Thursday, October 9, 2014

Polluting a Red Supergiant Star with Heavy Elements

A recent study by Levesque et al. (2014) found that the red supergiant star HV 2112 in a nearby galaxy known as the Small Magellanic Cloud is enriched with various peculiar heavy elements. As a result, HV 2112 is postulated to be a Throne-Zytkow Object (TZO). Basically, a TZO is a red supergiant star with a neutron star at its center. The neutron star most likely got there by in-spiralling within the envelope of the red supergiant star, all the way down to the core. The neutron star destroys the red supergiant star’s core and part of the core forms an accretion disk around the neutron star. Temperatures and densities in the accretion disk are high enough to synthesize a range of peculiar heavy elements.

HV 2112 is observed to be enriched with heavy elements such as calcium, rubidium, lithium and molybdenum. Although most of these heavy elements can be synthesized by a red supergiant star on its own, the high calcium abundance requires HV 2112 to be a TZO since only in an accretion disk around a neutron star are the temperatures and densities sufficiently high to synthesize calcium. Nevertheless, a study by Sabach & Soker (2014) provides an alternative explanation whereby the high calcium abundance observed for HV 2112 came from a companion star that had exploded as a core collapse supernova (CCSN). During the supernova event, HV 2112 was already a large red supergiant star and could intercept more of the supernova’s ejecta, including the calcium that was synthesized in the supernova.

HV 2112 and its companion star started of as a pair of massive stars circling one another. The companion star is slightly more massive than HV 2112. Since more massive stars evolve quicker, the companion star first evolves to a red giant star, puffs up, and transfers some of its mass to HV 2112. Eventually, HV 2112 becomes more massive than its companion star and also evolves to become a red giant star. The companion star subsequently explodes as a CCSN. When that happens, HV 2112 has already evolved further to form a red supergiant star. Its large size allows HV 2112 to intercept a good fraction of the supernova ejecta that is rich in heavy elements, including calcium. As a result, HV 2112 could be a red supergiant star that was “polluted” by ejecta from a supernova, rather than a TZO.

- Levesque et al. (2014), “Discovery of a Thorne-Zytkow object candidate in the Small Magellanic Cloud”, arXiv:1406.0001 [astro-ph.SR]
- Sabach & Soker (2014), “A super asymptotic giant branch star enriched with calcium by a supernova as the origin of HV2112, rather than a Thorne-Zytkow Object”, arXiv:1410.1713 [astro-ph.SR]