Astronomers have discovered a star like none seen before. They think it may be the remnant of an incomplete supernova explosion, something that would explain one mystery, but create enough new ones to keep them happily hunting for a long time.
LP 40-365 is a 15th magnitude star, making it far too faint to see with the naked eye, but much brighter than the vast majority of objects recorded in sky maps. Catalogs of stars registered the fact that it appears to be traveling very rapidly compared to the rest of the galaxy, and interested by such speedster stars, Professor Lilia Ferrario of the Australian National University and some fellow astronomers decided to check it out.
What they found, Ferrario told IFLScience, was more fascinating than anyone expected. LP 40-365’s light spectrum clearly marks it as a white dwarf, but one smaller than any we have seen before – just 0.14 times the mass of the Sun. “White dwarfs are extremely compact stars, where gravitational collapse is prevented by the pressure exerted by a gas of degenerate electrons. A teaspoon of white dwarf material would weigh many tonnes on Earth,” Ferrario said in a statement.
White dwarfs are the remnants of medium to large stars that have run out of fuel. They are called dwarfs because they are very compact, but, even with all the material they have lost, their masses are close to that of the Sun.
LP 40-365 “must have interacted with something to end up the way it is,” Ferrario told IFLScience.
The explanation Ferrario and her colleagues have presented in Science is that LP 40-365 is the result of what is called a Iax supernova. Type Ia supernovas are thought to occur when a white dwarf star that is part of a binary system with another star draws gas away from its companion until it has so much it explodes, although a subclass is triggered by the collision of two white dwarfs.
Iax explosions have a very similar spectrum, but are substantially fainter. Astronomers have speculated that these represent incomplete explosions, created by similar conditions to normal Type Ias, but leaving some of the precursor star behind. LP 40-365 looks to be the first good evidence we have to back this theory up, making it a truly wonderful find.
The explosion gave LP 40-365 its estimated speed of 546 kilometers per second (340 miles per second), enough to ensure it will eventually escape gravitational pull of the galaxy, and heated it to hundreds of thousands of degrees. The dwarf fragment has now cooled to a temperature of about 10,000 Kelvin – still hotter than the Sun, despite the fact that it is not generating any new energy. Ferrario told IFLScience there are wide error bars on the rate of cooling for an object so much smaller than other white dwarfs, but the explosion happened sometime between 5 and 50 million years ago.
As to what would cause a Ia explosion to not reach completion, no one knows, and Ferrario said the authors will attempt to model the event. Since they are fainter than other supernovae, there are probably a lot of Iax events we don't see, so estimating how common they are compared to the complete version is also not yet possible.
Ferrario doubts there are other large fragments from the explosion that produced LP 40-365 to be found, but thinks the companion star that triggered its explosion probably also flew off at high speed. Finding it, she added would allow us to establish the time and place of the location, but will be like looking for the proverbial needle in a haystack. Not only would the companion presumably be traveling in roughly the opposite direction and therefore much further away than LP 40-365's current distance of 970 light years from Earth, but we have only a faint idea of where to look.
1