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Astronomers from Bonn and Moscow Discover Extremely Rare Stellar Merger Product

Astronomers from the University of Bonn and their collaborators from Moscow have discovered a strange celestial object. It is most probably the outcome of the fusion of two stars that died a long time ago.

WISE 22 micron infrared images at different intensity scales (panels a and b) compared with an optical IPHAS H alpha image where the nebula is not visible (panel c). (Image credit: Vasilii Gvaramadse/Moscow University)

The purported white dwarfs circled around each other for billions of years before merging and rising from the dead. Possibilities are that their lives could finally come to an end in the near future, with a huge bang. Currently, the researchers have described their findings in the journal Nature.

Researchers from the University of Moscow identified this extremely rare merger product. They spotted a gas nebula with a bright star at its center from images captured by the Wide-field Infrared Survey Explorer (WISE) satellite. However, astonishingly, the nebula discharged almost entirely infrared radiation and no visible light.

Our colleagues in Moscow realized that this already argued for an unusual origin.

Dr Götz Gräfener, Argelander Institute for Astronomy (AIfA), University of Bonn

The spectrum of the radiation that the nebula and its central star emitted was investigated at Bonn. Thus, the AIfA scientists could demonstrate that the mysterious celestial object did not contain hydrogen or helium—a typical characteristic of the interiors of white dwarfs. Stars such as the Sun produce their energy by means of hydrogen burning, the nuclear fusion of hydrogen. Once the hydrogen is used up, the burning of helium starts. But they cannot fuse heavier elements since their mass is inadequate to create the required high temperatures. As soon as all the helium is consumed, they stop burning and cool down, turning into what are called white dwarfs.

Typically, their life ends at this point. However, this is not the case for J005311—the name given by the scientists for their new discovery in the constellation Cassiopeia located 10,000 light-years from Earth.

We assume that two white dwarfs formed there in close proximity many billions of years ago. They circled around each other, creating exotic distortions of space-time, called gravitational waves.

Dr Norbert Langer, Professor, AIfA, University of Bonn

They slowly lost energy in this process. As a result, the distance between them reduced more and more until they merged eventually.

Only Five of These Objects in the Milky Way

Now their total mass was adequate to fuse elements heavier than hydrogen or helium. The stellar furnace started burning again. “Such an event is extremely rare,” reiterates. “There are probably not even half a dozen such objects in the Milky Way, and we have discovered one of them.”

An Extreme Stroke of Luck

However, the scientists are confident that their interpretation is correct. Firstly, the brightness of the star at the center of the nebula is 40,000 times that of the sun, much more brighter compared to that of a single white dwarf. Moreover, the spectra suggest that J005311 has a very strong stellar wind—a stream of material emanating from the stellar surface. It is run by the radiation produced at the time of the burning process. With a speed of 16,000 km/second, J005311’s stellar wind is so fast that just this factor is not sufficient to describe it. However, it is expected that merged white dwarfs will have an extremely strong rotating magnetic field. “Our simulations show that this field acts like a turbine, which additionally accelerates the stellar wind,” stated Gräfener.

Unfortunately, the rebirth of J005311 will not last for a long time. The star will have converted all elements into iron and fade again within just a few thousand years. Since its mass has increased to over 1.4 times that of the Sun in the process of merger, its end will be phenomenal. The star will breakdown under the impact of its own gravity. Simultaneously, the protons and electrons that constitute its matter will merge into neutrons. The ensuing neutron star will have only a fraction of its earlier size, measuring just a few kilometers in diameter, while weighing more than the entire solar system.

However, J005311 will not fade without a final salute. Its breakdown will be accompanied by a huge bang, what is called a supernova explosion.

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