Astronomers rarely see teardrop -shaped stars

Artist’s impression of the HD265435 system some 30 million years from now, where a smaller white dwarf transforms a hot sub-dwarf into a distinctive “teardrop” shape. Credit: University of Warwick / Mark Garlick

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• An international team led by the University of Warwick has created a rare sight of a binary star system heading for a supernova
• The fate of the star system is determined by an unusual optical difference, a sign of the distortion of a single star into a teardrop shape by a massive white dwarf companion.
• Supernovae from such star systems can be used as “standard candles” to measure the expansion of the universe

Astronomers have made the rare sight of two stars rising to their demise by finding warning signs of a teardrop-shaped star.

This tragic shape is caused by a large white dwarf nearby distorting the star with its strong gravity, which will also be the catalyst for a supernova that will eventually eat both of them up. Discovered by an international team of astronomers and astrophysicists led by the University of Warwick, it is one of the few star systems discovered that will one day see a white dwarf rekindle its core.

New research was published by the team on July 12, 2021 in natural astronomy He confirmed that both stars are in the early stages of a spiral that is likely to end in a Type Ia supernova, the kind that helps astronomers determine how fast the universe is expanding.

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

HD265435 lies about 1,500 light-years away and consists of a hot semi-dwarf star and a white dwarf that orbits close together at a speed of about 100 minutes. A white dwarf is a “dead” star that has burned all its fuel and collapsed on its own, making it small but very dense.

It is generally believed that Type Ia supernovae occur when the core of a white dwarf ignites, resulting in a thermonuclear explosion. There are two scenarios where this could happen. In the first case, the white dwarf will gain enough mass to reach 1.4 times the mass of our Sun, which is known as the Chandrasekhar limit. HD265435 fits into the second scenario, where the total mass of star systems close to some stars is close to or above this limit. Only a few other star systems have been discovered that will reach this threshold and lead to a Type Ia supernova.

Lead author Dr Ingrid Bellisoli of the University of Warwick’s Department of Physics, previously affiliated with the University of Potsdam, explained: “We don’t know exactly how the supernova exploded, but we knew that it had to happen because we saw it happening elsewhere. in the universe.

“One way is if the white dwarf gains enough mass from the hot sub dwarf, then when the two orbit each other and get close, matter will start to escape from the hot sub dwarf and fall into the white dwarf. Another way is that because they lose energy due to the emission of gravitational waves, they will get closer until they merge. Once the white dwarf gains enough mass, it will turn into a supernova.”

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

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 needed to create a Type Ia supernova. Since the two stars are already close enough to start rotating together, the white dwarf is bound to turn into a supernova in about 70 million years. A theoretical model produced specifically for this study predicts that the hot sub-dwarf will shrink into a white dwarf as well before merging with its companion.

Type Ia supernovae are important to cosmology as “standard candles”. The brightness is constant and for a given type of light, meaning that astronomers can compare the brightness they should have with what we observe on Earth, and from that figure out how far away they are with a good degree of accuracy. By observing supernovae in distant galaxies, astronomers combine what they know about how fast these galaxies are moving with our distance from the supernova and calculate the expansion of the universe.

Dr. Pelisoli added: “The more we understand how supernovae work, the better we can calibrate our standard candles. This is very important today because there is a difference between what we get from these standard types of candles and what we get through them. another method.”

“The more we understand about how supernovae form, the better we can understand whether this difference we see is due to new physics we didn’t realize or account for, or simply because we reduced the uncertainty at that distance.

“There are other differences between the estimated and observed supernova rates of galaxies, and the number of progenitors we see. We can predict how many supernovae there will be in our galaxy by observing multiple galaxies, or by what we know from stellar evolution, and these numbers are consistent. But if we’re looking for an Object that can become a supernova, we don’t have enough. This discovery is very useful in making estimates of what contribution the hot sub-dwarf and white dwarf binary can make. It still doesn’t seem like much, and none of the channels we’re observing seem to be enough.”.