Knowing Our Neighbors:
Fundamental Properties of Nearby Stars

Dissertation Abstract

Within human experience, knowing our neighbors provides a sense of community. Similarly knowing more about the stars in our solar neighborhood gives us a sense of place within both our Milky Way Galaxy and the larger universe. The stars within a limited distance of the Sun make up the one sample of stars that we can hope to know thoroughly. This sample provides the basis for the stellar luminosity function and the mass-luminosity function. It also helps us understand the stellar component of the mass of the Milky Way and the history of our Galaxy. However, our understanding of the Milky Way relies on an incomplete survey of our solar neighborhood. Most of what we know and study in astronomy reflects the substantial, luminous objects that we can see, but we know many more undersized, dim ones also exist. In addition, the cosmic distance scale rests on the measurement of trigonometric parallaxes and intrinsic luminosities, but, for many objects within the reach of ground-based telescopes, these measurements have not yet been made.  The four studies reported here contribute to the census of the region within 25 parsecs (82 light-years) of our Sun through the application of astrometric techniques.

Astrometry is the discipline within astronomy that involves the precise measurement of stellar positions and motions; its most important technique is trigonometric parallax, which is also the only direct method of measuring the distances to stars. Parallax measures the very small angular variations in the position of stars as the Earth moves around the Sun, permitting the astronomer to triangulate on the star and, thereby, measure the distance. The angular variations are very small—at 25 parsecs (82 light-years), the star will shift only 0.04 seconds of arc—and their measurement requires careful collection and evaluation of data. Through the measurement of parallaxes, we not only establish distances to the specific stars observed, but we also calibrate and evaluate other indirect methods of distance measurement, such as photometric estimates, spectroscopic estimates, main-sequence fits, and standard candles. While measuring a parallax, we must also consider the proper motion of the star and separate the actual movement of the star through space from the parallactic shift caused by the orbit of the Earth. In addition, the presence of companions, including planets, will also cause a star to “wobble” as it and its companions orbit their common center of mass.

First, among nearby stars of special note is Barnard’s Star, a particularly interesting M dwarf, both because it is the second closest star to our Sun and because it has the highest known proper motion. It captured popular imagination during the 1960’s and 1970’s after Peter van de Kamp announced that it had at least one planet. Leander McCormick Observatory at UVa has the second largest collection of material on Barnard’s Stars. Expanding upon my master’s thesis (published abstract of technical poster). I have measured the relative parallax, proper motion, and secular acceleration of Barnard’s Star using more than 900 photographic plates taken at the Observatory since 1969; the results are comparable with measurements obtained using more modern equipment. Knowing these relative motions, the resulting residuals can be tested for any periodic motion, or “wobble,” that would indicate the presence of a low-mass companion. A time-series analysis of residuals using the Lomb-Scargle method found no evidence of planets; this study would have detected planets with 2.2 Jupiter masses or greater. 

Second, although Barnard's Star may not host substellar companions, an increasing number of extrasolar planets are being discovered. However, we do not know what fraction of stars within our solar neighborhood host such planetary systems. A similar approach was used to evaluate the astrometric residuals to stars observed by the UVa Southern Parallax Program. Of these, LHS 288 displays an intriguing signal, which might be caused by a very low mass companion and should be investigated further. Twelve other stars (LHS 34, 271, 337, 532, 1134, 1565, 2310, 2739, 2813, 3064, 3242, and 3418) demonstrate no astrometric perturbations. 

Third, while astrometry could reveal the presence of unseen companions, distances from trigonometric parallax define our solar neighborhood and identify its members. As part of the Cerro Tololo Inter-American Observatory Parallax Investigation (CTIOPI), I am obtaining astrometric, photometric, and spectroscopic observations of forty-three stars. Based on less accurate photometric and spectroscopic distance estimates, these stars are candidates for membership in the solar neighborhood. Preliminary parallaxes indicate that twenty-eight stars are actually within 25 parsecs (82 light-years), including three stars—LP 991-84, LHS 6167, and LP 876-10—that probably lie within 10 parsecs (33 light-years). Three more stars are close to the 25-pc boundary and their final parallaxes may qualify them as members. One recently established neighbor, LP 869-26, is a potential binary. This study of nearby stars is part of a larger effort to identify and characterize fully all stars within 10 parsecs of our Sun, led by Todd J. Henry of Georgia State University, Atlanta. For many stars in this third project, preliminary photometry (V, R, and I bands), spectroscopy, and proper motions are also available.

Finally, UVa was once a leader in the measurement of stellar parallaxes and proper motions but does not currently have an observing program. The parallax programs at Fan Mountain and Leander McCormick Observatories ceased observing in the 1990’s. The Southern Parallax Program concluded observations in 2002; the results of which are currently in preparation. However, the new infrared camera mounted on the refurbished 31-inch (79-centimeter) telescope at Fan Mountain Observatory provides an opportunity to renew astrometric research here. The measurement of parallaxes and proper motions requires a multi-year commitment of observing time; CTIOPI measurements typically require four seasons of observations spread over 2.5 years. Before UVa commits time and resources to such a program, I established the feasibility of measuring parallaxes in the infrared from this location. Such a program could provide much needed distances for brown dwarfs and very low mass stars.

Through this and similar efforts, we are establishing the foundations for understanding our Milky Way Galaxy, including its component stars and populations.

• Title Page
• Abstract
• Table of Contents
• Dedication
• Chapter 1 The Solar Neighborhood: Still Under Development
• Chapter 2 Barnard's Star: Parallax, Proper Motion, and Possible Planetary Perturbation
• Chapter 3 Planets in the South: Astrometric Search for Companions to Stars on the University of Virginia Southern Parallax Program
• Chapter 4 Solar Neighborhood Census: Identifying and Characterizing New Nearby Stars
• Chapter 5 Continuing the Solar Neighborhood Census:  Infrared Parallax Program at Fan Mountain Observatory
• Chapter 6 The Solar Neighborhood: Future Development
• Acknowledgements
• References
• Appendix A Finding Charts for Possible Nearby Stars