Russian and foreign astronomers have discovered an extremely unusual "carbon" star in the constellation Cassiopeia, which arose several tens of thousands of years ago as a result of the merger of large white dwarfs. In the near future, it will explode and turn into a pulsar, according to an article in the journal Nature Astronomy.
White dwarfs are the remnants of old "burned out" stars of small mass, devoid of their own energy sources. White dwarfs appear at the final stage of the evolution of stars with a mass not exceeding the solar mass by more than 10 times. Ultimately, our star will also turn into a white dwarf.
Astrophysicists are interested in such "dead stars" for several reasons. First, they are the progenitors of type I supernovae, which allow very accurate estimates of distances in space. Secondly, they consist of exotic superdense matter, the properties and structure of which scientists have not yet fully understood.
The answer to this question is important because it determines what should happen when white dwarfs merge. Now scientists believe that if the combined mass of two aged luminaries exceeds the so-called Chandrasekhar limit, which is 1.4 times the mass of the Sun, then the product of their merger becomes unstable and turns into a different type of object.
Depending on the speed and other parameters of the merger, this process can generate both a powerful thermonuclear explosion, a type I supernova explosion, or lead to the formation of a neutron star.
For a long time, scientists believed that the merger of all large white dwarfs, whose mass significantly exceeds the Chandrasekhar limit, is almost guaranteed to end in a supernova explosion. Belief in this idea was shattered in 2003 when astronomers from the United States and Canada recorded an extremely unusual flash in the sky. It had all the features of a supernova of the first type, but at the same time it was generated by an object whose mass exceeded the solar mass at least twice.
Its discovery caused a lot of controversy, since the theories that existed at that time could not explain not the mechanism of its birth, and the very existence of such an outburst contradicted the well-established idea that all supernovae of the first type have the same power and other properties tied to the Chandrasekhar limit.
Gvaramadze and his colleagues have discovered a highly unusual star, whose existence supports one of the explanations for how the 2003 burst and several other anomalously powerful supernovae recorded in the following years could have occurred.
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Initially, as astronomers explain, they were looking not for white dwarfs and traces of their collisions, but for nebulae that arise in the vicinity of large aged stars in the last stages of their lives. To do this, scientists studied images of the night sky taken by the WISE infrared orbiting telescope and other observatories of this type.
Their attention was attracted by a small nebula J005311, located about 10 thousand years away from us in the direction of the constellation Cassiopeia. When Gvaramadze and his team tried to find the parent star using the BTA telescope at the Special Astrophysical Observatory in Nizhny Arkhyz, they were in for a surprise.
In the center of this nebula lived an extremely unusual star, outwardly similar to the so-called Wolf-Rayet stars, the most restless and short-lived stars in the universe. Like its alleged "cousins", the star at the center of J005311 was incredibly hot - its surface temperature exceeded 200 thousand Kelvin. At the same time, she threw out huge amounts of gas into the environment, accelerating it to 16 thousand kilometers per second, or 5% of the speed of light.
On the other hand, it was about four times fainter than even the most modest Wolf-Rayet stars, but its spectrum was completely different from even the most active forms of such stars. In addition, its interior was almost entirely composed of two elements - oxygen and carbon, while hydrogen and helium were completely absent in the interior of J005311 and in its gas shroud.
These inconsistencies made scientists wonder how such an object arose. Referring to the famous Hertzsprung-Russell diagram, Russian and foreign astronomers noticed that stars formed as a result of the merger of large white dwarfs should have similar properties.
Theorists, as the researchers note, have long predicted the existence of such objects, whose mass is noticeably higher than the Chandrasekhar limit, but until now no one has found them.
As shown by these calculations, such superheavy objects can form if the bowels of white dwarfs are heated quickly enough during their merger. In this case, carbon will have time to "ignite" in the bowels of the new star even before it is strongly compressed.
This will stop the thermonuclear explosion, generate a small nebula of incandescent oxygen and neon, and inside it a unique superhot object will appear, which will live for several tens of thousands of years. After the "reborn" star has exhausted all its reserves of carbon and oxygen, it will contract even more, leading to the birth of a faint supernova and a small neutron star.
As the calculations of Gvaramadze and his colleagues show, this should happen in the very near future. J005311 is now about 16 thousand years old, which means that it is in the last stages of its "new life." It is possible that humanity will witness this momentous event, scientists conclude.
