A dead star that is careening across the Milky Way at 900,000 kilometers per hour is likely a “supernova survivor” that blasted itself out of an orbital bond with another star, according to a new study.
The speedy object, which is known as SDSS J1240+6710, suggests that some stars might blow up in half-cocked “partial” supernova events that are too short-lived for our telescopes to detect.
SDSS J1240+6710, which is located about 1,430 light years from Earth, has been puzzling scientists ever since it was first discovered in 2015. It’s a white dwarf, which is a type of dense stellar husk that is left over from the deaths of stars like the Sun. Most white dwarfs have atmospheres composed of light elements, such as hydrogen and helium, but this one has an atmosphere made of almost pure oxygen, which suggests that it has an exotic origin.
An international team led by Boris Gaensicke, an astrophysicist at the University of Warwick in the UK, used the European Space Agency’s Gaia spacecraft to clock the breakneck speed of the white dwarf. In addition to traveling very quickly, SDSS J1240+6710 is moving in the opposite direction of the Milky Way’s rotational flow, which implies that it was sent hurtling through the galaxy by a pyrotechnic blast.
“We tentatively suggest that SDSS J1240+6710 is the partially burned remnant of a low-mass white dwarf that underwent a thermonuclear event,” the researchers said in their paper, which was published online on Tuesday in Monthly Notices of the Royal Astronomical Society.
Gaensicke and his colleagues conducted follow-up observations of the object in 2017 with the Hubble Space Telescope, which revealed its “unique atmospheric composition,” according to the study. In addition to the strange oxygen-dominant atmosphere, SDSS J1240+6710 appears to lack heavy elements such as iron, cobalt and nickel, which would normally be present in a white dwarf that had undergone a successful supernova.
The most likely explanation, according to Gaensicke’s team, is that this white dwarf used to closely orbit a companion star in a tight binary system. An explosion some 40 million years ago broke the bonds of this pair and sent SDSS J1240+6710 and its unknown companion rocketing off in opposite directions.
Unlike neutron stars or black holes, which are formed by massive stars blowing up as supernovae, white dwarfs are the exposed cores of stars that have ejected their outer layers without the fanfare of a supernova blast. After their progenitor star dies, however, white dwarfs can undergo a type of explosion called a type Ia supernova, assuming that they are part of a binary star system.
These special supernovae are sparked when white dwarfs pull gassy material off of their companion stars, a process that builds pressure on the dwarfs until they burst into extremely luminous flare-ups.
SDSS J1240+6710 is about 40 percent as massive as the Sun, which is relatively light for a white dwarf. The combination of its high speed, low mass, and lack of heavy elements add up to an origin story in which the white dwarf half-exploded, but was repelled into space before it burned out its heaviest elements.
This partial supernova would have only lasted a few hours, in comparison to normal type Ia supernova that shine for days or weeks. As a result, it would be “extremely challenging” to try to detect these short explosions, the team said in the study, and we may only be able to learn about such partial ignition events by finding survivors like SDSS J1240+6710.
"We are now discovering that there are different types of white dwarf that survive supernovae under different conditions,” said Gaensicke in a statement.
“There is clearly a whole zoo out there,” he added. “Studying the survivors of supernovae in our Milky Way will help us to understand the myriads of supernovae that we see going off in other galaxies."