Teardrop shape reveals supernova fate

Research Highlight August 2021

August 01, 2021

Astronomers have made the rare sighting of two stars spiralling to their doom by spotting the tell-tale signs of a teardrop-shaped star. The tragic shape is caused by a massive nearby white dwarf distorting the star with its intense gravity, which will also be the catalyst for an eventual supernova that will consume both. Found by an international team of astronomers and astrophysicists led by the University of Warwick, it is one of only very small number of star systems that has been discovered that will one day see a white dwarf star reignite its core. Astrophycists at MPA confirmed the ultimate fate of the star with theoretical modelling.

Artist’s impression of the HD265435 system at around 30 million years from now, with the smaller white dwarf distorting the hot subdwarf into a distinct ‘teardrop’ shape.

The system called HD265435 is located roughly 1,500 light years away and comprises a hot subdwarf star and a white dwarf star orbiting each other closely at a rate of around 100 minutes. White dwarfs are ‘dead’ stars that have burnt out all their fuel and collapsed in on themselves, making them small but extremely dense.

A type Ia supernova is generally thought to occur when a white dwarf star’s core reignites, leading to a thermonuclear explosion. There are two scenarios where this can happen. In the first, the white dwarf gains enough mass to reach 1.4 times the mass of our Sun, known as the Chandrasekhar limit. HD265435 fits in the second scenario, in which the total mass of a close stellar system of multiple stars is near or above this limit. Only a handful of other star systems have been discovered that will reach this threshold and result in a Type Ia supernova.

Using data from NASA’s Transiting Exoplanet Survey Satellite (TESS), the team were able to observe the hot subdwarf, which is much brighter than the white dwarf, which was not directly observed. However, that brightness varies over time, which suggested the star was being distorted into a teardrop shape by a nearby massive object. Using radial velocity and rotational velocity measurements from the Palomar Observatory and the W. M. Keck Observatory, and by modelling the massive object’s effect on the hot subdwarf, the astronomers could confirm that the hidden white dwarf is as heavy as our Sun, but just slightly smaller than the Earth’s radius.

Predicted evolutionary tracks for hot subdwarfs with a mass of 0.63 M and different masses of the hydrogen envelope. The purple line represents the favoured model (no wind) starting from the beginning of the binary run. The model including a weak wind is represented by the dashed orange line. The observed position of the hot subdwarf in the diagram is as indicated, with error bars representing the systematic uncertainty of the obtained spectral parameters.

Theoretical models produced by researchers based at MPA reveal that the subdwarf star is currently about halfway through its expected lifetime, at the end of which it will collapse to become a white dwarf as well. In around 70 million years, this will lead to a supernova when it finally merges with the other white dwarf, as both stars combined have the mass needed to cause a Type Ia supernova. (The total mass of the system is found to be 1.65±0.25M⊙.) On the way to becoming a white dwarf itself, mass transfer from the subdwarf star will lead to a series of nova outbursts – less energetic cousins of the cataclysmic supernovae – as a precursor to the eventual explosive fate of the binary star.

Type Ia supernovae are important for cosmology as ‘standard candles’. Their brightness is constant and of a specific type of light, which means astronomers can compare what luminosity they should be with what we observe on Earth, and from that work out how distant they are with a good degree of accuracy. By observing supernovae in distant galaxies, astronomers combine what they know of how fast this galaxy is moving with our distance from the supernova and calculate the expansion of the universe. As the theoretical predictions show, this system impressively demonstrates that binary systems of a white dwarf with a subdwarf have to be taken into account in the hunt for the – still elusive – progenitor of Type Ia supernovae.

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