Most stars circle the galaxy's core at a relatively stately pace, but some are leaving like they're afraid a cosmic police force is chasing them. A newly discovered member of this group, LP 40-365, turns out to have many other unusual characteristics that make it a feast for astronomical investigation.
Fast-moving stars are thought to result from being close by when a supernova occurred, but LP 40-365 shows evidence of being the remnant piece of a supernova that only partially happened.
The idea LP 40-365 was accelerated by a supernova explosion was proposed in 2017, and is so widely accepted it has been used as a comparison for other candidates for the class. Now, in Astrophysical Journal Letters, astronomers explored the question of its likely role in the explosion.
“LP 40−365 […] is an exceptionally peculiar star: it is one of the most metal-rich stars known, with an atmosphere dominated by oxygen and neon that is also abundant in heavy elements from partial oxygen and silicon burning, with no detectable hydrogen or helium,” the paper reports.
The fact it is a white dwarf – a stellar core where fusion has ceased – with a mass 28 percent that of the Sun doesn't rate among its oddities. It's exiting the galaxy at 850 kilometers per second (1.9 million mph), but unlike other “hypervelocity stars” it didn't originate from the galactic center.
LP 40-365's speed is attributed to a Type Iax supernova explosion, thought to occur when a white dwarf draws material off a companion star, but doesn't fully explode as an ordinary Type Ia would.
This explanation was once theoretical, but the discovery of both LP 40-365 and an apparent remnant nebula have increased astronomers' confidence. The shockwave from the supernova sends the two stars speeding in opposite directions, rather than consuming the companion star entirely. This opens the question of whether a fast-moving star is the one that partially exploded or the “donor” that material was drawn from to produce the conflagration.
The paper reports that the Hubble and TESS space telescopes reveal a 1 percent variation in LP 40-365's visible brightness on a nine-hour cycle – considerably larger in the ultraviolet.
Although TESS was designed to look for planets blocking their star on a regular basis, this doesn't look like that kind of scenario. Instead, the authors think LP 40-365 is rotating every nine hours and has a “surface inhomogeneity” whose appearance and disappearance shapes its brightness.
The rotation speed leads researchers to conclude LP 40-365 was probably the star that partially exploded, rather than the smaller neighbor.
"To have gone through partial detonation and still survive is very cool and unique, and it's only in the last few years that we've started to think this kind of star could exist," study author and Boston University graduate student Odelia Putterman said in a statement.
"By understanding what's happening with this particular star, we can start to understand what's happening with many other similar stars that came from a similar situation."