This project is an observational program to obtain precise measurements of various types of binary stars in order to test models of stellar structure. In general, most theoretical models are calibrated using the properties of the Sun; the models for other stars are therefore extrapolations in stellar mass or composition. The main goal for this project is to obtain spectroscopic, photometric, and astrometric data for certain nearby, low-mass stars to improve the empirical mass-radius relation to a precision of a few percent.
Data from this project will lead to improved models of stellar evolution, which can be applied to a variety of problems in stellar astrophysics including the accurate determination of the properties of extrasolar planets. The project will also provide opportunities for undergraduate researchers.
The theory of stellar evolution is at the foundation of much of astrophysics. Models of how stars evolve with time are used to predict quantities that are difficult to determine by observation, such as the mass and size, or that depend entirely on theory, such as the age. Ages of various populations of stars are essential to piece together the history of our Galaxy and the Universe, and also to understand planet formation. Stellar evolution models have been shown in previous studies to be in disagreement with accurate observations of stars less massive than the Sun, in the sense that real stars in that range are often larger and cooler than predicted for their mass. This is generally believed to be caused by magnetic activity and/or starspots on these objects. Other classes of stars also show disagreements with theory. This project focused on binary stars because the mass of a star (its most fundamental property) can only be measured if it is in a binary system, through the gravitational interaction with the companion. The goal of the project was to enable improvements in the models by observing and measuring the properties of stars in selected binary systems to high accuracy (1-2%), and comparing them against theory. This enables us to discover gaps in our knowledge, and guide the work of theorists to improve their models. For this project we determined accurate properties (masses, radii, temperatures, luminosities) for the stellar components of more than two dozen binary systems, including the chemical composition in some cases. These measurements revealed a number of discrepancies with theory, and also pointed the way to possible sources for the differences. For low-mass stars we confirmed the anomalies described above, and found that new models that take into account magnetic fields are able to match the observed stellar properties with plausible surface field strengths. This showed that we are on the right track to understanding the discrepancies for stars less massive than the Sun. Other disagreements with theory were found for more massive and evolved stars in binaries that have nearly exhausted their central hydrogen fuel. In this case indications are that the problem has to do with the treatment of convection (turbulence) in the interiors of these stars, which is probably too simplistic in current models. For very young stars in eclipsing binaries that are only 1 to 20 million years old the comparison with theory showed generally poor agreement, and suggested that the presence of more distant third components in these systems may be to blame. The findings resulting from these studies were all published in peer-reviewed journals. The impact of these results goes beyond the discipline of stellar astrophysics, and directly influences other scientific endeavors such as the quest for Earth-size planets in the habitable zone of their parent stars. This is because the planetary properties can only be established after we know the properties of the host stars, and those in turn are critically dependent on the use of stellar models. Thus, errors in the stellar models can lead scientists to infer the wrong properties for the planets, confusing the picture and misleading further efforts to study what could turn out to be much larger planets (and potentially less interesting, at least from the perspective of finding life on other worlds) than previously thought. This project also provided excellent opportunities for student training. Over the 4-year life of the award the PI worked with eight students and postdocs, including minorities and women, on individual studies of binary systems related to the project. In many cases this resulted in publications that featured the students as first authors.
