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The first contracting white dwarf

White dwarfs are the remnants of solar-like stars that have exhausted the reservoir of fuel for the nuclear fusion reactions that powers them. It is widely believed, based on theoretical considerations, that young white dwarfs should experience a phase of contraction during the first million years after their formation. This is related to the gradual cooling of their interior which is not yet fully degenerate. In several million years the central temperature of a white dwarf decreases from several hundred million degrees to few tens of millions and also the surface temperature cools down. During this period the size of the white dwarf can become few hundred (or few tens, depending on mass and exact time interval) kilometers smaller. However, there were no observational indications for this important effect up to now. First, because most of the known white dwarves are much older, second because we do not have a direct and precise way to measure the radii and their variations in these stars.

However, we might have found the first observational evidence for a contracting white dwarf by studying the enigmatic X-ray source HD49798/RX J0648.0-4418, which is located at a distance of 2,000 light years, in the Puppis constellation. Despite it being known since the seventies of the last century that the 8th magnitude hot star HD49798 is a member of a binary with orbital period of 1.5 days, the nature of its companion remained a mystery for a long time. In 2009, thanks to observations with the XMM-Newton X-ray satellite, Mereghetti et al. found that the peculiar properties of this binary can be explained if the companion of HD 49798 is a rapidly rotating and fairly massive (1.3 solar masses) white dwarf, which emits X-rays due to the accretion of matter captured from the stellar wind of its companion star.

With further X-ray observations, we recently discovered that the rotational velocity of this white dwarf has been steadily increasing over the last twenty years. Its spin period of 13.2 seconds is decreasing by about seventy nanoseconds every year. This might seem a very small change, but it is actually a very large effect for a body weighing more than our Sun, but with a radius smaller than the Earth’s. Indeed, such a large spin-up rate could not be easily explained in standard ways (i.e. by the captured angular momentum of the accreting matter).

Artist’s impression of the binary system HD 49798, consisting of a fastly rotating white dwarf (the perl-like star surrounded by an accretion disc) that captures mass from the stellar wind of its hot subdwarf companion (shown in blue as it is a helium star with large surface temperature and high luminosity). Image credit: Francesco Mereghetti, background image: NASA, ESA and T.M. Brown (STScI). Used with author’s permission.

So what is causing this phenomenon? We found that, just as figure skaters would bring their arms closer to their bodies to rotate faster, the high spin-up rate can be easily explained by the contracting of the white dwarf. The calculations of the previous evolution of this compact object made with a computer code and presented in our paper show that the white dwarf has an age of only about two million years. The contraction rate of about one centimeter per year expected for this age is exactly the correct amount to explain the measured spin-up rate, showing that this is the first contracting white dwarf ever identified.

Computer simulations of the formation and evolution of binary stars, performed by Alexander Kuranov and Lev Yungelson, demonstrate that there are several dozens of systems very similar to HD49798 in our Galaxy. In addition, there may be several hundreds of systems of this type with slightly different parameters. Thus, it is not surprising to find one of them within 2,000 light years from us. The discovery of new systems of this kind will allow us to learn more about the youth of white dwarfs and to advance our understanding of these peculiar compact stars ruled by the laws of quantum mechanics.

Featured image credit: White dwarf in AE Aquarii by Casey Reed / NASA. Public Domain via Wikimedia Commons.

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