Optical studies of X-ray sources in the old open cluster M 67 Figures 1.1 and 8.1, the images in the chapter headings and the images at the end are created from the Digitized Sky Survey c 2001 Maureen van den Berg Alle rechten voorbehouden. Niets in deze uitgave mag worden verveelvoudigd, opgesla- gen in een geautomatiseerd gegevensbestand, of openbaar gemaakt, in enige vorm, zonder schriftelijke toestemming van de auteur ISBN 90-393-2679-7 Optical studies of X-ray sources in the old open cluster M 67 Optische studies van rontgenbronnen¨ in de oude open sterrenhoop M 67 (met een samenvatting in het Nederlands) Proefschrift ter verkrijging van de graad van doctor aan de Universiteit Utrecht op gezag van de Rector Magnificus prof. dr H.O. Voorma ingevolge het besluit van het College voor Promoties, in het openbaar te verdedigen op woensdag 21 maart 2001 des ochtends te 10.30 uur door Maureen Constance van den Berg geboren op 6 april 1972 te Maastricht PROMOTOR prof. dr Frank Verbunt Dit proefschrift werd mede mogelijk gemaakt met financiele¨ steun van de Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO). Contents 1 Introduction 1 1.1 Blue stragglers, yellow stragglers and sub-subgiants 1 1.2 X-ray observations of old open clusters 3 1.3 Summary: peculiar X-ray sources in M 67, NGC 752 and NGC 6940 5 2 Optical spectroscopy of X-ray sources in the old open cluster M67 9 2.1 Introduction 10 2.2 Observations and data reduction 12 2.2.1 Low-resolution spectra 14 2.2.2 High-resolution spectra 14 2.3 Data analysis 15 2.3.1 Determination of Ca II H&K emission fluxes 15 2.3.2 Determination of projected rotational velocities 17 2.4 Results 19 2.4.1 Comparison with RS CVn binaries 19 2.4.2 Activity indicators 21 2.4.3 Individual systems 22 2.5 Discussion and conclusions 26 3 Photometric variability in the old open cluster M 67 I. Cluster members detected in X-rays 31 3.1 Introduction 32 3.2 Data and analysis 32 3.2.1 Observations 32 3.2.2 Data reduction and light curve solution 33 3.2.3 Search for variability 36 3.3 Results 37 3.3.1 Periodic variables 38 3.3.2 Non-periodic variable: S 1063 44 3.4 Discussion 44 3.5 Conclusion 47 i CONTENTS 4 Photometric variability in the old open cluster M 67 II. Other stars 51 4.1 Introduction 52 4.2 Data and analysis 53 4.2.1 Observations 53 4.2.2 Data reduction and light curve solution 54 4.2.3 Search for variability 56 4.3 Results 57 4.3.1 Colour-magnitude diagrams 57 4.3.2 Photometric variability 57 4.4 Discussion 62 4.4.1 Stars below the cluster main sequence 63 4.4.2 Stars on the cluster main sequence 64 4.4.3 Stars on the cluster binary sequence 64 4.4.4 Giant stars 65 4.4.5 Blue stragglers 66 4.5 Conclusions 66 5 The blue straggler S 1082: a triple system in the old open cluster M 67 71 5.1 Introduction 71 5.2 Observations and data reduction 73 5.2.1 Photometry 73 5.2.2 Spectroscopy 73 5.3 Results 75 5.3.1 Eclipse light curve 75 5.3.2 Spectral line profiles 75 5.3.3 Radial-velocity curves 77 5.4 Parameter estimation for the eclipsing binary 82 5.5 Discussion 90 6 Sub-subgiants in the old open cluster M 67? 95 6.1 Introduction 96 6.2 The stars 98 6.3 Observations and data analyses 99 6.3.1 Speedometry and orbital solutions 99 6.3.2 Photometry and variability 100 6.4 Properties of the stars 104 6.5 Discussion 111 6.5.1 Membership 111 6.5.2 Origins 112 6.6 Summary 114 ii CONTENTS 7 An optical study of X-ray sources in the old open clusters NGC 752 and NGC 6940 117 7.1 Introduction 118 7.2 Observations and data reduction 120 7.2.1 Spectroscopy 120 7.2.2 Photometry 123 7.3 Results 124 7.3.1 Photometric variability 124 7.3.2 Spectral classification 125 7.3.3 Ca II H&K emission 126 7.3.4 Radial and rotational velocities 129 7.3.5 Lithium 6708 A˚ 130 7.4 Discussion and conclusions 132 8 Samenvatting 135 8.1 Sterren en sterrenhopen 135 8.2 Rontgenstraling¨ van sterren 137 8.3 Dubbelsterren 137 8.4 Rare rontgenbronnen¨ 138 8.5 Dit proefschrift 138 iii CONTENTS iv Chapter 1 Introduction 1.1 Blue stragglers, yellow stragglers and sub-subgiants Galactic clusters, also called open clusters, are bound groups of stars, consisting of a few hundred to a few thousand members. Their ages range from millions of years (e.g. the 4 Myr old IC 1590, Guetter & Turner 1997) to billions of years (e.g. the 6 Gyr old NGC 188, Dinescu et al. 1995), where they start to approach the typical age of the older, more