Using the NASA/ESA Hubble Space Telescope and NASA’s Transiting Exoplanet Survey Satellite (TESS), astronomers have detected 8.9-hr variability in both optical and ultraviolet light curves of the recently-discovered hypervelocity white dwarf LP 40-365.
An artist’s impression of a thermonuclear supernova event. Image credit: ESA / ATG Medialab / C. Carreau.
LP 40-365 is currently located some 2,000 light-years away in the constellation of Ursa Minor.
Also known as GD 492, it belongs to a class of partly burnt runaway stars that have been ejected from a binary system by a thermonuclear supernova event.
“This star is moving so fast that it’s almost certainly leaving the Milky Way Galaxy. It’s moving almost 852 km/s,” said Dr. J.J. Hermes, an astronomer in the Department of Astronomy & Institute for Astrophysical Research at Boston University.
“But why is this flying object speeding out of the Milky Way? Because it’s a piece of shrapnel from a past explosion that’s still being propelled forward.”
“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,” added Odelia Putterman, also from the Department of Astronomy & Institute for Astrophysical Research at Boston University.
By looking at various kinds of light data from Hubble and TESS telescopes, the astronomers found that LP 40-365 is not only being hurled out of our Galaxy, but based on the brightness patterns in the data, is also rotating on its way out.
“The star is basically being slingshotted from the explosion, and we’re observing its rotation on its way out,” Putterman said.
“All stars rotate — even our Sun slowly rotates on its axis every 27 days. But for a star fragment that’s survived a supernova, nine hours is considered relatively slow,” Dr. Hermes added.
Based on LP 40-365’s relatively slow rotation rate, the team feels more confident that it is a shrapnel from the star that self-destructed after being fed too much mass by its partner, when they were once orbiting each other at high speed.
Because the stars were orbiting each other so quickly and closely, the explosion slingshotted both stars, and now we only see LP 40-365.
“Our paper adds one more layer of knowledge into what role these stars played when the supernova occurred, and what can happen after the explosion,” Putterman said.
“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.”
“Stars like LP 40-365 are not only some of the fastest stars known to astronomers, but also the most metal-rich stars ever detected,” Dr. Hermes said.
“Stars like our Sun are composed of helium and hydrogen, but a star that has survived a supernova is primarily composed of metals, because what we’re seeing are the by-products of violent nuclear reactions that happen when a star blows itself up, making star shrapnel like this especially fascinating to study.”
The findings appear in the Astrophysical Journal Letters.
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J.J. Hermes et al. 2021. 8.9 hr Rotation in the Partly Burnt Runaway Stellar Remnant LP 40-365 (GD 492). ApJL 914, L3; doi: 10.3847/2041-8213/ac00a8
