Astronomers discover a massive white dwarf that has arisen from two others

Astronomers from the University of Warwick in the United Kingdom say that the massive white dwarf they observe with the peculiar carbon-rich atmosphere may actually be the result of a combination of two other white dwarfs.
Researchers have discovered this unusual, extremely massive white dwarf 150 light years from Earth. The star is surrounded by an atmosphere unlike any other observed so far. This is the first discovery of a white dwarf in history resulting from the merger of two smaller objects of this type.

The discovery, which was announced in an article published in the journal Nature Astronomy, raises new questions for scientists about the evolution of massive white dwarfs and the number of supernova explosions in our galaxy.

The described star (designation WDJ0551 + 4135) was discovered in data from the Gaia space telescope. After its discovery, astronomers decided to perform its spectroscopic analysis using the Herschel Space Telescope. By analyzing the spectrum of radiation it emits, the researchers determined the chemical composition of the star's atmosphere and were surprised to notice an unusually high carbon content.

This star was different from the others. Usually you can expect an outer layer of hydrogen, sometimes mixed with helium, or a mixture of helium and carbon. However, no one expects a mixture of hydrogen and carbon without helium. Something was wrong here - says Dr. Mark Hollands, lead author of the study.

White dwarfs are the remains of sun-like stars that have already burned their fuel and discarded all outer layers. Most white dwarfs have a mass of about 0.6 solar mass, but this particular has a mass of 1.14 solar mass, which is almost twice as high. Despite the mass higher than the mass of the Sun, the white dwarf is only 2/3 of the Earth's size.

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The age of white dwarfs is also a guide. Older stars orbit the center of the Milky Way faster than young ones, and this object moves faster than 99% of other white dwarfs in its surroundings. That means he's older than he looks.

We have a chemical composition that we cannot explain with our knowledge of star evolution, a mass twice as average, and a more advanced kinematic age than what comes from the cooling rate of the star. We know quite well how a white dwarf is formed, and this is completely different from the observations of this object. The only explanation is the joining of two white dwarfs.

According to the theory, when in a binary system consisting of two sun-like stars, one of them passes into the red giant stage, it absorbs the other, which is now circling inside it, and gradually the distance between the centers of both stars decreases. The same happens later when the other star begins to grow and devours his companion. Over the billions of years, the entire system emits gravitational waves, while tightening the orbit of both objects around a common center of mass. At some point, the white dwarfs merge into one massive object.

Most of these two-star connections occur in our galaxy between stars of very different masses. In this case, however, it appears that two stars of almost identical mass have merged. What's more, there is a theoretical limit on the maximum mass that a white dwarf can reach - it's 1.4 solar mass. If the object exceeds this mass, it explodes as a supernova. The object discussed here is very close to this border.

Because in the process of joining two white dwarfs, the process of their cooling begins again, it is difficult to determine the age of this object. Although the fusion occurred about 1.3 billion years ago, the two components that fused could exist billions of years ago.

So far, we have not discovered too many such massive white dwarfs, although some of them could probably have arisen in a joining process like this. In the future, we hope to use asteroseismology techniques to learn about the chemical composition of this white dwarf nucleus - it would be an opportunity to independently confirm the theory that it arose as a result of merging two objects - adds Dr. Holland.

Astronomers discover a massive white dwarf that has arisen from two others