The field of stellar astrophysics is the study of the origin, formation, evolution, and fate of stars and of the mechanisms (either nuclear, atmospheric, or exterior interactions with companions/other objects) by which they work and interact with their stellar environment.
Photometry and Cluster Populations: For the past twenty years, Boyle and his collaborators have been using the 1.8 meter VATT, the US Naval Observatory 1 meter telescope in Flagstaff, Arizona, and the Moletai Observatory Maksutov telescope in Lithuania to refine a system of specially chosen filters, the Vilnius seven-filter intermediate-band photometric system, to classify stars in stellar clusters. This work not only involves observing the clusters with this filter set but also testing this system with more traditional observations to confirm that it provides consistent classification.
Accurate photometric classification of stars (in temperature and surface gravity and luminosity and metallicity), even in interstellar dust-reddened fields, has allowed this team to determine stellar populations, distances, reddening and total extinction AV (and metallicities in some cases) for members of open clusters and field stars. Their photometric studies have determined how to distinguish, from stars found near each other in a particular field of view, which stars belong to clusters and which are field stars. This allows for a comparison of different clusters in the Galaxy, which can lead to a better idea of mass distribution in the Galaxy. They have also made similar photometric studies for fields defined for the Kepler space mission, which has been key in the detection of earth-sized planets.
The Vilnius photometric system can be of immense use in a survey of multiple stellar populations in globular clusters (and some open clusters). Determining the multiple main sequences in such clusters, and finding the distribution of EHB stars along the horizontal branch in the Hertsprung-Russell diagram (of such clusters), can help discriminate among various possible evolutionary mechanisms by which such groupings arose. These extensive studies in Strömvil band-passes will further benefit future international astronomy research because the 14-filter intermediate-band filter set for the upcoming GAIA space telescope will include seven filters virtually identical to the Strömvil band-passes and seven other filters. This means that the GAIA and Strömvil photometric systems will be among the systems most used to make measurements of stars.
Stellar Periodicity: Using the high speed Galway UltraFast Imager (GUFI) camera on the VATT, Boyle and collaborators have determined the optical periodicity in ultracool radio pulsating dwarfs. Similarly, they have participated at the VATT in a large multi-wavelength campaign to investigate anomalous radio emissions in brown dwarfs, which has shed light on the mechanism producing these radio emissions and established their association with the presence of magnetic spots on the surface of brown dwarfs.
Hot Subdwarf Stars: Brown has been involved in the study of formation channels of hot subdwarf (sdB/sdO) stars/EHB stars, in both the Galactic field and also in globular clusters. In recent years, surveys like Kepler have yielded a wealth of information, especially through asteroseismology. Brown has used theoretical modeling using stellar evolution codes and population synthesis programs to try to understand the exact formation mechanisms of these stars, producing models that can be tested in part by the results of those surveys.
These hot subdwarf stars are very much intricately linked with a number of issues in astrophysics. These include the second parameter problem and multiple stellar population puzzles in globular clusters; binary evolution via Roche Lobe Overflow; common envelope evolution; and white dwarf mergers. On a galactic scale understanding these stars is key to explaining the ultraviolet upturn phenomenon in giant elliptical galaxies and bulges in spiral galaxies, concerning which the GALEX mission has yielded many useful results. However the exact formation mechanism of such sdB stars remains a mystery.
Spectroscopy: Corbally is currently participating with researchers from Belgium, China, Italy, Poland, and the USA in a spectroscopic follow-up to exoplanet surveys, such as that conducted by Kepler. The aim is to use spectral ground-based data for these stars, including data from the Chinese-administered 4-m LAMOST telescope, to determine the physical parameters of stars in the Kepler data. Gray (Appalachia State University) and Corbally will complement this effort by obtaining the stars’ spectral classifications, as a check on those obtained by LAMOST. Given the high number of stars, they have developed an automatic classification code, MKCLASS, for this task. This code surpasses any previous auto-classification method in accuracy and scope.
Corbally has been a key participant in the Nearby Stars (NStars) project, the spectral analysis of the 3600 stars within 40 parsec of the Sun and earlier than M0 spectral type. The goal of this program is to determine new, precise spectral types and basic physical parameters (Teff, log10g, [M/H]) for such objects. These results can be applied to problems ranging from the measure of the chromospheric activity of these stars, to better understanding the basic physical parameters of stars with exoplanets. For example, one result suggests that stars with planets (extra-solar planets) have a higher heavy element abundance than those without planets.
Corbally and collaborators have been studying the spectroscopy of peculiar stars and binaries, including those in the Aries cloud, to determine precise spectral classes, luminosities, and any peculiarities. From this a distance and extinction study could be completed for star-forming regions of our Galaxy. Another spectroscopy project by this group is looking for planetesimal belts around binary systems, by examining debris disks in main sequence binary systems using the Multiband Imaging Photometer on the Spitzer Space Telescope. In addition, extensive work is being done on peculiar stars such as the lambda Boötis type and Barium II (Ba) stars, which have yet to be explained by standard theories of stellar formation and evolution. And finally, Corbally is working on an encoded system for classification of stellar spectra which are stored in individual databases.