An international team of astronomers has found that M31N 2008-12a, a frequently erupting ‘nova’ located in the Andromeda Galaxy, is surrounded by a 400-light-year-wide super-remnant. Larger than almost all known remnants of even supernova explosions, the existence of this super-remnant demonstrates that M31N 2008-12a has erupted with high frequency for millions of years.
The Steward 2.3-m Bok Telescope image of M31N 2008-12a and its surrounding super-remnant. Image credit: Darnley et al, doi: 10.1038/s41586-018-0825-4.
“A white dwarf is the dead core of a star. When it is paired with a companion star in a binary system, it can potentially produce a nova explosion,” said Liverpool John Moores University researcher Matt Darnley and co-authors.
“If the conditions are right, the white dwarf can pull gas from its companion star and when enough material builds up on the surface of the white dwarf, it triggers a thermonuclear explosion or ‘nova,’ shining a million times brighter than our Sun and initially moving at up to 10,000 km per second.”
Dr. Darnley and colleagues used Hubble Space Telescope imaging, accompanied by spectroscopy from telescopes on Earth, to uncover the nature of a super-remnant surrounding the M31N 2008-12a nova.
“Our result is significant, as it is the first such remnant that has been found around a nova,” said Dr. Steven Williams, an astronomer at Lancaster University.
“This nova also has the most frequent explosions of any we know — once a year. The most frequent in our own Galaxy in only once every 10 years.”
“It also has potential links to Type Ia supernovae, as this is how we would expect a nova system to behave when it is nearly massive enough to explode as a supernova.”
“A Type Ia supernova is caused when the entire white dwarf is blown apart when it reaches a critical upper mass, rather than an explosion on its surface as in the case of the nova in our work.”
M31N 2008-12a and its surrounding super-remnant. Left: the Liverpool Telescope image of the region surrounding M31N 2008-12a; the position of M31N 2008-12a is marked and the offset from the geometric center is indicated by the black line. Center: the Hubble image of the same region; the large square shows the bright western shell, and the small square the eastern ‘knot.’ Right: zoomed-in Hubble image showing the region within the large red box; at the top of this panel three long ‘nested’ filaments are discernible. Image credit: Darnley et al, doi: 10.1038/s41586-018-0825-4.
The scientists simulated how such a nova can create a vast, evacuated cavity around the star, by continually sweeping up the surrounding medium within a shell at the edge of a growing super-remnant.
The models show that the super-remnant is consistent with being built up by frequent nova eruptions over millions of years.
“Studying M31N 2008-12a and its super-remnant could help us to understand how some white dwarfs grow to their critical upper mass and how they actually explode as a Type Ia Supernova once they get there,” Dr. Darnley said.
“Type Ia supernovae are critical tools used to work out how the Universe expands and grows.”
The findings appear in the journal Nature.
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M.J. Darnley et al. A recurrent nova super-remnant in the Andromeda galaxy. Nature, published online January 9, 2019; doi: 10.1038/s41586-018-0825-4
