forefront of astronomical research. http://www.ing.iac.es/