Title

Modeling the White Dwarf Luminosity Function

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Physics and Astronomy

Date of Award

Summer 2012

Abstract

White dwarfs are the final stage of stellar evolution for the vast majority of stars in the Galaxy. The number of white dwarfs as a function of their luminosity (the white dwarf luminosity function, or WDLF) is the convolution of several key astrophysical processes, including the star formation history of a stellar population, the cooling physics of white dwarfs, and the stellar evolution of white dwarf progenitor stars. A software capable of incorporating different white dwarf and stellar evolution models, assumptions of star formation rates a function of time, and the relationship between the masses of white dwarfs and that of their progenitor stars is constructed to produce a model WDLF suitable for comparison to observed WDLFs. Once the validity of the calculations is confirmed, the tool is used to explore the observed WDLFs of a selection of intermediate-age open star clusters with well-studied white dwarf populations. These clusters have a known star formation history, a single burst at a known point of time, and the white dwarfs are young enough that the white dwarf cooling models are believed reliable. Previous studies have hinted that many open star clusters are missing significant numbers of white dwarfs. This study will, for the first time, use the differences between observed and predicted WDLFs to (a) confirm this deficit of white dwarfs, and (b) explore the timescale over which this deficit arises, thereby testing various proposed dynamical mechanisms for the white dwarf deficit. In addition, these calculations are applied to the star formation history of different stellar populations in the galaxy and the validity of the white dwarf cooling physics in a new generation of observed WDLFs. In particular, a forthcoming study by astronomers at the US Naval Observatory Flagstaff Station, Dr. Williams, and other collaborators is producing an empirical WDLF with two orders of magnitude more field stars than previous observed WDLFs. Using these models, the observed and theoretical WDLFs are compared to determine any discrepancies and potential underlying causes. This study will explore a suite of reasonable, parametrized star formation histories of the Galactic disk to create model WDLFs that best fit the observed WDLF. Remaining discrepancies will hint at possible shortcomings in white dwarf cooling physics.

Advisor

Kurtis Williams

Subject Categories

Physical Sciences and Mathematics | Physics

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