Dr. Orosz will undertake the observation and modeling of a wide variety of close binary systems using in large part telescopes at San Diego State University's Mount Laguna Observatory. Binary stars offer the best opportunity for accurate measurements of the radii and masses of stars, measurements essential to verify stellar evolution theory and to determine the properties of other diverse objects such as white dwarfs, neutron stars, black holes, and extra-solar planets. Here, accurate physical parameters for a wide variety of close binary systems will be determined, including main sequence stars and black holes. A specific goal of this project will be to measure accurate masses and radii for low-mass main sequence stars in order to provide needed constraints on evolutionary models for the lower main sequence (namely the mass-radius relation).
This program builds upon recent advances in computer models for binary systems made by Dr. Orosz, most notably the incorporation of specific intensities from model atmosphere computations into a light curve synthesis code and a fitting code based on a genetic algorithm. It has been shown that in some cases the observed light curves can be reproduced much better with these tools, and that the resulting masses and radii of the component stars can be significantly different from what was previously thought.
This project also makes use of NSF-funded improvements to the Mount Laguna Observatory, including a completed high-speed Internet connection between San Diego State University and the observatory, substantial upgrades to the existing 1-meter telescope at the observatory, and a robotic 1-meter telescope that will be constructed using new mirror technology.
It is expected that fundamental parameters will be found for a large number of binary systems, with a broad impact across diverse areas of astrophysics. The modeling code will be made publicly available, to the benefit of the wider astronomical community. This research will involve advanced San Diego State undergraduate students, graduate students working on Master's Theses, and undergraduate students participating in the NSF-funded Research Experiences for Undergraduates program at San Diego State. Dr. Orosz will also discuss his research in his classes, and to the wider community through outreach events on campus and at the Mount Laguna Observatory.
One of the basic goals of stellar astrophysics is to understand how stars work. Observers measure basic stellar properties such as the mass, the diameter, the surface temperature, the luminosity, and the chemical composition. Theorists can construct computer models of stars by accounting for all of the relevant physical processes such as gravitation, nuclear energy generation, energy transport, and so on. The models need to be compared with observations. It has been found that the most important property of a star that influences its overall structure is its initial mass, so the usual way to compare observational data with model predictions is compare the observed diameter with the predicted diameter, given the observed mass. If there is a disagreement between the observations and the models, then both the observations and the models need to be checked. Perhaps there are problems with the observations, or perhaps some important physical process is missing from the models. Over time, the models and the observations converge on agreement. The best way to measure the masses and diameters for stars is through the study of eclipsing binary systems, which contain a pair of stars in a relatively close orbit that is viewed edge-on. The orbital motions of the stars (which can be measured using spectroscopy) can be used to deduce the masses. If the orbit is seen edge-on from Earth, the stars will eclipse each other every orbital period. During an eclipse, the star in front will block part of all of the light from the star in back, leading to a drop in the overall brightness. By carefully analyzing the drop in brightness over time, it is possible to deduce the diameter of each star. Note that only a small number of stars end up in an eclipsing binary that is suitable for study from Earth. In the case of a binary star, it is expected that the two stars will have the same age and chemical composition. This leaves the initial mass as the main variable. If the masses and diameters are available for both stars, then the theoretical models should match both stars at once. If one star's diameter agrees with the model predictions but the other one does not, then the discrepancy cannot be due to differences in the age or chemical composition. For stars with masses similar to the Sun's mass, the agreement between the measured diameters and the model predictions is found to be excellent. On the other hand, for low-mass stars (defined here as stars with a mass less than 80% of the Sun's mass), the agreement between the measured diameters and the model predictions is much worse. In general, the observed diameters are 10% to 15% larger than the model predictions. It is generally thought that some physical process is not completely included in the theoretical models, and that abnormally high levels of stellar activity might be to blame (many stars are observed to have spots analogous to sunspots but much larger). When this project was started, there were only a dozen or so known low-mass stars in eclipsing binaries that are suitable for study. The main goal of the project was to collect additional observations of the known low-mass eclipsing binaries, and to find new low-mass eclipsing binaries. The photometric observations were carried out at Mount Laguna Observatory. Several students from San Diego State University and elsewhere were trained to carry out the observations. A few of the previously known systems like GU Bootis were observed several times in order to understand the systematic errors caused by spots on the stars. Eclipses from about a dozen new low-mass eclipsing binaries discovered by NASA's Kepler mission were also observed. Many of the student observers were also trained to analyze the observations to produce measured masses and diameters for the stars under study. For the main result of this project, it was found that for many of the systems with repeated measurements, the observed diameters of the stars were systematically 10% to 15% larger than the model predictions for the measured masses. In addition, it was found for GU Bootis that the star spots in that system limit the precision to which we can measure the stellar diameters to a few percent (in many other eclipsing binaries, it is possible to measure the diameters to much better than 1% accuracy). On the other hand, there was one binary discovered by NASA's Kepler mission where the measured diameters of each star agreed with the model predictions. The stars that are inflated relative to model predictions generally have high levels of stellar activity. The stars in the Kepler binary that were not inflated seem to have much lower levels of stellar activity. This supports the notion that increased levels of stellar activity may account for the disagreement between the observed predicted diameters.
