Optical studies of X-ray sources in the old open cluster M67

Abstract

M67 is an old open star cluster with an estimated age of ~4 billion years. It is one of the best studied open clusters. This thesis contains an optical study of X- ray sources in M67; in addition several sources in the old clusters NGC752 and NGC6940 are studied. X-ray observations of old open clusters have detected many magnetically active binaries. This is not unexpected: at ages higher than ~1 billion years single late-type stars are believed to rotate too slowly to emit detectable X-rays. Tidal interaction in a close binary is therefore required to induce rotation at a higher rate than is typical for single stars. However, in this work I mainly studied stars whose X-rays are not as easily explained: binaries with orbits too wide for tidal interaction, and stars whose evolutionary statuses are badly understood; in these cases the X-rays might provide a clue to the nature of the stars. Many of the peculiar X-ray sources that I investigated are binaries. It is found that the properties of some systems are difficult to understand in the context of binary evolution alone. Therefore, one of the conclusions is that interactions between cluster stars have produced these peculiar systems and that the properties of the stars that are the outcome of such interactions are still poorly understood. One of the stars that is studied in detail is the blue straggler S 1082 in M67 which is probably a good example of a system that is the product of one or multiple encounters. The present study solved its apparently contradictory properties: eclipses with a period of ~1 day had been observed but the radial- velocity variations of the narrow lines in the spectrum indicate that the star moves in an orbit of ~1000 days. We concluded that S 1082 must be triple, and indeed found the signature of three stars. As we find that two stars in the system are blue stragglers on their own account and that the total mass of the inner binary is about three times the turnoff mass of M67, probably five stars were involved in the formation of S 1082. Two other stars in M67 that are studied are the sub-subgiants S 1063 and S 1113. Even though the peculiar positions of these binaries in the colour- magnitude diagram are so similar, we found that their different orbital properties make it difficult to find one explanation for their origin. In fact, we have great difficulty to explain the properties of these stars at all, and are left to conclude that they were involved in a recent interaction with other cluster stars.