Rare Stellar Populations in the Magellanic Clouds: Identification of Super-Asymptotic Giant Branch Star and Yellow Supergiant Binary Candidates

In this thesis, I present a series of observational studies in which I identify and characterize multiple new populations of massive stars in rare or elusive phases of stellar evolution---thereby demonstrating the power of large surveys and systematic observation campaigns to constrain models of massive star evolution. First, I investigate the true nature of the peculiar star HV2112. Previous studies have identified HV2112 as the first Thorne-Zytkow object (TZO) candidate --- supergiants with with neutron star cores. However, others have argued its properties are more consistent with that of a super asymptotic giant branch star (sAGB), which are the most massive stars that do not undergo iron core collapse. I characterized the extreme variability of HV2112 and identified a population of 11 stars that possessed the same photometric and variability properties as HV2112. I derived physical properties and rates to uncover the true identity of this population. I find that all analyses are consistent with predictions for sAGB stars, while HV2112 itself remains ambiguous. This increases the population of sAGB star candidates by an order of magnitude, providing important information on the transition between low and high mass stellar evolution. I also demonstrate the feasibility of identifying yellow supergiant (YSG) binary systems using photometric data. Binarity plays a vital role in the evolution of massive stars: ~70% of massive stars should interact with a companion. Observational constraints on massive star binary populations in therefore crucial, but the binary fraction of YSGs is poorly-constrained. I demonstrate that optical photometry can be effective at discriminating between single YSGs and YSG+B-star binaries cooler YSG temperatures, while UV photometry can be effectively used to probe systems with hotter YSGs. I use these methods to identify hundreds of YSGs in the Magellanic Clouds that show signs of a blue/UV photometric excess. This demonstrates that optical/UV photometry can be a robust way to obtain better constraints on YSG binarity using large photometric surveys. Spectroscopic follow-up will still be necessary to understand the true nature of these systems, thus I also present some predictions and a pilot study of optical spectroscopy.

Department dc:contributor.department

Astronomy and Astrophysics

Year dc:date.issued

2023

Author dc:creator

• O'Grady, Anna Julia Gorman

Advisors dc:contributor.advisor

• Drout, Maria R
• Gaensler, Bryan M

• binary stars
• massive stars
• peculiar stars
• stellar astrophysics
• stellar populations

Statement dc:rights

• Attribution-NonCommercial 4.0 International

Licence dc:rights.uri

• http://creativecommons.org/licenses/by-nc/4.0/

Handle dc:identifier.uri

http://hdl.handle.net/1807/130063

OAI identifier oai:identifier

oai:utoronto.scholaris.ca:1807/130063