T. D. Oswalt

Stellar Evolution: For most of my career I have been interested in wide binary stars (“fragile binaries”) that were discovered during various proper motion surveys. More than 200 competitively awarded observing nights at the national observatories have provided spectra and BVRI, JHK photometry for several hundred newly identified white dwarfs in our sample. New techniques have been developed to determine the spectral classes, luminosities, temperatures and gravities of faint white dwarfs. Our project has produced a definitive luminosity function for white dwarfs in the solar neighborhood from which a lower limit of ~10 Gyr for the age of the Galactic disk has been determined. We are also measuring the gravitational redshifts of white dwarfs relative to main sequence companions, which provide intrinsic radial velocities, masses, and complete space motions for one of the largest known samples of white dwarfs. The goal is to make a definitive test of the mass-radius and initial-final mass relation for white dwarfs, which sets firm constraints on the mass recycling and star formation history of the Galaxy. Stellar Chromospheric Activity: My longest-standing research interest involves multi-wavelength observations of tidally interacting binary star systems, to monitor period changes caused by low levels of mass loss and/or enhanced chromospheric activity. A main goal is to identify periodicities in the light curves caused by photospheric spots, from which rotation rates and ages can be determined. We use cooling ages of white dwarf stars in wide binaries to calibrate the chromospheric activity vs. age relation in their distant late type main sequence companions. Our observational effort is conducted mostly at the national observatories. We expect to extend the rotation-activity-age calibration for G,K,M stars to far beyond that provided by stellar clusters (perhaps up to 10 Gyr). Minor Planets. I have long-standing side interest in photometric observations of asteroid, comet, and planetary satellite occultations. Such observations are the least expensive means of determining accurate sizes and shapes for these bodies, since they are in general too far away to be resolved optically.This program has involved several dozen students and has contributed useful data on over two dozen occultation events. Early in this program I organized an expedition to the Caribbean to cover a stellar occultation by Ceres that provided the first accurate determination of the size of the largest known asteroid (see Millis et al. 1987) more recently, I supervised observations of a stellar occultation by Pluto used to measure the current state of its atmosphere, in support of the New Horizons mission (see Bosh et al. 2015).