A white dwarf star called SDSS J124043.01+671034.68 (SDSS J1240+6710) is traveling at 900,000 km/h (559,234 mph) through our Milky Way Galaxy. It also has a particularly low mass for a white dwarf — only 40% the mass of our Sun — which would be consistent with the loss of mass from a partial supernova. According to new research carried out by Boris T. Gänsicke and colleagues, SDSS J1240+6710 was most likely a member of a binary system that survived a so-called thermonuclear supernova event, which sent it and its companion flying through the Milky Way in opposite directions.
An artist’s impression of a thermonuclear supernova: the material ejected by the supernova will initially expand very rapidly, but then gradually slow down, forming an intricate giant bubble of hot glowing gas; eventually, the charred remains of the white dwarf that exploded will overtake these gaseous layers, and speed out onto its journey across our Milky Way Galaxy. Image credit: Mark Garlick / University of Warwick.
White dwarfs are the remaining cores of red giants after these huge stars have died and shed their outer layers, cooling over the course of billions of years.
The majority of white dwarfs have atmospheres composed almost entirely of hydrogen or helium, with occasional evidence of carbon or oxygen dredged up from the star’s core.
SDSS J1240+6710, which was discovered in 2015, lies 1,432 light-years away from us in the constellation of Draco.
Also known as WD 1238+674 and LSPM J1240+6710, the star was previously found to have an oxygen-dominated atmosphere with significant traces of neon, magnesium, and silicon. It 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.
Using the Cosmic Origin Spectrograph onboard the NASA/ESA Hubble Space Telescope, Professor Gaensicke and colleagues identified carbon, sodium, and aluminum in the atmosphere of SDSS J1240+6710, all of which are produced in the first thermonuclear reactions of a supernova.
However, there is a clear absence of what is known as the ‘iron group’ of elements, iron, nickel, chromium and manganese.
These heavier elements are normally cooked up from the lighter ones, and make up the defining features of thermonuclear supernovae.
The lack of iron group elements in SDSSJ1240+6710 suggests that the star only went through a partial supernova before the nuclear burning died out.
The authors theorize that the supernova disrupted the white dwarf’s orbit with its partner star when it very abruptly ejected a large proportion of its mass.
Both stars would have been carried off in opposite directions at their orbital velocities in a kind of slingshot maneuver. That would account for the star’s high velocity.
The best studied thermonuclear supernovae are the Type Ia. But there is growing evidence that thermonuclear supernovae can happen under very different conditions.
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.
The study of thermonuclear supernovae is a huge field and there’s a vast amount of observational effort into finding supernovae in other galaxies. The difficulty is that we can see the star when it explodes but it’s very difficult to know its properties before it exploded.
The fact that such a low mass white dwarf went through carbon burning is a testimony of the effects of interacting binary evolution and its effect on the chemical evolution of the Universe.
References: Boris T Gänsicke, Detlev Koester, Roberto Raddi, Odette Toloza, S O Kepler, “SDSS J124043.01 + 671034.68: the partially burned remnant of a low-mass white dwarf that underwent thermonuclear ignition?”, Monthly Notices of the Royal Astronomical Society, Volume 496, Issue 4, August 2020, Pages 4079–4086..