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The Galactic Globular Cluster Ngc 3201: Primordial Binaries and Tidal Dynamics

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The Galactic Globular Cluster Ngc 3201: Primordial Binaries and Tidal Dynamics THE GALACTIC GLOBULAR CLUSTER NGC 3201: PRIMORDIAL BINARIES AND TIDAL DYNAMICS By Patrick Cote B.Sc.. ~LSc. A TheJi" Submitted to the Faculty of Graduate Studies in Partial Fulfillment of the Requiremenf., for the Degree Doctor of PhilOJophy McMaster University June 1994 (c) Copyright by Patrick Cote, 1994. DOCTOR OF PHILOSOPHY (1994) ~[d1.-\STER t-~lVERSITY {.\STROPHYSICS) Hamilton, Ontario TITLE: The Galactic Globular Clustt'r :\GC 3201: Prllllordial Binaries and Tidal Dynamics .-\t-THOR: Patrick Cote B.Sc. (l-niwrsity of \\'estern Ontario) ~LSc, (~Ic~laster Cnh'ersity) SCPER\lSOR: Dr. D. L. \Yekh Nl"~lBER OF PAGES: xv, 125 ii ACKNOWLEDGElVIENTS I am grateful to ),Idlaster Cniversity and the Department of Physics £.: .-\5- tronomv. for continued fundin!!- I. and XSERC for cominued in:miration... I. \Yithout ),Iarg, Wendy. CheryL Rosemary and Jackie to guide me through the paperwork. I might never have escaped the confines of graduate schooL ending up. instead. like a fossil entombed in GH-llO. The entire ensemble of students \\-ith whom I ha';e shared an office over the last four years - Ste\'e Butterworth. Pat DurrelL Phil Fis- cher, Dave Fleming, Steve Holland. Dean ),IeLaughlin. Kanan Patel. Jeff Secke:- and Mark \Valker - are hereby acknowledged for not just their counsel and tolerance but also for their unwavering eagerness to "talk shop"'. Especially profound thanks are reserved for Phil Fischer. who expertly served as a sounding board throughout the preparation of tbs thesis. The continuous encouragement of my family has helped to ease the pain that ine"-itably accompanies the pursuit of a career in as- tronomy. Thanks to the astronomy staff at MdIaster - Bill Harris. Ralph Pudritz. Peter Suthcrland~ Doug \Velch and Christine \Vilson - for almost never reminding me of the fine line between professor and student. And to Doug \Yelch - ad,,;sor extraordinaire and contagious enthusiast of everything astronomical - my sincere gratitude. But. most of all. thank you Abby for your constancy and understanding in all things great and small. iii For Abby iv \Ve can conclude that a project as grand as the scientific-mythical construction of victory over human limitation is not something that can be programmed by science. Even more, it comes from the \;tal energies of masses of men sweating within the nightmare of creation - and it is not even in man's hands to program. \Vho knows what form the forward momentum of life \\;ll take in the time ahead or what it will make of our anguished searching. The most that anyone of us can seem to do is to fashion something - an object or ourselves - and drop it into the confusion, make an offering of it, so to speak, to :he life force. - Ernest Becker \' v TABLE OF CONTENTS ..\.bstract .................................................................. .. IX List of Important Symbols.. ............................................ .. Xl List of ..\.cronyms xiii List of Figures xiv List of Tables. ........................................................... .. x'· Section 1 Overture . 1.1 Astronomical Context and Division of Labour , . 1.2 Observational Basics 8 Section 2 The Detection of an Extended Moving Group Near the Galactic Disk 12 2.0 .-\bstract 12 2.1 Introduction............ ............................................ .. 12 ')_._.) Observatlons. ..... .... .... .......... ... .............................._. 1" 2.3 Monte Carlo Experiments. .......................................... .. 13 2.-1 Ta.....onomy and ~Io,;ng Group Parameters.. ......................... .. l-l 2.5 Conclusions and Future \Vork '" 15 Section 3 A Search for Binaries in Globular Cluster NGC 3201 16 vi :to Abstract , 16 3.1 Introduction.... .................................................... .. 16 :3.~ Ohsen<l.tions and Reductions 1j' 3.3 The Radial Velocities.......... ..................................... .. 19 3.-1 The Binary Candidates 35 3.5 )'lodeling the Binary Population. ................................... .. 39 3.6 Discussion 40 3. j Summary 45 Section -l Dynamics of the Galactic Globular Cluster NGC 3201 4j ·4.0 .-\. bstract 48 4.1 Introduction 49 4.2 Observations and Reductions 51 ·4.2.1 Surface Photometry and Star Counts 51 4.2.2 Radial Velocities 53 4.2.3 Structure in the Velocity Field....... ......................... .. 54 4.3 Dynamical :\Iodels.. ................................................ .. 59 4.3.1 Single- and )'(ulti-Mass Models 59 4.3.1.1 Luminosity-to-Mass Ratios 62 4.3.1.2 Fitting the ~Iodels ...................................... .. 63 4.3.1.3 ~lonte Carlo Simulations 65 4.3.2 Xonparametric ).Iodels. ........... .. ....................... .. 