In the mid-1970s, theoretical astrophysicist Kip Thorne, working with collaborator Anna Zytkow, postulated the existence of a bizarre form of star. Now known as Thorne-Zytkow objects (TZOs), these bodies were the product of the merger of two separate stars: one a giant star, the second a neutron star. They were able to calculate several likely properties of these stars, making predictions for what they might look like. But in the intervening years, none have been discovered.

Anna Zytkow, however, did not give up the search. And now, 40 years later, she may have spotted one. She and three collaborators (Phil Massey, Nidia Morrell, and Emily Levesque) have reported what may be the first observational evidence that TZOs exist.

Neutron stars are the cores of massive stars that have undergone a supernova. Their massive gravity compresses matter so much that an object the mass of the Sun can squeeze into a sphere about 20 km across. At these densities, matter is compressed down to neutrons—and possibly even a sea of subatomic particles.

To form a star massive enough to undergo a supernova generally requires a dense cloud of gas, which often forms additional stars. These companions can exchange mass with the neutron stars in various ways, but Thorne and Zytkow suggested that they may do more than interact—the companion can swallow the neutron star. Many stars evolve through a giant phase in which their envelope expands significantly. Should the neutron star be orbiting close enough, this expansion could cause the giant star to envelop its companion.

When this happens, the neutron star's orbit would rapidly be slowed down by its interactions with the gas, causing it to spiral toward the star's center. Once there, it would displace the normal core of the star, in effect taking over the center of the object. Although its intense gravity would draw matter in, its equally intense heat would drive it off, creating a stable balance—the TZO.

On the exterior, the object would look much like any other red giant star. But there would be differences. The intense heat near the neutron star would trigger different fusion reactions from those normally found at the center of a red giant. Based on then-current models of convection inside these stars, Thorne and Zytkow predicted that some of the elements that result from these unusual processes would make their way to the star's surface, creating a distinctive signature that we can detect using spectroscopy.

The team, including Zytkow herself, continued searching for this signature, even though spectroscopy on distant stars requires a high-powered telescope in order to resolve the appropriate wavelengths of light. In the new report, she and her collaborators took data from a total of 62 red supergiants in the Milky Way and its companion dwarf galaxies, the Large and Small Magellanic Clouds.

To look for a TZO, the team searched the spectrums for some of the elements predicted to be produced in high quantities in these objects: lithium, rubidium, strontium, vanadium, zirconium, and molybdenum. Not all of these are easy to image, so they focused on lithium, rubidium, and molybdenum. They compared the levels of these elements with elements that create spectral features nearby but aren't expected to be enhanced in TZOs: potassium, calcium, iron, and nickel.

Most of the stars clustered together, having similar levels of these elements. But a single star, HV 2112, was a consistent outlier, having ratios of elements that were generally off by three standard deviations compared to the other ones. Although HV 2112 had been observed before, no one had ever obtained a detailed enough spectrum to determine that it had such an unusual composition.

There are two caveats to this work. One is that the star appears to be in the Small Magellanic Cloud. If it's not, then some of the measurements of its properties will be off, meaning it may not be a red supergiant—and thus not a TZO. It would still be unusual, just not this particular type of unusual.

The second thing is that the object doesn't show all the properties predicted for TZOs, and it has a few additional ones that weren't predicted back in 1975. But the authors argue that it's probably time to revisit that work, since models of the convection inside stars have improved dramatically in the last 40 years. Once we redo these predictions, we'll have a much better idea of whether HV 2112 is the first member of this class of objects.

Monthly Notices of the Royal Astronomical Society, 2014. DOI not yet available.