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Abstract

Supervisors: Dr. C. D. Scarfe, Dr. T. J. Davidge

The open clusters NGC 2141, NGC 6791, NGC 6819 and NGC 7142,

all suspected of having ages greater than 2 billion years (Gyr), were ob-

served at optical and near-idkared wavelengths. The images were reduced

using standard IRAF routines, and magnitudes for the stars were determined

using DAOPHOT (Stetson, 1987). These data were used to construct colour-

magnitude diagrams ( CMDs) for each duster, as well as two-colour diagrams

( J - K , V - K),(J- H , H - K ) ofthegiants.

Colour excesses were redetermined by comparing the optical CMD main

sequences to semi-empirical ZAMS calibrations (VandenBerg and Pou, 1989;

this work) and are as follows: E(B - V) = 0.32 f. 0.04, 0.23 f 0.03, 0.11

& 0.03 and 0.29 f 0.04, for NGC 2141, NGC 6791, NGC 6819 and NGC

7142, respectively. Apparent distance moduli for the clusters listed above

were found to be (rn - M)v = 13.93 f 0.13, 13.52 k 0.13, 12.10 f 0.13 and

12.96 it 0.16.

The optical CMDs were compared to sets of theoretical isochrones to

ascertain ages and test whether canonical or convective overshooting models

best represent the data. I t was found that isochrones which allowed for

convective overshooting provided the best fits, resulting in ages of 2.5 Gyr,

10 Gyr, 2.5 Gyr and 2.5 Gyr for NGC 2141, NGC 6791, NGC 6819 and

NGC 7142, respectively. Two sets of overshooting isochrones (Bertelli et

al., 1994; Dowler and VandenBerg, 1996) yielded ages within 0.5 Gyr. The

MAR method (Anthony-Twarog and Twarog, 1985) pIaced the three younger

clusters a t an approxhate age of 3 Gyr.

In theory, the two-colour diagrams may be used to distinguish between

cluster giants and field stars. However, in practice this is not an easy task

since the a a r e d observations are not always accurate enough to separate

the cluster members and field stars. This was the case for these data, since

a problem with the H magnitudes resulted in colours offset from what was

expected.

The a a r e d (V, V - K and K, V - K) CMDs were usefd in defining the &nt branch locus based on the position of cluster members. (V- K)o colours

were computed for each giant suspected of being a member. These were used

to determine effective temperatures and bolometric luminosities, which in

turn were used to produce an HR diagram for each cluster. These were

compared to HR. diagrams of other open and globdar clusters (Houdashelt

et al., 1992; Frogel et al., 1983), as well as evolutionary tracks (Bertelli et

al., 1994). The giant branch loci of the near-solar abundance and metal-poor

clusters were found to lie between those defined by the clusters M67 and 47

Tuc. The cornparison between the cluster HR diagrams and evolutionary

tracks indicated that the theoretical temperatures may be too hot.

The new cluster results were plotted on the age-metalIicity relation de-

fined by Houdashelt et al.'s (1992) and Friel and Janes' (1993) sample of open

clusters, and confirmed the lack of correlation between these two quantities.

The galactocentic distances (calculated from the distances given above) for

the clusters studied here were determined and used with the cluster metal-

licities to support the presence of a metallicity gradient (- -0.09 dex kpc-')

in the galaxy.

Acknowledgement s

First and foremost, 1 would like to thank Colin Scarfe for convincing me

to undertake this endeavour and then agreeing to CO-supervise me for yet

another degree, and Tim Davidge for CO-supervising me as well despite being

already busier than is humanly possible. Their patience, knowledge, guidance

and fiiendship is deeply appreciated, and they have made the past four years

an immense learning experience, both scholastically and pusondy.

