White dwarfs are by far the most common end-products of stellar evolution. In addition to a general interest in their final physical properties, such as mass, chemical composition, and radius, they give important information on the cumulative mass lost by stars through various stages of their evolution. The Principle Investigator (PI) will continue past studies to clarify the physical properties of a statistical sample of white dwarfs. These investigations will focus on two specific areas. First, clusters of stars formed at the same time ("galactic clusters") will be surveyed for white dwarfs with modern spectroscopic means, using Charged Couple Device detectors. This survey will lead to a better estimate of a special delimiting mass for precursor main sequence stars (now thought to be about six times the mass of the Sun). Stars below this limit are thought ultimately to become white dwarfs, whereas stars just above it probably become supernovae and neutron stars or pulsars. The PI will use modern theoretical stellar atmosphere techniques to estimate the values of several mutually dependent characteristics of white dwarf atmospheres, such as their light element chemical abundances and surface gravities. It is hoped that this will solve one of the current astrophysical riddles about white dwarfs: why are most white dwarfs observed to have so low an abundance of hydrogen compared to what stellar models "evolved" to their white dwarf states predict?