dense and populous globular clusters. The field of an open cluster is usually not too crowded so that a probability of cluster membership for individual stars projected in the field can be established by means of proper-motion and radial-velocity measurements with ground-based instruments. Only recently can accurate proper motions also be measured for stars in globular clusters, with the Hubble Space Telescope (e.g. King et al. 1999). A basic fingerprint of a cluster is its colour-versus-magnitude diagram: comparison of the observed locations in this diagram of the cluster stars with theoretical isochrones provides an estimate of the age, distance and metallicity of the cluster. The advantages of studying stars in clusters are therefore many: we can derive basic stellar parameters like mass, temperature and radius of the individual members from comparison with models. On the other hand, clusters have been used extensively to test stellar evolution models. By studying the properties of clusters of different ages the evolution in time of those properties can be investigated. Also, as the spatial distribution of open clusters is concentrated towards the disc of the Galaxy, they are used to probe the properties and evolution of the Galactic disc. The open cluster that plays a central role in this thesis is M 67 (NGC 2682, Fig. 1.1). M 67 is one of the best-studied open clusters as a result of several factors: it is a rich cluster, at 850 ¢ pc it is relatively nearby, and due to its high Galactic latitude (b ¡ 32 ) the reddening in the direction of M 67 is low and the field is little contaminated by field stars; also, it is one of the few clusters older than several Gyr. The stars in the colour-magnitude diagram can be fitted with a 4 to 5 Gyr isochrone of solar or slightly subsolar metallicity (e.g. Fan et al. 1996). Even after the non-members are removed, the colour-magnitude diagram of M 67 still features many stars that straggle from the theoretical isochrone (see Fig. 1.2). This can be 1 CHAPTER 1 Figure 1.1: Central 25 x 25 arcmin2 of M 67. accounted for when we are actually seeing the light of two cluster members in an unresolved binary. For instance, the line of stars that lies above and parallel to the main sequence is likely due to a binary population. However, this cannot be the case for the two sub-subgiants below the base of the giant branch, for the blue stragglers that are brighter and bluer than the main- sequence turnoff and for some of the yellow stragglers that lie between the blue stragglers and the red giants. What makes these stars so interesting is that we have no explanation for their properties which only involves the evolution of a single isolated star. Therefore, they offer the possibility to study the products of interactions between stars – either between the companions in a binary or between stars in the cluster. Encounters and collisions between single and binary stars in the high-stellar density environment of a cluster have been invoked to explain the formation of strange stars like low-mass X-ray binaries or blue stragglers, especially in globular clusters (see review by Bailyn 1995). The interest in these systems is not only restricted to the desire to understand interacting stars: the exchange of energy that results from interactions between cluster stars can also affect the evolution of the cluster as a whole (Hut et al. 1992). Two ROSAT observations of M 67 detected many of the peculiar stars in X-rays (Belloni et al. 1993, 1998), possibly providing an important clue to their origin or present state. These X-ray sources were selected for an optical follow-up study, and the results can be read in this thesis. As the X-ray emission of these stars was unexpected (see next Section) our first 2 INTRODUCTION Figure 1.2: Colour-magnitude diagram in which the binaries (Whelan et al. 1979, Mathieu et al. 1990, Gilliland et al. 1991, Latham et al. 1992, Latham et al. private communication) and the X-ray sources (Belloni et al. 1998) are indicated. Only stars with a proper-motion membership probability £ 80% (Girard et al. 1989, for several stars Sanders 1977) are plotted. Photometry is mostly taken from Montgomery et al. (1993), in a few cases from Sanders (1989). question was whether we can find spectroscopic (Chapter 2) and photometric (Chapter 3) sig- natures that explain the X-rays.