Stars can get flung hither and yon through a variety of processes, and in a new paper in the Monthly Notices of the Royal Astronomical Society with lead other Boris Gansicke, a truly odd star has been caught fleeing at breakneck speeds across our own galaxy. This object is traveling at 900,000 km/hr and appears to be the survivor of a supernova explosion.
This tiny star – tiny even for a white dwarf – is about the size of the Moon and 40% the mass of the Sun, and its surface appears composed of an unusual mix of oxygen, neon, magnesium, and silicon with a side of carbon, sodium, and aluminum. All these elements can be produced in a thermonuclear supernova.
According to Gainsicke: This star is unique because it has all the key features of a white dwarf but it has this very high velocity and unusual abundances that make no sense when combined with its low mass. It has a chemical composition which is the fingerprint of nuclear burning, a low mass, and a very high velocity: all of these facts imply that it must have come from some kind of close binary system, and it must have undergone thermonuclear ignition. It would have been a type of supernova but of a kind that we haven’t seen before. If it was a tight binary and it underwent thermonuclear ignition, ejecting quite a lot of its mass, you have the conditions to produce a low mass white dwarf and have it fly away with its orbital velocity.
To get this kind of an event, two stars need to be in a close binary, where a white dwarf can feed on its companion star until it reaches a mass at which it can ignite as a supernova. In this situation, however, it appears that ignition was incomplete and worked more like a nuclear bomb-powered rocket engine attached to the small star. As the press release summarizes: SDSSJ1240+6710 may be the survivor of a type of supernova that hasn’t yet been “caught in the act”. Without the radioactive nickel that powers the long-lasting afterglow of the Type Ia supernovae, the explosion that sent SDSS1240+6710 hurtling across our Galaxy would have been a brief flash of light that would have been difficult to discover.
More Information
University of Warwick press release
“SDSS J124043.01+671034.68: The Partially Burned Remnant of a Low-Mass White Dwarf that Underwent Thermonuclear Ignition?” Boris T. Gänsicke et al., 2020 July 15, Monthly Notices of the Royal Astronomical Society (Preprint on arxiv.org)
