The best way these stars look, a supernova is inevitable

Sometimes loud explosions are easier to deal with when you know they are coming. They are also easier to observe. When University of Warwick astronomers found a rare teardrop-shaped star named HD265435, they knew they were waiting for a potential new supernova waiting to happen. The only caveat – it might actually not happen in 70 million years.

That may be an absurdly long time in human terms, but astronomically it is the blink of an eye. Fortunately, TESS, the telescope used to collect the data for the paper describing the system published in Nature Astronomy, doesn’t have to blink. Originally developed to record transits of exoplanets, TESS is particularly good at quantifying slight drops in the brightness of a star. Sometimes these dips are caused by planets moving in front of the star. But there are also other explanations for a drop in brightness.

Current picture from HD 265435 in the star field.
Credit – Simbad / University of Strasbourg

An alternative explanation for the difference in brightness is if the star has been deformed. In the case of HD265435, it is more like a teardrop than the spherical fireball of traditional stars. So what would deform a star into such a shape? The answer seems to be – another star. The star that is visibly losing brightness is called the “hot subdwarf” – a type of star that is slightly less bright than the sun. Calculations made by Dr. Ingrid Pelisoli, the lead author of the article, employed with radial velocity data from Palomar and the Keck Observatory, showed that the relatively bright, hot sub-dwarf star is actually likely to be hiding a much darker companion of the white dwarf.

Given the proximity of the two stars, it seems inevitable that they will eventually collapse. They have already started spinning around each other, spinning around each other every 100 minutes. But what makes this combination particularly interesting is the fact that one of the companions is a white dwarf.

UT video discussing what makes a supernova happen so quickly.

White dwarfs are believed to cause Type 1a supernovae when their core is reignited and burns with a fire so bright and uniform that it helps astronomers determine the rate of expansion of the universe. This can potentially happen if the total mass of the white dwarf and its surrounding stars has a total mass greater than the Chandrasekhar limit, or about 1.4 times the solar mass. The combined mass of the hot sub-dwarf (about 0.6 times the mass of our sun) and the white dwarf (similar to the mass of our sun) exceeds this limit. As such, the system should become a supernova when the mass in the core of the white dwarf is crushed by gravity.

This system is one of the few that is expected to produce supernovae in the (relatively) near future. And this is the only one that contained a hot binary system of sub-dwarfs and white dwarfs. Estimates for the total number of supernovae expected in a certain area can be obtained both from methods of observing other galaxies and from modeling the evolution of the stars. But given these estimates of how many supernova researchers expect in our own galaxy, there aren’t many candidates for a system in which to expect them. That is, while we occasionally find systems where it is clear that something is going to happen, we might get some unexpected explosions.

UT video about supernovae in general.

Learn more:
University of Warwick Teardrop Star Reveals Hidden Supernova Doom
Natural Astronomy – A Hot Subdwarf White Dwarf Super Chandrasekhar Candidate For Supernova Ia Precursors
Live Science – Rare teardrop star and its invisible partner are doomed to explode in a massive supernova
Sci-News – HD 265435 will explode as a Type Ia supernova in 70 million years, astronomers say
UT – White dwarf measured before it exploded as a supernova

Mission statement:
Artist’s impression of the drop shape of the HD265435 system.
Credit – University of Warwick / Mark Garlick

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