PHOTOMETRIC PARALLAXES AND SUBDWARF IDENTIFICATION FOR M-TYPE STARS A THESIS SUBMITTED TO THE GRADUATE SCHOOL IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE MASTER OF SCIENCE BY DAYNA L. THOMPSON ADVISOR: DR. THOMAS ROBERTSON BALL STATE UNIVERSITY MUNCIE, INDIANA JULY 2012 Acknowledgements First and foremost, I would like to thank my advisor, Dr. Thomas Robertson, for his endless guidance and patience throughout this project. I would also like to thank the Physics and Astronomy faculty and students, my family and friends, and everyone who put in observing time for this research. ii Abstract Photometric data on the Kron-Cousins photometric system have been obtained for 118 new late K to middle M-type stars with known distances. These data have been used to obtain absolute red magnitudes, to construct a color-magnitude diagram, and to compute a polynomial function for disk dwarf stars in the color range 1.5 ≤ R-I < 2.0, which can be used to compute absolute red magnitudes to be used for photometric parallaxes. Such photometric parallaxes allow new stellar distance estimations that are essential when modeling the spatial distribution of stars in our Galaxy. This is especially important for M-type stars, as they make up more than half of the mass of the Milky Way. Intermediate-band CaH observations have also been obtained in an ongoing effort to distinguish stellar luminosity classes and populations; R-L and R-I colors are used to identify possible subdwarf stars. A total of seven possible new subdwarfs and three previously known subdwarfs have been identified with this method. iii Table of Contents Acknowledgements ........................................................................................................... ii Abstract ............................................................................................................................. iii Table of Contents ............................................................................................................. iv List of Tables .................................................................................................................... vi List of Figures .................................................................................................................. vii 1. Introduction ..................................................................................................................1 1.1 Red Dwarf Stars .................................................................................................1 1.2 Parallax Methods ...............................................................................................2 1.3 Subdwarf Population ..........................................................................................4 1.4 Applications of Photometric Parallax and Subdwarf Identification ..................8 2. Methodology ...............................................................................................................10 2.1 Observations ....................................................................................................10 2.2 Photometric Calibrations .................................................................................11 2.3 Photometric Consistency .................................................................................13 3. Data Reduction ...........................................................................................................16 3.1 Binary Systems and Interstellar Reddening .....................................................16 3.2 Lutz-Kelker Corrections ..................................................................................16 3.2 Flare and BY Draconis Variable Stars .............................................................19 4. Data Analysis ..............................................................................................................21 4.1 Error Analysis ..................................................................................................21 4.2 Photometric Parallaxes.....................................................................................23 4.3 Two-Color Diagram .........................................................................................26 iv 4.4 Subdwarf Identification ...................................................................................28 5. Results and Discussion ...............................................................................................31 5.1 Photometric Parallax ........................................................................................31 5.2 Subdwarf Main Sequence ................................................................................32 5.3 CaH and TiO Absorption in Late M Dwarfs ...................................................36 6. Conclusions .................................................................................................................37 Appendix ...........................................................................................................................39 A.1 (ΔMR/σ, (R-I)) Standardized Differences in MR vs. R-I .................................39 A.2 Photometric Data............................................................................................40 References .........................................................................................................................44 v List of Tables Table 1 Pass-band Characteristics ...............................................................................5 Table 2 Transformation Coefficients .........................................................................15 Table 3 Possible Subdwarfs ........................................................................................35 Table A.2 Photometric Data ...........................................................................................40 vi List of Figures Figure 1 Two-Color Diagram for Warm Stars and Cool Dwarfs and Red Giants ...7 Figure 2 Parallax Star Proper Motion Distributions.................................................18 Figure 3 Lutz-Kelker Corrections ...............................................................................19 Figure 4 Photometric Consistency ...............................................................................22 Figure 5 (MR, (R-I)) Color-Magnitude Diagram........................................................25 Figure 6 ((V-I), (R-I)) Two-Color Diagram ................................................................27 Figure 7 ((V-R), (R-I)) Two-Color Diagram...............................................................28 Figure 8 ((R-L), (R-I)) Two-Color Diagram ...............................................................30 Figure 9 (MR, (R-I)) Color-Magnitude Diagram Identifying Subdwarfs ................32 Figure 10 ((R-L), (R-I)) Two-Color Diagram Identifying Subdwarfs .......................33 Figure A.1(ΔMR/σ, (R-I)) Standardized Differences in MR vs. R-I .............................39 vii 1. Introduction 1.1 Red Dwarf Stars If you open an astronomy textbook to learn about red dwarf stars, you may be surprised that there is little, or in some cases, no information. This might give one the impression that these M-type stars are insignificant; however, they make up more than half of the stellar mass of our galaxy. In fact, they make up four fifths of the stellar population. Yet, detecting M dwarf stars and finding their place in the Milky Way galaxy is a difficult task (Farris et al. 2012). These red dwarf stars that are no bigger than 0.6 solar masses go undetected due to their low luminosity. It is important to note that essentially all light produced in our galaxy comes from high luminosity stars, although they are few in number. Their domination in astronomical catalogs reflects this fact. Although early-type red dwarfs (K7 to M3) can be sampled out to 2-3 kpc, late-type red dwarfs (M4 to M9) are limited to samples within 100 pc. However, complete samples for late dwarf stars are available only out to 25 pc (Reid et al. 1995). To add a bit of perspective to these distances, consider that the diameter of our galactic plane is roughly 30 kpc. It is because of their large numbers that M-type stars reflect the galactic structure of our Milky Way. In addition, their long lifespan affords data concerning their formation and evolution. These are vital pieces for putting together the galactic puzzle (Spengler et al. 2012). The ability to determine distances to these stars will aid in defining their spatial distribution within our galaxy. 1.2 Parallax Methods Determining the distances to stars is a difficult and time consuming process regardless of the technique used. Astronomers can measure distances to nearby stars using their annual periodic apparent displacement, a technique called trigonometric parallax. Using the star’s change of location in the night sky in comparison to distant background stars, a parallax angle (measured in arcseconds) is found. Since both the Sun and target star are moving in the Milky Way, their relative positions to one another change with time. As a result, one requires observations over a few years to solve for these so-called proper motions. Once the parallax angle is known, basic trigonometry can be applied such that the following relation for distance is obtained ( ) 1.1 ( ) where p(“) is the parallax in arcseconds and d(pc) is the distance to the star in parsecs. Trigonometric parallax can be used for stars out to distances of ~200 pc. This study