These objects would consist of a red supergiant or a Wolf Rayet star with a neutron star inside.
The two astrophysicists have proposed several ways in which this very particular category of stars could form. For example, this object can form in certain binary systems with components separated by a small distance. First the most massive star, exploding into a supernova, would become a neutron star. Then the second component, in the final stages of its life, would begin to enter the red giant stage and would begin to expand to incorporate the neutron star.
Thorne-Żytkow objects could also form within globular clusters, when a supergiant star collides and incorporates a stray neutron star, although this scenario is very unlikely.
To date, no Thorne-Żytkow objects have been discovered, although there have been several candidates.
The most famous is definitely HV 2112, a red supergiant variable star visible in the direction of the Small Magellanic Cloud.
In 2014, a study found that HV 2112 was more than 100,000 times brighter than the Sun (much brighter than a normal red supergiant) and contained high concentrations of elements such as lithium, molybdenum and rubidium. The presence of such elements in the observed quantities could only be explained by the hypothesis that HV 2112 was a subject of Thorne-Żytkow.
In 2018 a new study repeated the analyses made in 2014 with more advanced tools. This time there was no abundance of the chemical elements mentioned above and it was noticed that the star was much closer to Earth than thought. Its brightness was therefore not 100 thousand times that of the Sun, but only a thousand times that of our star.
As far as we know today HV 2112 should be a simple red supergiant and is very unlikely to be an object of Thorne-Żytkow.
Credit: NASA, ESA, Hubble.
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