66 4.4 Results 68 4.4.1 Previous \Vork on NGC 3201 68 4.4.2 ~Iass-to-Light Ratios 69 4.4.3 )'lass Function Slope. ......................................... .. 69 4.4.4 .~nisotropy ................................................... .. ;1 4.5 Summary. .......................................................... .. ;2 vii Section 5 Future \Vork 103 Section 6 Appendix: Radial Velocity Simulations 106 Section 7 References 1:?4 viii ABSTRACT 1859 radial velocities (median precision ~ 1 km S-I) have been measured for 1318 stars in the direction of the Galactic globular cluster XGC 3201. Since the systemic radial velocity for NGC 3201 is 494.2 km 5-1 , the field and cluster samples separate unambiguously into two distinct samples. Analysis of the radial velocities and APMjCCD photometry for the 879 field stars in our sample has revealed the presence of a probable mm-;ng group of'" 13 stars '\vith radial velocity 75 km S-1 at a distance of 6 - 12 kpc. For the cluster members, we have multiple velocities for 279 stars (known photometric variables excluded) spread over a ma."cimum of six years. Comparison of the observed velocities to simulated datasets containing knovm. numbers of binaries has yielded u.pper limi~ to the true binary fraction (for binaries with 0.1 ~ P ~ 5 - 10 years and mass ratios in the range 0.1-1.0) of 0.06-0.10 (circular orbits) and 0.15-0.18 (eccentric orbits), consistent with the corresponding incidence among nearby solar-type stars and among a sample of six other Galactic globular clusters with measured binary fractions. NGC 3201 appears to be rotating with an apparent projected rotation amplit11de of 1.22±0.25 km S-I. The observed increase in ostensible rotation amplitude with distance from the cluster core may, however, be due to either the projection of the cluster space velocity onto the plane of the sky, or the preferential stripping of stars on prograde orbits near the limiting radius due to the disk-shocking instability identified by Oh and Lin (1992). BV CCD images have bef:n used to derive cluster surface brightness profiles which extend out to ~ 18'. These profiles and the 399 mean radial velocities for non­ variable cluster members have been analysed using both single- and multi-mass King-Michie models and nonparametric techniq~es. Both methods suggest that the cluster mass-to-light ratio is relatively fiat in the range 1.5 - 10 pc: M/Ls ~ ix ~I!L\.. = 2.0±0.2 in solar units. The best-fit mass function has a spectral index of I :::: 0.75 ± 0.25, consistent \\;th recent findings that the fomi of the mass function depends on the position relative to the potential of the Gala.'\."}o. x LIST OF IMPORTANT SYMBOLS Amplitude of Projected Rotation Curve a Orbital Semi-:"lajor Axis Right Ascension 1,J.~ B j , B. R, I Broadband Photometric ~Iagnitudes c Concentration Parameter Surface Brightness uncertainty Anisotropy ~'Ieasure Declination e Eccentricity ErB- V) Colour Excess in B- V E Ellipticity 1J Dimensionless Velocity Dispersion [Fe/HI Logarithmic Iron-to-Hydrogen Abundance f(E,J) Distribution Function I(e) Eccentricity Distribution G Gra\;tational Constant £3 Scale Luminosity MB Absolute Blue Magnitude Mv Absolute Visual Magnitude (m.M}o Reddening-Corrected True Distance Modulus (m.M)\! Apparent Visual Distance ~Iodulus M/L Mass-to-Light Ratio Surface Brightness Degrees of Freedom [O/Fe] Logarithmic Oxygen-to-Iron Abundance p Orbital Period II,E),Z Velocity Components in Galactic Rest Frame xi Position Angle Nwnber of Stars per l7 nit ~lass S~condary-to-Primary :Mass Ratio Stellar Radius Galactocentric Distance Solar Radius r Projected Radius Anisotropy Radius Scale Radius Tidal Radius Po Central }.-Iass Density Ph Mean Density within Half-Mass Radius Pt Mean Density within Tidal Radius Luminosity Density Velocity Uncertainty; Core Velocity Dispersion til Cluster Disruption Timescale trO Central Rela."<ation Time trh Half-Mass Relaxation Time Td,O Central Binary Disruption Time < v~ >1/2 Central rms Velocity Mean Cluster Radial Velocity Vrot Projected Rotation Velocity Scale Velocity 'Wo Dimensionless Cluster Potential x Mass Function Spectral Index Binary Fraction Perpendicular Distance from Galactic Disk xii LIST OF ACRONYMS AAS American Astronomical Society ADU Analog-Digital Conversion ("nits APM Automatic Plate )"Ieasuring AU Astronomical Unit AGB Asymptotic Giant Branch CCD Charge Coupled De,,;ce CMD Color ~lagnitude Diagram CTIO Cerro Tololo Inter-American Observatory HB Horizontal Branch HST Hubble Space Telescope lAS Institute for Advanced Study IAU International Astronomical 'Union IMF Initial Mass Function lRAF Image Reduction and Analysis Facility LMC Large Magellanic Cloud LSR Local Standard of Rest NLTT New Luyten Two Tenths QE Quantum Efficiency RGB Red Giant Branch SBP Surface Brightness ProfilE' VDP Velocity Dispersion Profile , j' xiii LIST OF FIGURES Figure 1.1 BV Transmission Curves and Stellar Spectrum
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