THE STELLAR POPULATIONS OF THE GLOBULAR CLUSTER M55 By Georgi Mandushev B.Sc. Hon. (Physics) Sofia Univerisy St. Kliment Okhridski M.Sc. (Astronomy) Saint Mary's University A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES PHYSICS & ASTRONOMY We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA October 1998 © Georgi Mandushev, 1998 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Physics &; Astronomy The University of British Columbia 129-2219 Main MaU Vancouver, Canada V6T 1Z4 Date: Abstract New broad-band, ground-based photometry in four filters (UBVI) for two fields in the sparse, metal-poor Galactic globular cluster M55 (NGC 6809) is presented and analyzed. New values are derived for the reddening (EB_V = 0.13 ± 0.02 and Ev_i = 0.17 ± 0.02), distance modulus {{m~M)v = 14.02 ± 0.08) and age (14 ± 1.2 Gyr) of M55. The main-sequence luminosity function of M55 is found to be different from the luminosity functions of the metal-poor clusters M15, M30 and M92 and this difference is interpreted as a deficiency of low-mass stars by about 50% compared to the other three clusters. The mass function of M55 for masses below O.4A40 is found to be fairly flat and consequently low-mass stars do not dominate the cluster mass. The red giant branch of M55 has been observed from nearly its tip to the subgiant branch. In all passbands the observed luminosity of the red giant clump is lower than the predictions of theoretical models. The ratios of the number of stars on the red giant branch, the horizontal branch and the asymptotic giant branch are found to be in a good agreement with theoretical models. Neither the K-band nor the /-band luminosity functions for the evolved populations in M55 show any significant deviation from the theoretical luminosity functions. In particular, no evidence is found for a deficiency of main-sequence stars compared to the number of stars on the subgiant and giant branches. M55 is the only well-studied, metal- poor cluster for which no discrepancy between observations and canonical luminosity functions is found. A large sample of blue stragglers in the core of M55 is identified and analyzed. It is concluded that the blue stragglers in M55 are born with helium-enriched cores but not n envelopes, thus resembling stars that have already evolved away from the main sequence. It is also suggested that the observed blue straggler sequence represents the equivalent of a core helium-enriched main sequence where the blue stragglers spend most of their lives. The observations agree qualitatively with unmixed collisional or merger models, although the former are less likely in the low-density environment of M55. in Table of Contents Abstract ii List of Tables vii List of Figures ix Acknowledgements xii 1 Introduction 1 1.1 Globular Clusters and Stellar Populations 1 1.2 The Colour-Magnitude Diagram and the Evolution of Low-mass Stars . 5 1.3 Structure of Globular Clusters 13 1.4 Earlier Studies of M55 14 2 The Main Sequence of M55 17 2.1 Observations and Preprocessing 17 2.1.1 Flat Fields, Overscan Subtraction and Cosmic Ray Removal ... 18 2.2 Photometry 21 2.2.1 Instrumental Magnitudes 21 2.2.2 Aperture Corrections 22 2.2.3 Transformation to the Standard System 23 2.2.4 Comparison with Earlier Photometry 28 2.3 The Color-Magnitude Diagram of M55 31 2.3.1 Morphology and principal sequences 31 iv 2.3.2 The Stars of the Sagittarius Dwarf Galaxy and the Galactic bulge 36 2.4 Photometry of the comparison field 40 2.5 The Reddening and Metalhcity of M55 44 2.6 The Distance and Age of M55 47 2.6.1 Distance Modulus 47 2.6.2 The Age of M55 59 2.7 Luminosity and Mass Functions for the Main Sequence of M55 63 2.7.1 Completeness Corrections 66 2.7.2 Luminosity Functions 71 2.7.3 Mass Function 79 3 The Evolved Populations of M55 88 3.1 Observations and Preprocessing 90 3.2 Photometry 92 3.2.1 Instrumental Magnitudes and Aperture Corrections 92 3.2.2 Transformation to the Standard System 93 3.2.3 Quality of the UB VI Photometry: Errors and *2 99 3.3 The Color-Magnitude Diagram of Evolved Stars 106 3.3.1 Fiducial Sequences 110 3.4 The Horizontal Branch of M55 Ill 3.5 The Red Giant Branch of M55 119 3.5.1 The Gap at the Base of the Giant Branch and the RGB Clump . 