* Astronomy

Title: A super asymptotic giant branch star enriched with calcium by a supernova as the origin of HV2112, rather than a Thorne-Zytkow Object
Author: Efrat Sabach, Noam Soker (Technion, Israel)

We explain the high calcium abundance of the recently discovered red supergiant (RSG) star HV2112 in the Small Magellanic Cloud by the pollution from a more massive companion that had exploded as a core collapse supernova (CCSN). This is an alternative explanation to HV2112 being a Thorne-Zytkow object (TZO). We examine the evolution of binary systems where the primary star is slightly more massive than its secondary companion. The primary star evolves first to a red giant and transfers some of its outer envelope to its companion. When the primary explodes as a CCSN the secondary is already a RSG that intercepts a large fraction of the ejecta. The metal-rich ejecta from the explosion pollutes the secondary to a level that is compatible with the enhanced calcium abundance of HV2112. We use Modules for Experiments in Stellar Astrophysics (MESA) to present the evolution of two stars with initial masses of 13.5 Mo and 13 Mo that might potentially explain HV2112. We estimate that on average at any given time tens of such objects are present in the Galaxy, and ~10 such objects are present in the Magellanic Clouds. This scenario adds to the variety of peculiar astrophysical objects related to explosions and eruptions.

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