title: Observational probes of helium ignition in low-mass stars
Creators: name: Manning, EM
Publisher: name: University of Tasmania
Year: 2019

description: The later evolutionary stages of Solar-type stars are governed by nuclear reactions that successively convert helium into carbon and carbon into oxygen. The relative numbers of highly evolved, low-mass stars as a function of luminosity and temperature can be analysed as a way to gauge the conditions under which these elements are synthesised and the energy released in their reactions. The most extreme event in the life cycle of low-mass stars is the explosive ignition of helium under electron degenerate conditions. This "helium-flash" involves the complete rearrangement of the stellar structure in a turbulent and non-equilibrium process. Although the theoretical reasoning for the occurrence of this violent event is comprehensive, observational evidence is virtually non-existent because the effects on the surface properties are subtle, and it occurs on a significantly shorter timescale than surrounding stellar evolutionary stages. Due to this, the precise details of the helium-flash (HeF) remain unclear. The physical conditions and processes occurring in low-mass stars during these advanced stages of their evolution are explored as part of this project. The initial aim is to test the predictions of red giant stellar evolution models by looking at the expected "death rate" of red giant branch (RGB) stars at the onset of core helium-burning by accounting for the lifetime and initial mass function calculations. Simulated population densities of red giants are created from PAR-SEC isochrones (Padova and Trieste Stellar Evolution Code) to calculate simple correction factors for over- and under-represented populations, depending on age and metallicity. These results have an immediate application of correcting spectroscopic samples, since the age- and metallicity-dependences of RGB lifetimes require careful modelling. Comparing the simulated number of stars populating the RGB and horizontal branch (HB) allows for the calculation of approximate evolutionary rates through the HeF phase for different stellar ages and metallicities. These data are combined with evolution tracks generated by mesa (Modules for Experiments in Stellar Astrophysics) to improve estimates. The clear trend in evolutionary rates is that longer times are associated with higher metallicities, and with the exception of younger stars, evolution rates do not vary significantly with age. These results are in agreement with previous studies where applicable, however this project covers a wider range of ages and metallicities than previously explored in this area. The calculated locations of the HeF in colour-magnitude coordinates derived here, could aid future observational searches for this elusive population of stars. The second stage of this project investigates whether post-HeF stars, before they settle on the HB, can be identifid in current stellar variability surveys, testing the predictions from the first stage. Data from the Optical Gravitational Lensing Experiment (ogle) are used here, since it is the most complete catalogue of variable stars currently available. Using Lomb-Scargle period analysis,182 variables in the Small Magellanic Cloud (SMC) are identified with changing periods (from a sample size of 2,383), including 20 of which are likely to be post-HeF candidates based on their negative rate of change in periodicity and their location in relation to the HB. The two methods for determining change in periodicity used here are tested against previous detections in similar star types, with excellent agreement. These candidates warrant follow-up observations in order to determine the cause of their changes in periodicity, and possibly give the first observational insight into a star's evolution immediately following a helium-flash.