Studying a fire from its ashes: white dwarfs as probes of Milky Way evolution

Abstract

As the remnants of stars with initial masses < 8 M⊙, white dwarfs contain valuable information regarding the formation histories of stellar populations. This dissertation focuses on using white dwarfs as tracers of Galactic evolution by first creating a self-consistent model of the Milky Way’s white dwarf population and comparing the results of various inputs to observational white dwarf catalogues. The model is applied to data from the Canada France Imaging Survey to derive the star formation histories of the thin disk, thick disk, and stellar halo. The results show that the Milky Way disk began forming stars (11.3 ± 0.5)Gyr ago, with a peak rate of (8.8 ± 1.4)M⊙ yr−1 at (9.8 ± 0.4)Gyr, before a slow decline to a constant rate until the present day — consistent with recent results suggesting a merging event with a satellite galaxy. Studying the residuals between the data and best-fit model shows evidence for a slight increase in star formation over the past 3 Gyr.

The halo star formation history is relatively unconstrained owing to the relative rarity of halo white dwarfs. A complementary method to determine the age and star formation history is to obtain masses and temperatures to derive individual ages for a sample of halo objects. Using a sample of 18 spectra obtained at the Gemini Observatories the age of the inner halo is determined to be 9.3 ± 1.4 Gyr using the Cummings et al. (2018) IFMR and MIST isochrones, or 10.8 ± 0.6 Gyr using the relation from Kalirai (2012), however, the study determined that a bias is present in the mass determinations at low signal-to-noise and suggests that a larger, high signal-to-noise follow-up will be required to more accurately characterize this population.

Finally, the future of white dwarf astronomy will be in good hands with the imminent start of the Legacy Survey for Space and Time (LSST) on the Vera C. Rubin Observatory, as well as several new space telescopes expected to begin operations later in this decade. The white dwarf population synthesis model is modified to simulate the WD populations in four upcoming wide-field surveys (i.e., LSST, Euclid, the Roman Space Telescope and CASTOR) and use the resulting samples to explore some representative WD science cases. The results confirm that LSST will provide a wealth of information for Galactic WDs, detecting more than 150 million WDs at the final depth of its stacked, 10-year survey. Within this sample, nearly 300,000 objects will have 5σ parallax measurements and nearly 7 million will have 5σ proper motion measurements. This sample will be used to detect the turn-off in the halo WD luminosity function for the first time, allowing for an accurate determination of the age and star formation history of the Milky Way at its earliest epoch.