Stellar exotica in the field and globular clusters

Abstract

Stellar exotica are unusual stars or binary systems which are distinctive due to variability in their brightness, or by their emission in the ultraviolet and X-ray wavelengths which indicate high energies. These stars and systems demonstrate some of the most ferocious physics known to astronomy, such as colliding stars, mass accretion and nova explosions. Some stellar exotica form due to the interactions between binary components, for example blue straggler (BS) stars, cataclysmic variables (CVs) and X-ray binaries. Others show fluctuations in their luminosities from instabilities in their interiors, such as RR Lyrae variables and M-dwarf stars undergoing the convective kissing instability (CKI). An understanding of how these stars defy the norm broadens our knowledge of stellar evolution, and the extreme physics taking place deepens our fascination of the profound and diverse universe. <br /> This thesis explores these stars and systems by processing far-ultraviolet (<em>FUV</em>) exposures taken by the <em>Hubble Space Telescope</em> of the globular cluster M30, and computing models using Modules for Experiments in Stellar Astrophysics (MESA). The sources in M30 are catalogued and provided to the VizieR database at the Centre de Donnees astronomiques de Strasbourg (CDS). The blue and red sub-populations of the double BS sequence in M30 appear mixed in the ultraviolet, and the radial distribution of BS stars indicates that mass segregation has taken place, confirming that M30 is a core-collapsed cluster. Measuring the magnitudes over time reveal several sources demonstrating variability. One BS star is a known W-UMa contact binary and one of the gap objects is a known CV identified in this work to be experiencing a dwarf nova event. Two variable horizontal branch stars are previously known RR Lyrae variables of types RRab and RRc, and one is potentially a new RRab classification. Assessing stars in colour magnitude diagrams (CMDs) of different wavelengths identifies other interesting sources. Within the <em>FUV</em> field of view are ten known X-ray sources, to which six confident counterparts are made, consisting of two CVs, one RS CVn, one red giant with strong <em>FUV</em> emission and two sources only detected in the <em>FUV</em>. <br /> A recent discovery in the CMD from the <em>Gaia</em> datasets has revealed a new type of stellar exotica: M-dwarf stars undergoing the CKI. To investigate this phenomenon, models are calculated with MESA using very fine mass increments and a maximum timestep. CKI stars experience repeated merging events of their convective core and envelope resulting in fluctuations in luminosity and temperature. This leads to a discontinuity in the luminosity-mass relation and loops in the evolutionary tracks which explains the low-density region of the M-dwarf gap. The CKI is reduced with increasing amounts of overshooting but sustained with semi-convection. The width of the MS decreases over time along with the difference in MS width between masses higher and lower than the CKI. The parallel offset between the upper and lower MS also change with time, providing a potential age-dating method for single stars and stellar populations.