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Two dead stars have merged

White dwarfs are stellar corpses, the remains of dead stars where the thermonuclear reactions have stopped. Very rarely, they can reignite when two white dwarfs have merged by colliding. This must have happened for the star found in an atypical planetary nebula.

The archives of the Wide-field Infrared Survey Explorer mission (WISE) are full of surprises. They had already allowed the Brazilian astronomer Denilso Camargo to make several discoveries concerning the star clusters in the Milky Way but today, it is an international team of astronomers who announced having made a discovery with these data.

As the researchers explain in an article published in the journal Nature, also available on arXiv, it all started with the detection of an unusual planetary nebula in the infrared by WISE. Cataloged as J005311, it is located in the constellation Cassiopeia, about 10,000 light-years from the Solar System in the Milky Way. At its center, there is a star that at first sight looks like a classic white dwarf.

Normally a planetary nebula produced by the end of a little massive star on the main sequence and which has become a white dwarf is not only bright in the infrared range. It can also be seen in the visible domain, as the famous example of the Nebula of the Lyre (in English Ring Nebula) shows, photographed by the Hubble telescope, inscribed in the Catalog of Messier under the name M57.

This is not the case of J005311. Moreover, a fine study of the composition of the atmosphere of the central star, and also of the nebula itself, does not show the presence of nuclei of hydrogen and helium. The stellar atmosphere models can account for other characteristics of spectral data observed by bringing a surface temperature of about 200,000 K and a muzzle velocity of winds stellar record of about 16,000 km/s, powerful facilitating the acceleration of these winds.

Half a dozen zombie stars in the Milky Way

These characteristics are also atypical and one can convince oneself, as the astrophysicists, that one is really in front of a rare case when one learns that the mass of the central star which behaves anyway like a white dwarf is greater than the Chandrasekhar mass limit. For researchers, there is little doubt we would have discovered by chance a type of celestial body of which it is estimated that there is only a handful in the Milky Way, a white dwarf with a mass of Super Chandrasekhar type.

This type of star is not inert from the point of view of thermonuclear reactions. It would be formed by the fusion of two white dwarfs in a binary system that collided for the same reason as the pairs of black holes and fused neutron stars, detected by Ligo and Virgo: loss of energy by emission of gravitational waves.

By losing energy, the orbits of the white dwarfs would have decreased radius until the collision. Ordinarily, when this happens and the Chandrasekhar mass of the resulting object is exceeded, the resulting object collapses gravitationally on itself, exploding immediately.

But in rare cases, the re-energized thermonuclear reactions temporarily stop the collapse so that the star begins to fuse nuclei, and it appears as an atypical white dwarf whose mass slightly exceeds the limit mass initially calculated by Chandrasekhar, he almost 90 years ago. Since the two initial stars have a kinetic momentum and a magnetic field, the associated conservation laws then imply that the resulting star rotates rapidly on itself and has a large magnetic field, which is what we observe.

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