A stellar dance in a characteristic style: astronomers have discovered a pair of stars in which the smaller partner apparently takes on a teardrop shape. This is a sign that the “cosmic tango” will come to an explosive end: The masses of the two stars show that a type Ia supernova will occur in this system. The binary star system can thus provide information about the formation processes of these star explosions, which are used in astronomy as “standard candles” to determine distances.
Some cosmic luminous dots are more complex than it initially seems – this is also the case with the HD265435. This system, about 1,500 light years away from us, is the target of the scientists working with Ingrid Pelisoli from the University of Warwick. As they report, it emerged from their observations that it is a binary star system consisting of a hot “subdwarf” and a white dwarf star that orbit each other closely. White dwarfs are “dead” stars that have burned all of their fuel and collapsed on themselves, making them small and pale but extremely dense.
An explosive duo in their sights
A particularly interesting aspect of the white dwarfs is that under certain circumstances they can re-ignite, resulting in a type Ia supernova. A certain characteristic leads to the thermonuclear explosion: the white dwarf has to reach 1.4 times the mass of our sun, which is known as the Chandrasekhar limit. This can result from the fact that he receives material from the outside. As Pelisoli and her colleagues report, this is exactly the case with the HD265435 binary star system.
As part of their investigations, they targeted the duo with NASA’s Transiting Exoplanet Survey Satellite (TESS). As they explain, only the hot dwarf of the system can be observed, but not the white dwarf, as it is outshone by its partner. However, it makes itself clearly noticeable indirectly: with the help of radial velocity and rotational velocity measurements and by modeling the effect of the massive object on the hot lower dwarf, the astronomers were able to infer the mass of the white dwarf. It is therefore as heavy as our sun, but only about the diameter of the earth.
This mass alone would not be sufficient for a Type Ia supernova – but an increase is emerging, as the astronomers report. The TESS data shows variations in brightness over time, suggesting that the hot subdwarf is being dragged into a teardrop shape by the nearby massive object. Together with the mass of the hot sub-dwarf, which has a little more than 0.6 times the mass of our sun, the two can together reach the Chandrasekhar limit and thus trigger a type Ia supernova, the scientists say. So far, only a few other star systems have been discovered that will probably reach this limit and thus end in a type Ia supernova.
Mass growth up to the bang
“In the current case, one possibility is that the white dwarf is already absorbing enough mass from the hot sub-dwarf, in that matter escapes from it and falls onto the white dwarf. Another possibility is that they will get closer to merging because they are losing energy from gravitational wave emissions. As soon as the white dwarf gains enough mass in both scenarios, it then becomes a supernova, ”explains Pelisoli. The researchers’ modeling shows that the white dwarf and its partner will inevitably end up in the thermonuclear inferno in around 70 million years.
As the scientists point out, information about the evolution of Type Ia supernovae has far-reaching implications for astronomy. Because these explosions are used as so-called “standard candles” in cosmology: Since they have a uniform brightness and emit a certain type of light, their distances to earth can be determined relatively precisely. By observing these supernovae in distant galaxies, astronomers can also gain clues about the expansion processes of the universe.
However, recently there have been doubts about the accuracy of the information that Type Ia supernovae can provide. “The more we understand how supernovae work, the better we can calibrate the standard candles,” says Pelisoli. “This is important because there has been a discrepancy between the information we get from these types of standard candles and what we get from other methods. The more we know about how supernovae form, the better we can understand what this discrepancy is based on. The current observations can now contribute to this, ”says the scientist.
Source: University of Warwick, specialist article: Nature Astronomy, doi: 10.1038 / s41550-021-01413-0