A strange white dwarf star rushing through the Milky Way may be the survivor of a “partial supernova,” according to a new study.
White dwarfs are the cool, wispy Earth-sized cores of dead stars that remain after medium-sized stars have run out of fuel and ripped off their outer layers. Our sun will one day become a white dwarf, just like more than 90% of the stars in the Milky Way.
Previous research found that white dwarfs generally have internal structures layered. Its nuclei are mainly carbon and oxygen, which is generally surrounded by a helium layer and then a hydrogen layer. Astronomers examining white dwarfs generally see only hydrogen, only helium, or sometimes a mixture of helium and carbon (although there are exceptions that suggest unusual pasts).
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In a new study, scientists focused on the white dwarf SDSS J1240 + 6710, located about 1,430 light years from Earth. Discovered in 2015, previous work found that this white dwarf had an unusual atmosphere that seemed to possess no hydrogen or helium, but was made up of a strange mixture of oxygen, neon, magnesium, and silicon.
The researchers in this new study used the Hubble Space Telescope to take a closer look at the white dwarf and identified carbon, sodium, and aluminum in the object’s atmosphere. They say that this mix distinguishes this white dwarf from any other previously known.
In this study, scientists also discovered that the white dwarf was traveling at about 560,000 miles per hour (900,000 kilometers per hour) in the opposite direction from the way the galaxy rotates. Also, it had an especially low mass for a white dwarf – only about 40% of the mass of our sun.
“When we discovered that this unusual white dwarf had very little mass and was moving very fast, that really piqued my curiosity about what happened to it in the past,” said study lead author Boris Gänsicke, an astrophysicist at the University of Warwick in England. Space.com
What could explain all these strange details about this white dwarf? The researchers in this study think that a thermonuclear explosion did not completely destroy the white dwarf, but that a “partial supernova” destroyed what was left of the object through the Milky Way.
Supernovae are the most powerful stellar explosions in the universe, bright enough to momentarily overshadow entire galaxies. They can occur when a white dwarf dies from extracting too much mass from a companion star. All of the extra weight of the stolen mass squeezes the nucleus of the white dwarf, increasing the temperature and density of the nucleus high enough to trigger a thermonuclear chain reaction that explosively destroys the white dwarf.
In the case of SDSS J1240 + 6710, scientists noted that the elements seen in the white dwarf’s atmosphere could have been produced in the first thermonuclear reactions of a supernova. However, there is a clear absence of what is known as the group of elements of iron: iron, nickel, chromium, and manganese.
These heavier elements are normally cooked from lighter elements, and their absence in this white dwarf suggests that it only passed through a supernova, failing to reach the high temperatures and densities necessary to forge elements of the iron group.
“That is what makes this white dwarf unique: it suffered a nuclear fire, but stopped before reaching iron,” said Gänsicke. “When she had her supernova event, it was probably brief, maybe a couple of hours.”
The researchers suggested that SDSS J1240 + 6710 was small compared to white dwarfs that normally experience thermonuclear supernovae. As such, only a miniature supernova, an Iax type, could have occurred, a partial supernova weak enough to leave most of a white dwarf behind.
“In the old days, researchers would have thought that a thermonuclear supernova would destroy a white dwarf entirely, but in the past 10 to 15 years, scientists have discovered that a partial supernova may occur that leaves part of the white dwarf burned. and charred, “said Gänsicke. “The explosion is not powerful enough to totally disrupt the star.”
This explosion would have destroyed his partner’s SDSS J1240 + 6710, ripping matter from the small white dwarf and hurling it into deep space at the rate at which it was orbiting its partner, Gänsicke said. This scenario would help explain the speed, insignificant size, and strange atmosphere of the white dwarf.
Based on the mass and temperature of SDSS J1240 + 6710, scientists estimated that this partial supernova occurred about 40 million years ago. Much is still unknown about the white dwarf’s companion, but researchers think it could have been a white dwarf like SDSS J1240 + 6710.
Previous research into the origins of thermonuclear supernovae focused largely on the largest white dwarfs. Now that this new study suggests that smaller white dwarfs may experience similar explosions, future models could explore what these outbursts and their subsequent remains might look like, Gänsicke said.
“Thanks to the Gaia space mission, which was able to identify more than 50,000 white dwarf candidates, we can examine these white dwarfs to get a much better idea of what happens during this type of partial supernova, such as what the products of burning are” Gänsicke said. “Hopefully we can identify a few dozen similar systems. We can start to go from an unusual atypical to a small class of systems.”
Future research could also explore whether astronomers have already detected the brief flashes of light that Gänsicke and his colleagues suspect are related to this type of strange white dwarf.
“It will be interesting to see if they will be able to find these very short supernova-type events that were probably ruled out so far because they didn’t look like supernovae,” said Gänsicke. “Because they were so short, the chances of capturing one were very slim, and there was very little time to track whether any of those detections were real. But, in principle, the data suggesting that these events are real exist somewhere part”.
The scientists detailed their findings online July 15 in the Monthly Notices of the Royal Astronomical Society.
Follow Charles Q. Choi on Twitter @cqchoi. Follow us on Twitter @Spacedotcom and on Facebook.
