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Observe for the first time how a white dwarf shrinks

A team of astrophysicists has detected a young white dwarf that is part of a binary system in its contract phase.

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Astrophysicists at Moscow State University in Russia along with colleagues from Italy and the Russian Academy of Sciences have found the first test by observations of a white dwarf star contracting. White dwarfs are stellar remnants that are generated when a star of mass smaller than 9-10 solar masses has exhausted its nuclear fuel.

In this case, the researchers authors of the finding explain that a star of this type located in a strange binary system that rotates at a constant high speed can only occur if the white dwarf is shrinking, in its contraction phase.

The finding is published in the Monthly Notices of the Royal Astronomical Society. Thanks to this discovery, astrophysicists will be able to better study the evolution patterns of young white dwarfs, according to the main author of the research, the Russian astrophysicist Sergei Popov.

From theoretical considerations, it is widely accepted that young white dwarfs – those compact remnants of stars similar to the Sun – undergo a phase of contraction in their primitive life.

The radius of a typical white dwarf can be reduced by several hundred kilometers during its first million years of existence, but until now there has been no observation of this phenomenon. First, because most known white dwarfs are much older and secondly because astrophysicists do not have at this time a direct and precise method of measuring radius and its variations in these stars.

Now, while studying an enigmatic source of X-rays in a binary system, scientists may have come up with the first observational test of a white dwarf in the process of contracting. The binary system HD49798 / RX J0648.0-4418 is 2,000 light-years away, in the constellation Puppis, and has been extensively studied with optical, UV and X-ray telescopes.

It contains a massive white dwarf that rotates to a period of only 13 s, the fastest known in a star of this type, and emits X-rays thanks to the accretion of matter captured from the stellar wind of its companion star in the system.

Sandro Mereghetti, another co-author of the study, recently discovered that the rotational speed of the white dwarf had been increasing steadily over the past 20 years while the turning period of 13 s was decreasing at a rate of 7 nanoseconds per year. Although this seems like a minuscule change, in fact it produces a considerable effect for a body heavier than our Sun but whose radius is 5,000 kilometers smaller (even smaller than that of Earth). Such a wide frequency of rotation cannot be explained under normal conditions, or depending on the matter of accretion.

The solution to this puzzle is presented by the authors of the new study, for whom the high rotation coefficient is due to the white dwarf shrinking, as skaters do when they contract their arms against the body to spin faster. According to the calculations of the astrophysicists, the white dwarf is only 2 million years old, an age in which the frequency of rotation is exactly that presented by this star and that for the first time has been observed in the full phase of contraction.

This characteristic that young white dwarfs “lose weight” was clear at the theoretical level but it was necessary to observe it in real time. According to Popov, “we have to thank the unique characteristics of the binary system studied, because the white dwarf appeared literally illuminated thanks to the accretion of matter from the neighboring star, but although it stood out clearly, the matter of accretion did not affect the rotation – a truly strange phenomenon – ”.

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