ESA/Hubble. The Small Magellanic Cloud is rich in red giant stars, and the site of the first Thorne-Zytkow Object identified

It has been almost 40 years since Professor Kip Thorne and Dr Anna Zytkow first suggested it was possible for a red giant and a neutron star to merge. Now this theoretical, almost mythical, chimera has been found.

Neutron stars are the outcome of a supernova explosion. Red giants are the later stage of life for stars with between 0.3 and 8 times the mass of the sun. Having run out of hydrogen in their cores, red giants have taken to conducting fusion in a shell further out, which in turns causes their outer layers to expand thousands of times in size. They can often grow to the size of the Earth's orbit, and in some cases much larger.​

If the neutron star is in a close orbit with the red giant it would be swallowed by its companion as the outer layers expand. Frictional drag would cause the denser object to slowly spiral in to the center of the giant to become part of the core. 

Thorne and Zytkow noted that such a star would have a very different chemical signature from an ordinary red giant, and would also be powered in an unusual way.

Now a team, including Zytkow herself, have announced in the Monthly Notices of the Royal Astronomical Society Letters the discovery of the first Thorne-Zytkow Object (TZO) in the Small Magellanic Cloud (SMC).

The SMC is a dwarf galaxy neighboring the Milky Way, and one of just three other galaxies visible with the naked eye. A recent burst of star formation left the SMC rich in large stars and their remnants such as X-ray binaries. 

Examining the SMC's red giants, Philip Massey of the Lowell Observatory, noted the unusual features of one, HV 2112, commenting, “I don't know what this is, but I know that I like it!” The spectral lines of HV 2112 looked like nothing Massey had seen before, and on closer examination showed high concentrations of rubidium, lithium and molybdenum. Each of these sometimes appears in large quantities on its own, but the combination of lithium and heavy metals has not been seen before, except in theoretical models of TZOs.

"I am extremely happy that observational confirmation of our theoretical prediction has started to emerge," Żytkow said. "Since Kip Thorne and I proposed our models of stars with neutron cores, people were not able to disprove our work. If theory is sound, experimental confirmation shows up sooner or later. So it was a matter of identification of a promising group of stars, getting telescope time and proceeding with the project."

If experimental results always perfectly matched theories however, there would be no need for experimentalists. Massey acknowledges, “There are some minor inconsistencies between some of the details of what we found and what theory predicts. But the theoretical predictions are quite old, and there have been a lot of improvements in the theory since then. Hopefully our discovery will spur additional work on the theoretical side now."

Besides the satisfaction of proving the theory correct, TZOs have been sought because they would offer a unique insight into the internal workings of very large stars. Project leader Emily Levezque of the University of Colorado Boulder said, "In these interiors we also have a new way of producing heavy elements in our universe. You've heard that everything is made of ‘star stuff’—inside these stars we might now have a new way to make some of it.” Many heavy elements have been thought to only be produced in supernova explosions, but TZOs might offer an alternative site in which they might be forged. It is likely that TZOs would produce different abundances of certain heavy metals to supernovae, which could affect the evolution of planets formed around the next generation of stars.



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