Calibrating Stellar Chromospheric Activity and the Galaxy's Dark Matter Content Using White Dwarf Cooling Times

Florida Institute Of Technology, Melbourne FL

Investigators

Abstract

AST 0206115 Oswalt Dr. Terry Oswalt, at the Florida Institute of Technology, will carry out a spectroscopic study of wide binary star pairs that contain normal cool main sequence stars with distant non-interacting white dwarf companions. White dwarfs are the slowly cooling remnants of all previous generations of stars in the Galaxy. The wide binary sample contains white dwarfs whose cooling ages span the entire history of the Solar neighborhood (~10 billion years). Some may be members of the Galaxy's halo component. Dr. Oswalt's study has two main objectives: (1) How far beyond the Sun's age of ~4.6 billion years does the strong correlation between chromospheric activity and age in lower main sequence stars extend? What variables other than age (e.g. mass, metallicity) affect chromospheric activity? (2) What is the space density of cool white dwarfs stars, and how much does it contribute to the dark matter content of the Galaxy? What are the ages of the disk and/or halo derived from the white dwarf cooling times? For several decades it has been known that stellar chromospheric activity strongly depends upon stellar age in Solar-type stars. Recently, it has been recognized that this relationship breaks down, or at least depends on additional variables, in cool M-type stars less massive than about one fourth that of the Sun. However, all this prior work has been calibrated using stars in several nearby clusters whose ages were estimated from stellar evolutionary models. None are much older than the Sun. Stellar ages are among the most difficult to determine fundamental properties of stars. Cooling times for the white dwarfs in wide binaries, corrected for their progenitors' main sequence lifetimes, provide relatively robust estimates for the ages of their main sequence companions. In the course of a prior investigation Dr. Oswalt has determined cooling ages for several hundred white dwarfs in wide binaries. The uncertainty in age can be as small as 10-20% in pairs older than about 3-4 billion years, when the white dwarf mass is known. The recent discovery of a few halo white dwarfs has spurred intense interest because they are the remnants of the first generation of stars in the Solar neighborhood and may be a large component of the Galaxy's dark matter. Wide binaries may help to resolve the current dispute over the population membership of the candidates found to date. A main sequence companion provides a unique benchmark for the determination of the mass (via gravitational redshift), kinematics (via radial velocity) and chemical abundance (metallicity) that cannot be obtained from the near featureless spectra of single white dwarfs. ***