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Analysis of the Halo Globular Cluster M30 and Its Variable Stars

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Analysis of the Halo Globular Cluster M30 and Its Variable Stars ANALYSIS OF THE HALO GLOBULAR CLUSTER M30 AND ITS VARIABLE STARS Michael T. Smitka A Thesis Submitted to the Graduate College of Bowling Green State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE August 2007 Committee: Andrew C. Layden, Advisor John B. Laird Dale W. Smith ii ABSTRACT Andrew C. Layden, Advisor Photometry of the metal-poor globular cluster M30 is presented in B, V, R and I bandpasses. A color-magnitude diagram created from this photometry indicates that accurate magnitude measurements were obtained for stars from the red giant branch down to approximately 2.5 magnitudes fainter than the main sequence turn- off. Time-series photometry is presented for six RR Lyrae type variable stars, three of which are newly discovered. Four variable stars of other types, three of them newly discovered, are also discussed. A metallicity value of [Fe/H] = -2.02 was adopted for this study. Using the RR Lyrae stars' mean colors at minimum light, a reddening of E(B V ) = 0:053 0:010 was found for this cluster as well as an extinction value of − ± AV = 0:165 0:031. A distance modulus of µ = 14:504 0:127 and the corresponding ± ± distance of 7:958 0:147 kpc was also computed using the RR Lyrae stars' mean ± magnitudes. Isochrone fitting of the color-magnitude diagram yielded a cluster age of 15:8 1:8 Gyr. ± iii ACKNOWLEDGMENTS I would like to thank my advisor, Andrew C. Layden, for all of his help and patience with my undertaking of this project. His knowledge and guidance were crucial at every step and none could have been taken without him. I would also like to thank my family. Particularly Mom & Dad for their love, support and for putting up with having a child in college for longer than four years. I also wish to say thank you to Uncle Ray for his amazing generosity which helped me to get this far. I thank Karen as well for her love and support. Finally, I wish for years of happy marriage between Pig & Ben and Joe & Jen. iv TABLE OF CONTENTS Page CHAPTER 1. Introduction :::::::::::::::::::::::::::: 1 CHAPTER 2. Observations :::::::::::::::::::::::::::: 6 CHAPTER 3. Photometry ::::::::::::::::::::::::::::: 10 3.1 DAOPHOT Photometry . 10 3.2 ISIS Photometry . 12 CHAPTER 4. Calibration ::::::::::::::::::::::::::::: 14 4.1 BGSU Calibration Set . 15 4.2 Calibration of All Stars . 21 4.3 Calibration of Variable Stars . 27 CHAPTER 5. The Color-Magnitude Diagram :::::::::::::::::: 30 5.1 Variable Stars on the CMD . 35 CHAPTER 6. Variable Stars ::::::::::::::::::::::::::: 36 6.1 RR Lyrae Stars . 40 6.2 Other Variable Stars . 51 CHAPTER 7. Discussion ::::::::::::::::::::::::::::: 60 7.1 Reddening and Extinction . 60 7.2 Distance . 63 7.3 Age . 65 7.4 Oosterhoff Type . 67 v CHAPTER 8. Conclusion ::::::::::::::::::::::::::::: 72 REFERENCES ::::::::::::::::::::::::::::::::::: 75 vi LIST OF FIGURES Figure Page 1.1 CMD: V vs. B-I Labeled . 2 1.2 The Zero-Age Horizontal Branch . 3 1.3 RRab & RRc Light Curves . 4 2.1 Sample M30 Image . 7 4.1 Stetson Calibration Stars . 16 4.2 Alcaino Calibration Stars . 17 4.3 BGSU Standard Star Magnitudes . 20 4.4 Post-Calibration Residuals in V . 24 4.5 Post-Calibration Residuals in B . 25 4.6 Post-Calibration Residuals in I . 26 5.1 Post-Calibration Residuals in V for CMD Data . 32 5.2 CMD of M30 . 34 6.1 Variability Index vs. V Magnitude . 37 6.2 Magnitude vs. HJD for Variable Star . 38 6.3 Magnitude vs HJD for a Non-Variable Star . 39 6.4 CMD of M30 with Variable Stars . 43 6.5 Horizontal Branch of CMD with Variable Stars . 44 6.6 Light Curves for s161 (RRab) . 45 6.7 Light Curves for s212 (RRab) . 46 6.8 Light Curves for s178 (RRab) . 47 6.9 Light Curves for s184 (RRc) . 48 vii 6.10 Light Curves for s181 (RRc) . 49 6.11 Light Curves for s193 (RRc) . 50 6.12 Blazhko Effect . 52 6.13 Magnitude vs. HJD for s5474 SS Cygni . 54 6.14 Magnitude vs. HJD for s65 . 56 6.15 Magnitude vs. HJD for s27 . 58 6.16 Magnitude vs. HJD for s1045 . 59 7.1 Dereddened CMD . 64 7.2 Isochrone Fits for Z=0.0001 . 68 7.3 Isochrone Fits for Z=0.0001 (MS) . 69 7.4 Isochrone Fits for Z=0.0001 (HB) . 69 7.5 Isochrone Fits for Z=0.0004 . 70 7.6 Isochrone Fits for Z=0.0004 . 71 viii LIST OF TABLES Table Page 2.1 Subsets of images of M30 analyzed independently . 8 4.1 Standard Magnitude Stars . 15 4.2 Calibration coefficients and relative uncertainties of the linear fitting process for the data set gathered at BGSU. 19 4.3 Magnitude Calibration Coefficients and Statistics . 22 4.4 Color Calibration Coefficients and Statistics . 