Critical Tests of Stellar Evolution Theory

Smithsonian Institution Astrophysical Observatory, Cambridge MA

Investigators

Abstract

Despite significant advances in recent decades in our understanding of the evolution and internal structure of stars, many aspects of theory remain untested or have been shown to disagree with observations. Evolutionary models are widely used by astronomers to infer various properties of stars such as the masses, ages, or overall chemical abundances, but our confidence in these models is limited to some extent by those disagreements and by a lack of observational constraints. Carefully selected binary systems are ideal objects to test and validate theory through a comparison with accurate measurements of fundamental stellar quantities such as the stellar mass, the radius, the effective temperature, or the luminosity. The aim of this project is to obtain these measurements for a number of astrophysically important systems and to perform "critical" tests of stellar evolution theory, namely, tests that can distinguish between competing models and are able to rule out at least some of them. Only in this way can progress be made towards understanding in detail where theory fails and how stars work. Specific areas that will be tested are the theory of convection (mixing length theory, overshooting), internal structure (apsidal motion, including relativistic contributions), rotation, mass loss, and tidal evolution (rotational synchronization and orbit circularization due to tidal forces). This project will go beyond merely showing the overall agreement with one or another set of models. The goal is to provide so many observational constraints for a given binary system that only the most realistic sets of models (if any) may satisfy all the observations within the errors, simultaneously for both stellar components and at a single evolutionary age (since they are presumably coeval). This project will focus on low-mass stars (which have highlighted many problems with theory), evolved stars (giants and subgiants) and very young (pre-main sequence) stars for which virtually no constraints are available, as well as stars with chemical compositions very different from the Sun (metal-poor). High quality spectroscopic, photometric, and astrometric observations will be obtained and analyzed to make determinations of the absolute stellar properties with relative errors no larger than 1-2% in the case of the masses and radii, which is the current state-of-the-art. The dissemination of the results to the scientific community mainly in the form of peer-reviewed publications is an integral part of this project. It is expected that this work will significantly advance our understanding of several poorly known areas of stellar astrophysics as indicated above. Improving theory will not only allow much more reliable determinations of parameters such as the ages of stars (old and young), which have profound cosmological implications, but it will also provide a more solid framework for the study of the origin and evolution of substellar objects such as brown dwarfs and extrasolar planets, which in recent years have become a subject of great interest for the public and scientists alike. This project will also provide excellent opportunities for training by integrating front-line research into the experience of learning. Two Ph.D. students will work with Dr. Torres towards completing their dissertations on binaries, and two postdoctoral associates as well as several undergraduates are expected to participate actively in all stages of this project.