119 3.5.2 Intrinsic Width of the Red Giant Branch 125 3.5.3 Population Ratios and the Helium Abundance of M55 129 3.6 Luminosity Functions for the Evolved Populations in M55 132 3.6.1 Artificial Star Tests and Completeness Corrections 133 v 3.6.2 Luminosity Functions in V and /: Theory vs. Observations . 137 4 The Blue Straggler Population of M55 151 4.1 The Blue-Straggler Sample - Definition and Completeness 152 4.2 Radial Distribution 159 4.3 Origin and Evolutionary Status 161 5 Conclusions 167 5.1 The Main Sequence of M55 167 5.2 The Evolved Populations of M55 169 5.3 The Blue Straggler Stars in M55 172 5.4 Future Directions 173 Appendices 175 A Summary of some important parameters for M55 175 B Publications Related to the Thesis 176 C Photometry Software Used in the Thesis 178 Cl DAOPHOT II 178 C.2 ALLSTAR 178 C.3 ALLFRAME 178 C.4 Calibration and Transformation Programs 179 D Journal Abbreviations 180 References 181 vi List of Tables 2.1 Program and comparison field exposure information 20 2.2 List of the observed Landolt standards 25 2.3 Local standards in M55 29 2.4 The fiducial main sequence and subgiant branch of M55 36 2.5 The fiducial main sequence of the Sagittarius Dwarf Galaxy 37 2.6 Galaxy counts in the comparison field 43 2.7 Observed and derived data for nearby subdwarfs 50 2.8 References for the compiled subdwarf data 51 2.9 Comparison of My(RR) estimates from different My(RR) - [Fe/H] rela• tions 58 2.10 Star and galaxy counts in the V band 71 2.11 Star and galaxy counts in the / band 72 2.12 Mass function slopes 84 3.1 Core field exposure information 92 3.2 List of the observed Landolt standards 94 3.3 Local standards in M55 97 3.4 Fiducial points for the giant branch, subgiant branch and the main se• quence 112 3.5 Fiducial points for the asymptotic giant branch 113 3.6 Fiducial points for the horizontal branch 113 3.7 Known and Suspected RR Lyrae Stars in the Core Field 115 vii 3.8 Star Counts and Population Ratios 130 3.9 K-band and /-band differential luminosity functions 144 4.1 Photometry for the blue stragglers in the core of M55 157 A.l Some parameters of M55 as derived in this work 175 vni List of Figures 1.1 Principal sequences of a globular-cluster colour-magnitude diagram ... 7 2.1 The location of the deep field relative to M55 19 2.2 Finder chart for the local standards in the deep field 27 2.3 Residuals for the Landolt standards 30 2.4 Color-magnitude diagram for all objects in the deep field 32 2.5 Color-magnitude diagram for the deep field with %2 restrictions 34 2.6 The principal sequences of M55 and other populations 39 2.7 Colour-magnitude diagram for the comparison field 41 2.8 Plot of the average image-sharpness index r0 versus V magnitude .... 42 2.9 Colour-colour diagram for the central field of M55 45 2.10 Fit of the main sequence of M55 to nearby subdwarfs 54 2.11 Agreement between the shapes of the main sequence and the isochrones . 56 2.12 Isochrone match to the photometry around the turnoff 60 2.13 Comparison between the observed and artificial main sequence 68 2.14 Differences between the input and output magnitudes and colour 69 2.15 Completeness fractions in the V and I passbands 70 2.16 The F-band and /-band luminosity functions 73 2.17 Comparison of the ground-based luminosity function of M55 with HST- based luminosity functions - faint normalization 75 2.18 Comparison of the ground-based luminosity function of M55 with HST- based luminosity functions - bright normalization 77 ix 2.19 Comparison of the mass-luminosity relations in V and / 80 2.20 Comparison of low-mass isochrones with the main sequence of M55 ... 81 2.21 The mass function of M55 83 2.22 More comparisons of the ground-based luminosity function of M55 with HST-ha.sed luminosity functions 86 3.1 Location of the core field relative to M55 91 3.2 Finder chart for the local standards in the core field 96 3.3 Residuals for the Landolt standards 98 3.4 Magnitude and colour errors as a function of V magnitude 100 3.5 Plot of x2 versus V magnitude 102 3.6 Rejection criteria in magnitude and colour index 104 3.7 The effect of error and \2 restrictions on the colour-magnitude diagram .