Cool Stars and Contingencies: Stellar Characterisation in the Solar Neighbourhood

Abstract

The coming decades will see stellar parameters like temperature, radius, and metallicity produced for many millions of new stars as part of both current and upcoming massive stellar surveys. However, for any new insights into stellar or Galactic physics to reach their full potential, these parameters need to be calibrated by a library of benchmark stars whose properties are as fundamental and model independent as possible. Thus was the motivation for my PhD work as presented here: to extend the currently available library of stellar standards, and to use this library as part of the calibration strategy for upcoming stellar spectroscopic surveys.

For stellar temperature and radius, there is nothing more accurate or precise than when these properties are calculated from the angular diameters measured by long-baseline optical interferometry. Using VLTI/PIONIER, I inducted a new set of 10 stars into the ranks of stellar benchmarks, and confirmed another six with higher precision than ever before. These results, precise to the 1% level, will serve as critical benchmarks for all current and future surveys in Galactic archaeology.

One such survey was the planned FunnelWeb Survey. A low resolution spectroscopic survey of two million of the brightest stars in the southern hemisphere, FunnelWeb was built around the novel TAIPAN instrument and its ability to rapidly reconfigure its 150 optical fibres in parallel. This high-multiplex capability and broad range in targeted magnitudes combined to provide unique challenges in optimising survey efficiency and yield. Here I present a sky tiling algorithm for this class of survey, including a six step priority scale and overlapping magnitude bins, able to efficiently allocate observing fields to a high level of survey completeness.

A more traditional survey involves observing stars one at a time using a high efficiency spectrograph with greater resolving power. WiFeS on the ANU 2.3m Telescope is one such instrument, and is well suited to a survey of exoplanet host stars identified by NASA's TESS Mission. The result was the spectroscopic characterisation of 92 cool dwarfs and transit light curve modelling of 100 planet candidates. Given known complexities modelling cool dwarf atmospheres, I quantified model deficiencies at predicting optical fluxes, and developed an empirical photometric relation to determine cool dwarf metallicity independently of spectroscopy. This large and uniform sample will prove instrumental to future demographic studies of planets around cool dwarfs - a historically small, but now rapidly growing sample thanks to TESS.

My PhD research has provided new insight into the stars and planets of the Solar Neighbourhood and improved our ability to calibrate broader surveys of our Galaxy as a whole. While I have demonstrated the strength of tried and tested methods, my work has made apparent the need for non-traditional analysis techniques for dealing with challenging regions of the parameter space - techniques that should be well suited to the data-rich environment stellar astronomers will soon find themselves in. Show more