1 also would like to thank Don VandenBerg and David Efartwick for the

advice they gave me during several discussions regarding various aspects of

this work. 1 appreciate the time Pat Dowler took to generate some of his

isochrones for cornparison with my chsters, and would like to thank him for

explainhg all sorts of concepts associated with convective overshooting. 1 am

also gratefd to Dave Zurek for obtaining some of the observations of NGC

2141 and NGC 6791. All the observations were taken with the 1.8 m telescope at the DAO, and

this would not have been possible without the help from Les Saddlemyer,

Doug Bond and Frank Younger. They have been super night assistants,

and were always ready to help. 1 offer them my thanks. Of course, I could

not have observed so much had Robert McClure not been so generous with

telescope time! 1 have him to thank for so many observing nights. 1 do

not, however, thank the Victoria weather demons for being so uncooperative

during many of those nights.

With the work cornes stress, and much of the tension was relieved through

the friendship of the 4th floor crowd. 1 have the other grad students, Russ

Robb and Ann Gower to thank for listening to my problems and brightening

my mood. You all are better than any psychiatrist, and you don't cost as

much!

When the stress was too great to be reduced by hearing the sounds of

Homer Simpson or farm animals emanating fiom the workstations, or the

sounds of laughter, 1 knew 1 could count on karate to relieve me of it for at

least a few hours. My deepest thanks go to Sensei Greg, Sensei Erich, Sensei

Debra and Sensei Brendan for encouraging me to develop a side of myself I

never knew existed, and for guiding me down the path towards geater peace

and strength.

1 would like to thank my parents, and my brother and sister for their

support and encouragement and for trying to understand why 1 would want to

go to University for so many years (yes Dad I'm going to get a real job now),

and the feline members of my family for providing me with much-needed fuzz

therapy throughout my university career. Findly, 1 am eternally gratefd to

rny husband Myron for his never-ending well of patience, encouragement and

love. I'm sure he will be as glad as 1 that this work is completed!

Contents

Abstract

Acknowledgement s

Table of Contents vii

List of Tables ix

0..

List of Figures xlll

1 Introduction 1 1.1 Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2 Advantages of Near-Wared Observations . . . . . . . . . . . 5 1.3 StellarModels. . . . . . . . . . . . . . . . . . . . . . . . . . 7 1.4 Motivation and Goals . . . . . . . . . . . . . . . . . . . . . . . 11 1.5 Target Objects . . . . . . . . . . . . . . . . . . . . . . . . . . 12 1.6 Outline of the Thesis . . . . . . . . . . . . . . . . . . . . . . . 17

2 CCD and Infrared Array Photometry 19

2.1 CCD Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . 20 2.2 IR Array Detectors . . . . . . . . . . . . . . . . . . . . . . . . 24 2.3 Differences in Observing Techniques . . . . . . . . . . . . . . . 27

3 Observations and Data Reduction 29

3.1 Observational Details . . . . . . . . . . . . . . . . . . . . . . . 29

vii

CONTENTS

3.2 Photometry and Reduction to a Standard System . . . . . . . 36

4 Resdts 52 . . . . . . . . . . . . . . . . . . . 4.1 Colour-Magnitude Diagrams 52

. . . . . . . . . . . . . . . . . . . . . . . 4.1.1 Optical CMDs 53 . . . . . . . . . . . . . . . . . . . 4.1.2 Near-infrared CMDs 62

. . . . . . . . . . . . . . . . 4.1.3 Field Star Contamination 67 . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Cluster Parameters 86

. . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1 Metallicity 86 . . . . . . . . . . . . . . . . . . . . . 4.2.2 Distance Modulus 87

. . . . . . . . . . . . . . . . . . 4.2.3 Differential Reddening 94 . . . . . . . . . . . . . . . . . 4.2.4 Mean Cluster Reddening 98

5 Discussion 101 . . . . . . . . . . . . . . . . . . . 5.1 Stellar Evolutionary Theory 101

. . . . . . . . . . . . . . 5.2 Methods of Determining Cluster Ages 103 . . . . . . . . . . . . . . . 5.2.1 Isochrones and Cluster Ages 105

. . . . . . . . . . . . . . . . . . . . 5.2.2 The MAR Method 107 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Cluster Ages 10