Astrophysicists from Russia and Italy have uncovered the nature of the mysterious binary system HD 49798 in the constellation of Poppa, proving that one of its inhabitants is a young white dwarf whose matter is gradually contracting, and not a neutron star, as some scientists believed. Their findings were published in the journal MNRAS.
“This discovery has two important implications. On the one hand, there was a mysterious system. For several years, astronomers have been arguing about what is there - a white dwarf or a neutron star? We have explained this, although, perhaps, it is not so important for astrophysics in general. In terms of significance, the discovery can be compared with the disclosure of a cunning crime in a detective story - although the event is quite local, the viewer is still interested,”says Sergei Popov, an astrophysicist at the Sternberg State Astronomical Institute, Moscow State University.
The life of most relatively small stars, similar to the Sun in size, ends not with a supernova explosion, but with transformation into a white dwarf - a small and superdense lump of superhot exotic matter that continues to glow for many hundreds of millions of years due to the heat trapped in their depths.
Scientists have long believed that young white dwarfs gradually shrink after the outer shells of a deceased star are shed, "losing weight" by several hundred kilometers in the first million years of their independent existence. Evidence of this, as Popov explains, has not yet been found, since most of the white dwarfs we know are much older than a million years, and astronomers did not have accurate data on their diameter in order to "see" this compression process.
Russian and Italian astrophysicists, led by Popov, were able to obtain the first evidence that such a compression actually occurs by observing the mysterious double star HD 49798 in the constellation Korma, approximately 2,100 light years distant from us.
The object was discovered back in 1964 by Argentine astronomers, who initially thought they were dealing with an unusual white subdwarf, which contained unusually little hydrogen in its interior. Later, scientists discovered that HD 49798 is a binary star, one of the halves of which is a "normal" star, and the other is a neutron star or white dwarf.
The controversy was caused by the unusually high rotational speed of the second object - it makes one revolution around its axis in just 13 seconds. This is normal for neutron stars, but a record high white dwarf anomaly that is hard to explain.
Moreover, scientists recently discovered that the rotation of the second half of HD 49798 is accelerating, which probably indicates that a white dwarf or neutron star is actively "stealing" matter from its neighbor, accumulating it on its surface. This is extremely difficult to explain in the context of both versions of the nature of this star, which drove astronomers even more into a dead end.
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Russian astrophysicists have found an explanation for all these oddities by drawing attention to how the properties of stars change when they turn into neutron stars and white dwarfs.
Both the one and the other process have one thing in common - the volume of the star decreases sharply, and its density increases sharply. A decrease in the volume of a star, in accordance with the "school" law of conservation of angular momentum, will lead to the fact that the speed of its rotation will increase.
Accordingly, the rapid increase in the rotation rate of the second half of HD 49798 can be explained by the fact that this star is contracting and its volume gradually decreases. Guided by this idea, Popov and his team calculated how a white dwarf of similar size and age should shrink, and compared the computer model with real data.
As these calculations showed, a white dwarf 2 million years old should be compressed at a rate of about a centimeter per year, which ideally corresponds to the rate at which the rotation speed of the second half of HD 49798 is growing. Accordingly, we can say that now astronomers have an example of a “young The white dwarf, the study of which the authors of the article hope will help us better understand their nature.
“There is also a second sense of discovery. For decades, it was clear that young white dwarfs were contracting. We have never seen the actual compression phase "live". And the uniqueness of the system that we observed was that the white dwarf was as if “illuminated” due to the flow of matter from a nearby star. Most importantly, it was illuminated so neatly that it did not affect its rotation, and this is the rarity of the system. In other pairs of white dwarfs and main sequence stars, accretion is much more powerful: it already determines how the white dwarf rotates, which makes it impossible to notice the 'beauty of compression',”concludes Popov.