Astronomers have long used stellar clusters to learn about the orbital motions of stars in bound groups and as laboratories for testing ideas of star formation and stellar evolution. The two fields of dynamics and stellar evolution have developed independently, but with advances in detector technology and computing speed, it is now possible to integrate these two areas of research.
In this collaborative study, Dr. Robert Mathieu (University of Wisconsin - Madison) and Dr. Imants Platais (Johns Hopkins University) will undertake a series of observational and theoretical investigations to address several outstanding questions in stellar evolution and dynamics. The first objective is to obtain a comprehensive census of the single star and binary star populations in seven rich open clusters which have ages from 100 million to 7 billion years. This will be accomplished through precise measurements of minute apparent tangential motions and multi-epoch measurements of radial velocities. These data will provide highly reliable three-dimensional kinematic membership determinations for the entire evolved cluster population. The second objective is to characterize the populations of nearly 100 blue stragglers and other newly discovered anomalous stars in these clusters, focusing on their binary, rotation and stellar activity distributions. The third objective is to compute an array stellar motion simulations for open clusters with initial conditions, especially the binary population, guided by observations of young stellar clusters. The simulations will be performed in collaboration with Dr. J. Hurley (Swinburne, Australia), and Dr. A. Sills (McMaster, Canada) will collaborate on the production of anomalous stars through collisions. The union of comprehensive rotational, stellar activity and binary data for a large set of anomalous stars of different ages and masses will allow new insights on formation mechanisms, internal structures, and ages of these stars at the interface of stellar dynamics and stellar evolution.
The research plan will have a broader impact through enhancement of the research and teaching skills of future astronomers, both within and beyond the borders of the United States. This research will include students and a postdoctoral fellow, and is expected to yield three graduate dissertations, several undergraduate theses, and research experiences of summer undergraduate students. This work will be associated with the Center for the Integration of Research, Teaching, and Learning, an NSF Center for Learning and Teaching directed by Mathieu. Platais, through his association with the trilateral Taiwan-Baltic collaboration, will assist in the education, training and research of students and young researchers in recently disadvantaged countries that have fast developing infrastructure and intellectual potential.
Our understanding of the lives of stars has been primarily based on theoretical models of single stars. In fact, most stars are in orbit with stellar companions; the Sun is unusual in not having a stellar partner. At the same time, observations of star clusters show that many stars do not have properties that agree with these single-star models. Perhaps the most famous are called "blue stragglers", since they are brighter and bluer than most stars in the cluster and thus should have previously exhausted their fuel and faded away. Our observations have shown that blue stragglers have company; indeed, 25% of the evolved stars in old star clusters do not follow standard stellar evolution tracks. These stars are not anomalies; rather they represent a rich array of alternative stellar lifestyles. The overarching outcome of the research reported here is that the high frequency of binary stars and the presence of alternative paths of stellar evolution are intimately linked. Most significantly, our research has shown the origin of most blue stragglers to lie in the transfer of material from one star to another within progenitor binaries, and furthermore has delineated the origin stories for specific blue stragglers. Our observations first showed that many (75%) blue stragglers in the old open star cluster NGC 188 are themselves binary stars, a strong hint that interactions within binaries likely play a key role in their formation. We next showed that the masses of the companions to the blue stragglers were about half that of the Sun – typical of white dwarfs. With the acquisition of Hubble Space Telescope ultraviolet data and sophisticated analyses developed under this grant, we were able to directly detect the white dwarfs orbiting several blue stragglers and statistically infer that white dwarfs are companions to at least two-thirds of blue stragglers in NGC 188. White dwarfs are the expected remnants of the mass transfer process in binary stars. The higher mass star evolves to become a giant star and then transfers all of its material onto its companion, except for the dense, hot core at its center that is revealed as a white dwarf. Meanwhile, the originally lower mass star becomes more massive, and therefore bluer and more luminous. The white dwarf companions to blue stragglers that we detected are still very hot, and therefore this mass transfer must have completed very recently, less than 250 million years ago. This youth allowed the development of the first specific models of the mass-transfer formation processes creating specific blue stragglers. We also have completed extensive surveys of the entire binary star populations in a set of fundamental clusters, yielding the largest database of orbital solutions yet derived for binary stars in clusters. Star clusters are crucial to a broad expanse of stellar astrophysics and thus these data are an asset for the entire community; for example, one of our target clusters has been intensively surveyed for exoplanets by the Kepler satellite. We have used these findings to explore the dynamical evolution of these star clusters, in part because theorists have long wondered whether the interactions between binaries in clusters might lead to stellar collisions that also produce blue stragglers, and might influence the frequency of mass transfer events. We developed international collaborations to run forefront "N-body" computer simulations of our clusters. Our observations provided accurate initial starting points and detailed tests of the simulations against clusters spanning their entire life span. In fact, the simulations were not able to produce enough blue stragglers, while over-producing normal red giants with white dwarf companions. Clearly the astrophysical processes in the models were not accurately reproducing the real frequency of mass transfer. Thus the integration of our observations and the theoretical models are yielding new astrophysical understanding about the interactions of stars in binary systems, and about the evolutionary paths of nearly a quarter of the stars in our Galaxy. This program has enhanced the research skills of future scientists and their preparation for effective teaching as future STEM faculty. The PI mentored five graduate students and seven undergraduate students in astrophysical research, including REU students from underrepresented groups in STEM. In addition the PI directed the NSF-funded Center for the Integration of Research, Teaching and Learning (CIRTL), whose mission is to develop a national STEM faculty committed to implementing and advancing effective teaching practices for diverse learners. The graduate students participated in an array of mentored activities to develop their skills in teaching, outreach and communication. As examples, they participated in the Delta Program in Research, Teaching and Learning (the CIRTL learning community at the University of Wisconsin - Madison), the UW Universe in the Park outreach program, the UW Space Place (with an emphasis on under-represented children) and the Expand Your Horizons program (with an emphasis on middle-school girls).
