Dr. Kurtis A. Williams
Research

The vast majority of stars end their lives in one of two ways. Some stars explode as supernovae, leaving behind either a neutron star or a black hole, while most stars lose their mass more gently and form white dwarfs. The criterion determining the fate of a star is thought to be its mass; more massive stars go supernova, while lower mass stars form white dwarfs. However, the value of this critical mass is uncertain, lying somewhere between 6 and 10 solar masses. Assuming standard stellar mass functions, a burst of star formation will produce as many stars in this uncertain range as it will stars of higher mass, meaning that the number of supernovae resulting from that burst of star formation is uncertain by a factor of two. Since supernovae are a major source of many metals and since they inject a lot of energy into the interstellar medium, it is important to better constrain the mass dividing white dwarf and supernova progenitors.

My research uses white dwarfs as astrophysical probes of the fates of stars. At its simplest, stars that form white dwarfs did not explode as core-collapse supernovae. So, if I can determine which masses of stars make white dwarfs, I can infer which masses of stars go supernova.

More specifically, I use white dwarfs in open star clusters to constrain this crucial dividing line. Since open star clusters are simple stellar populations, we know the age and metallicity of each star, including the white dwarfs. These constraints allow us to determine the masses of each white dwarf's progenitor star. Based on the work of my collaborators and myself, we can confidently claim that stars less massive than about 6.5 solar masses form white dwarfs. I am now expanding this work to more star clusters to try and constrain this dividing line further.

A few years ago, Patrick Dufour discovered that a white dwarfs belonging to a rare spectral class (the "hot DQ" white dwarfs) have atmospheres dominated by carbon; previously white dwarfs were only known to have atmospheres dominated by hydrogen or helium. Around this same time, I discovered one of these hot DQ white dwarfs in the young open star cluster Messier 35.

The presence of one of these hot DQ white dwarfs in the open cluster means that hot DQs likely originate from some of the most massive stars that produce white dwarfs; therefore, hot DQ white dwarfs may be observable probes of the evolution of these stars. Such a probe is desperately needed, as the final evolution and fate of these intermediate-mass stars involves physics that current models struggle with: convection, rotation, magnetic fields, and extreme mass loss.

So far, my collaborators and I have uncovered many properties of these hot DQ white dwarfs, and many of these properties are quite mysterious. I led the observing team which discovered that some of these stars are variable; we believe these are stellar pulsations that could allow us to probe the internal structure of the white dwarfs. My collaborators and I also discovered that half of these hot DQs, including the variable DQs, have strong magnetic fields. In normal white dwarfs, the incidence of magnetism is a few to ten percent, and no other class of pulsating white dwarfs is observed to have a magnetic field. We continue to develop new observational programs to study and characterize these very strange stars.

Strong gravitational lenses are systems where a background galaxy or quasar is lensed into multiple images by and intervening galaxy. These systems have been used to study cosmological parameters (such as the dark energy density and value of the Hubble Parameter) and to study the structure of dark matter halos in the lens galaxy. One important but often ignored detail in the interpretation of strong lensing is that of the lens environment. Any nearby concentration of mass, whether a single galaxy, a group of galaxies, or a cluster of galaxies, whether physically near the lens or projected along the line-of-sight, has the potential to distort the lens images and therefore bias the inferred cosmological conclusions.

My collaborators and I are finishing analysis of a wide-field survey of nearly 60 strong gravitational lenses. We are finding that nearly half of all lens systems are in a group or cluster environment, and a significant number also have contributions from line-of-sight structures. We are presently modeling these effects and preparing summary papers on our findings.