23 6.1 Photometry of RR Lyrae Stars . 42 6.2 Data for RR Lyrae Stars . 45 7.1 Reddening Values from RRab Stars . 62 7.2 Metallicity Values . 62 7.3 Reddening Values . 62 7.4 Oosterhoff Type . 67 1 CHAPTER 1 Introduction For many years the observation of globular clusters has contributed vast amounts of knowledge about our galaxy and the universe as a whole. Globular clusters are densely packed, gravitationally bound groupings of stars found distributed around a galaxy. Believed to have formed at the earliest epoch of star formation, globular clusters provide astronomers with a powerful window into a galaxy's distant past as well as a useful tool for understanding stellar and galactic evolution models. One result of their old age is that the stars that comprise many globular clusters have very low metallicities. Typically, clusters are broken up into two categories based on their metallicity. Those with low metallicities ([Fe/H]< 0.8) lie in a spherical distribution − about the Galactic center, while higher metallicity clusters ([Fe/H]> 0.8) tend to − lie closer to the Galactic plane. It is thought that this distribution is the result of the metal-rich clusters forming after the metal-poor ones, after the Galaxy had been enriched with heavy elements and formed a disk shape. Another result of their old age is that globular clusters contain stars that are in greatly varying phases of evolutionary progress. This is most evident when viewed on a color-magnitude diagram (CMD) of a cluster (Fig. 1.1). One phase of stellar evolution that is of particular interest is when an individual star has ventured onto the horizontal branch of the CMD. This occurs after the core helium flash at the tip of the red giant branch and the star has migrated down the CMD toward dimmer and bluer values. Following this transition, the star then settles at a position somewhere along the zero-age horizontal branch (ZAHB) on the CMD. The position an individual star takes on the ZAHB is dependent upon the total 2 Figure 1.1: A CMD of M30 with the phases of stellar evolution superimposed. mass, core mass and chemical makeup of the star. Stars with total masses of roughly 0:7 0:8M are positioned on the horizontal branch (HB) nearby to an instability − strip inside which they can develop instabilities and begin to radially pulsate. A pulsating star could have its ZAHB location within the instability strip or it can have evolved there from a ZAHB location outside the instability strip (Fig. 1.2). The pulsation results from the energy output of the star's core cycling between ionizing atoms in the star's partial ionization zone, and thus increasing its opacity, and being released. This pulsation is observable as a change in the luminosity and color of the star and defines the characteristics of an RR Lyrae (RRL) type variable star. RRL stars have been observed to occur in three distinct classes, types RRab, RRc and RRd. The RRab type pulsate in the fundamental mode and display an asymmetric light curve with a steep rising branch, a pulsation period between 0.3-1.2 day and a 3 Figure 1.2: Evolutionary tracks moving away from the zero-age horizontal branch (ZAHB) are shown for stars of differing masses (in solar units). The dashed line rep- resents the ZAHB. The dotted vertical lines represent the boundaries of the instability strip. This plot was taken from RR Lyrae Stars by H. A. Smith (1995). variability amplitude between 0.5-2.0 magnitude in the V bandpass (Fig. 1.3(a)). RRc type stars pulsate in the first overtone mode and display a nearly sinusoidal shaped light curve, a pulsation period between 0.2-0.5 day and a variability amplitude of less than 0.8 magnitude in V (Fig. 1.3(b)). RRd type stars pulsate simultaneously in the fundamental mode and the first overtone. Their periods always appear very near the threshold value between RRab and RRc type stars. It is thought that RRd stars may be RRL's which are transitioning from an RRab type to an RRc type, or vice versa (Smith 1995). RRL variable stars are valuable to the astronomer because they function as a standard candle. A distance modulus can be calculated for an RRL with merely an observed mean apparent visual magnitude and an absolute visual magnitude, which 4 Figure 1.3: (a) A typical RRab light curve. (b) A typical RRc light curve. can be calculated based on metallicity (Chaboyer 1998). Distance measurements of this type have enabled the mapping of our own Galaxy and the distances of others, such as the Large Magellanic Cloud (Walker 1992). RRL stars are also valuable because reddening caused by the interstellar medium can be calculated by analyzing the star's color at minimum light (Blanco 1992).
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