Astrophysicists from Russia’s Moscow State University along with colleagues from Italy and the Russian Academy of Sciences have found the first evidence through observations of a contracting white dwarf star . White dwarfs are stellar remnants that are generated when a star with a mass less than 9-10 solar masses has exhausted its nuclear fuel .
In this case, the researchers who made 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 contracting 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, Russian astrophysicist Sergei Popov.
From theoretical considerations, it is widely accepted that young white dwarfs – those compact remains of stars similar to the Sun – undergo a contraction phase in their early 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 have been no observations of this phenomenon. First, because most of the known white dwarfs are much older and second because astrophysicists currently do not have a direct and precise method of measuring the radius and its variations in these stars.
Now, while studying an enigmatic X-ray source in a binary system , scientists may have stumbled upon the first observational evidence of a contracting white dwarf . The HD49798 / RX J0648.0-4418 binary system is located 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 at a period of only 13 s, the fastest known for 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 of the study’s co-authors, recently found that the white dwarf’s rotational speed had been increasing steadily for the past 20 years while the 13-s spin period decreased at the rate of 7 nanoseconds per year . Although this may seem like a tiny change, it actually produces a considerable effect for a body heavier than our Sun but whose radius is 5,000 kilometers smaller (even smaller than Earth’s). Such a wide turning frequency cannot be explained under normal conditions, or as a function of accretion matter.
The solution to this puzzle is what the authors of the new study have presented, for whom the high coefficient of twist is due to the fact that the white dwarf is shrinking , as skaters do when they contract their arms against the body to spin faster. According to the calculations of astrophysicists, the white dwarf is barely 2 million years old , an age at which the rotation frequency is exactly that of this star and which for the first time has been observed in full contraction.
This characteristic that young white dwarfs “lose weight” was theoretically clear but needed to be observed in real time. According to Popov, “we have to thank the unique characteristics of the binary system studied, because the white dwarf literally appeared illuminated thanks to the accretion of matter from the neighboring star. But although it stood out sharply, the accretion matter did not affect the rotation – a truly strange phenomenon. “
Image: Francesco Mereghetti
