Rare astronomers sighting of a teardrop star

• International team led by University of Warwick make rare sighting of binary star system heading towards supernova
• The fate of the star system has been identified from its unusual light variations, a sign that a star has been distorted into a teardrop shape by a huge white dwarf companion
• Supernovas from these star systems can be used as “standard candles” to measure the expansion of the universe.

Astronomers made the rare sighting of two stars spiraling to their loss by spotting telltale signs of a teardrop star.

The tragic form is caused by a massive white dwarf nearby warping the star with its intense gravity, which will also be the catalyst for a possible supernova that will consume both. Found by an international team of astronomers and astrophysicists led by the University of Warwick, it is one of the very few star systems discovered that will ever see a white dwarf star reignite its core.

New research published by the team on July 12, 2021, in Nature astronomy confirms that both stars are in the early stages of a spiral that will likely end with a Type Ia supernova, a type that helps astronomers determine how fast the universe is expanding.

This research received funding from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) and the Science and Technology Facilities Council, which is part of the UK Research and Innovation.

HD265435 is located about 1,500 light years away and includes a hot dwarf star and a white dwarf star in close orbit at a speed of about 100 minutes. White dwarfs are “dead” stars that have used up all their fuel and collapsed on their own, making them small but extremely dense.

It is generally believed that a Type Ia supernova occurs when the nucleus of a white dwarf star reignites, resulting in 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 into the second scenario, in which the total mass of a near multi-star star system is near or above this limit. Only a handful of other star systems have been discovered that will reach this threshold and give rise to a Type Ia supernova.

Lead author Dr Ingrid Pelisoli from the Department of Physics at the University of Warwick, and formerly affiliated with the University of Potsdam, explains: “We don’t know exactly how these supernovae explode, but we know it must be. produce because we see it happening. elsewhere in the universe.

“One solution is that if the white dwarf builds up enough mass from the hot sub-dwarf, so that the two orbit and come together, matter will start to escape from the hot sub-dwarf and fall onto the white dwarf. Another way is that because they lose energy from the emission of gravitational waves, they will come closer until they merge. Once the white dwarf gains enough mass with either method, it will become a supernova.

Using data from NASA’s Transiting Exoplanet Survey Satellite (TESS), the team was able to observe the hot sub-dwarf, but not the white dwarf, as the hot sub-dwarf is much brighter. However, this brightness varies over time, suggesting that the star was distorted into a teardrop shape by a massive object nearby. Using radial and rotational speed measurements from the Palomar Observatory and the WM Keck Observatory, and modeling the effect of the massive object on the hot dwarf, astronomers were able to confirm that the white dwarf hidden is as heavy as our Sun, but just slightly smaller than the radius of the Earth.

Combined with the mass of the hot sub-dwarf, which is just over 0.6 times the mass of our Sun, the two stars have the mass necessary to cause a Type Ia supernova. Since the two stars are already close enough to begin to move closer, the white dwarf will inevitably become a supernova in about 70 million years. Theoretical models produced specifically for this study predict that the hot sub-dwarf will contract to also become a white dwarf star before merging with its mate.

Type Ia supernovae are important to cosmology as “standard candles”. Their luminosity is constant and of a specific type of light, which means that astronomers can compare the luminosity they should be with what we observe on Earth, and from this, determine their distance with a good degree of precision. By observing supernovae in distant galaxies, astronomers combine what they know about the speed at which this galaxy is moving with our distance from the supernova and calculate the expansion of the universe.

Dr Pelisoli adds, “The more we understand how supernovae work, the better we can calibrate our standard candles. This is very important at the moment because there is a lag between what we get from this type of standard candle and what we get by other methods.

“The more we understand how supernovae are formed, the better we can understand whether this discrepancy we are observing is due to new physics that we are not aware of and taking into account, or simply because we are underestimating them. uncertainties of these distances.

“There is another gap between the estimated and observed galactic supernovae rate and the number of progenitors we are seeing. We can estimate how many supernovae are going to be in our galaxy by looking at many galaxies, or by what we know about stellar evolution, and that number is consistent. But if we are looking for objects that can become supernovae, we do not have enough. This discovery was very useful in estimating what hot dwarf and white dwarf binaries can provide. It still doesn’t seem like much, none of the channels we’ve seen seem to be enough.

Reference: “A supernova progenitor Ia hot hot dwarf white dwarf supernova candidate” by Ingrid Pelisoli, P. Neunteufel, S. Geier, T. Kupfer, U. Heber, A. Irrgang, D. Schneider, A. Bastian, J van Roestel, V. Schaffenroth and BN Barlow, July 12, 2021, Nature astronomy.
DOI: 10.1038 / s41550-021-01413-0