First Thorne-Zytkow Object Found 200,000 Light-Years Away

HV 2112 – a giant, variable star in a nearby galaxy known as the Small Magellanic Cloud – represents a long-sought class of stars called Thorne-Zytkow objects, says an international group of astronomers led by Dr Emily Levesque from the University of Colorado.

Thorne-Zytkow objects are a class of star in which a compact neutron star is surrounded by a large, diffuse envelope. This is an artist’s impression of a neutron star. Image credit: Casey Reed / Penn State University.

This image shows the giant, variable star HV 2112. Image credit: Digital Sky Survey / Centre de Données astronomiques de Strasbourg.

First proposed in 1975 by physicist Kip Thorne and astronomer Anna Zytkow,Thorne-Zytkow objects (TZOs) are hybrids of red supergiant and neutron stars.

They superficially resemble normal red supergiants, such as Betelgeuse in the constellation Orion, but differ in their distinct chemical signatures that result from unique activity in their stellar interiors.

TZOs are thought to be formed by the interaction of two massive stars – a red supergiant and a neutron star formed during a supernova explosion – in a close binary system.

While the exact mechanism is uncertain, the most commonly held theory suggests that, during the evolutionary interaction of the two stars, the much more massive red supergiant essentially swallows the neutron star, which spirals into the core of the red supergiant.

While normal red supergiants derive their energy from nuclear fusion in their cores, TZOs are powered by the unusual activity of the absorbed neutron stars in their cores.

Dr Levesque’s team used the 3.5-meter telescope at Apache Point Observatory to examine the spectrum of light emitted from 24 red supergiants in our Milky Way Galaxy.

They also used the 6.5-meter Magellan Clay telescope at Las Campanas Observatory to study 16 red supergiants in the Large Magellanic Cloud and 22 red supergiants in the Small Magellanic Cloud.

When the spectrum of one particular star – HV 2112 in the Small Magellanic Cloud (located about 200,000 light-years away from Earth) – was first displayed, the astronomers were quite surprised by some of the unusual features.

When they took a close look at the subtle lines in the spectrum of HV 2112, they found that it contained excess rubidium, lithium and molybdenum.

Past research has shown that normal stellar processes can create each of these elements. But high abundances of all three of these at the temperatures typical of red supergiants is a unique signature of TZOs.

“I am extremely happy that observational confirmation of our theoretical prediction has started to emerge,” said Dr Zytkow from the University of Cambridge, who is a co-author of the paper accepted for publication in theMonthly Notices of the Royal Astronomical Society Letters (arXiv.org version).

“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.”

The team is careful to point out that HV 2112 displays some chemical characteristics that don’t quite match theoretical models.

“We could, of course, be wrong. There are some minor inconsistencies between some of the details of what we found and what theory predicts,” said co-author Dr Philip Massey of Lowell Observatory in Flagstaff, Arizona.

“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.”