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.
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.
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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