Open Cluster Variables: Evasive, Invaluable Probes

Abstract

In this dissertation, I present my investigation of periodic variables in two open clusters --- M35 and NGC~2158 --- using photometric data supplied by the NASA K2 mission. To acquire high-precision photometry in these crowded stellar regions, these data were reduced using an image subtraction technique, which was tailored to address K2-specific systematics. Light curves for 3,960 sources were extracted and made publicly available. This work represented the first published image subtraction analysis of a K2 super-stamp. The detected variables were classified using light curve characteristics and stellar parameters obtained from the Gaia survey. This work culminated with the generation of a variable catalog containing 1,143 sources. Among the variable types included were 44 eclipsing binaries, 561 rotational variables, 251 pulsational variables, and two transiting exoplanet candidates. Despite the fact that other variability investigations had been conducted on this field, 46\% of my catalog sources were new detections.

The detection of cluster variables is an important scientific pursuit, as these coeval, isometallic stellar populations facilitate the investigation of stellar evolutionary processes with tight constraints. The implications of stellar evolutionary models extend far beyond the life cycles of stars, as these models play a critical role in our understanding of the formation and evolution of planetary companions, the structure and evolution of stellar systems, the ages and distances measured on extragalactic scales, and the cosmic evolution of nuclides.

In addition to the detection and characterization of cluster variables, this dissertation includes an investigation of the underlying mechanisms driving variability. High-precision photometry and high-resolution spectroscopy reveal sources that exhibit unexpected divergences from the predictions of standard stellar evolutionary models. One such anomaly is the class of rapidly-rotating, lithium-rich giant stars, which can be found in the field and within open cluster populations --- including the M35 system. Using lithium abundance measurements from the Galactic Archaeology with HERMES (GALAH) Survey and MESA stellar evolutionary models, I describe my investigation of one possible underlying mechanism: the engulfment of close-orbiting gas giants. Such a phenomenon would not be a rare occurrence. As stars evolve off of the main sequence, expanding hundreds to thousands of times in diameter, close-orbiting companions are engulfed. This is the fate awaiting the 1\% of sun-like stars that harbor hot Jupiters. My research showed that planetary engulfment events are capable of reproducing the enriched photospheric abundances of lithium-rich subgiants of $1.4-1.6$~\Msun{}. Moreover, I found that the lithium enrichment signatures observed among red giant stars cannot be explained by engulfment events and must be produced by other mechanisms, such as the Cameron-Fowler process or by the accretion of material from an AGB companion.

In addition to constraining the underlying processes responsible for anomalous observational signatures, there is utility in an unambiguous detection of a planetary engulfment event. Such a star would present a remarkable case study to probe the bulk composition of an exoplanet. The constraints offered by open cluster populations make these systems the ideal environments to search for such a source. Important next steps to my research efforts include modeling the rotational signatures induced by the transfer of angular momentum accompanying planetary engulfment, exploring the enrichment signatures of other key isotopes, such as $^9$Be and $^6$Li, and searching for these anomalous variables in open cluster systems using image subtraction techniques to mitigate blending.