The study of open clusters is extremely valuable for stellar astronomy: they provide coeval samples of stars located at the same distance, allowing to study the emission properties in a systematic way. Stellar activity is no exception: since activity depends crucially on stellar rotation (Pallavicini et al. 1981, Rosner et al. 1985), which decreases with age because of magnetic breaking (e.g. Skumanich 1972), open clusters are crucial to distinguish between truly evolutionary effects on stellar activity and effects primarily due to the rotation rate itself. From the observations of the Hyades and the Pleiades with the Einstein Observatory it was found that coronal emission is a common feature among the late-type stars of the clusters (Stern et al. 1981, Micela et al. 1990). With the advent of ROSAT, these two clusters were observed with a better sensitivity and larger solid angle (Stern et al. 1992, Stauffer et al. 1994); meanwhile various other nearby open cluster have been observed (Patten & Simon 1993; Randich & Schmitt 1995; Randich et al. 1995, 1996a,b; Dachs & Hummel 1996).
Since stellar activity is connected to stellar rotation and the latter decreases with age because of magnetic breaking, the attention has been concentrated mainly on young clusters (30 to 700 Myr). In contrast to younger clusters, clusters older than 1 Gyr are not expected to contain rapidly rotating single late-type stars, and therefore strong X-ray sources. In this respect, old and intermediate-age open clusters may appear to be of little interest to the X-ray observer. However, there are stars older than 1 Gyr that show rapid rotation: these are members of close binary systems, where tidal interaction prevents the stars from losing angular momentum; well-known examples are the RS CVn binaries. Another class of rapidly rotating but relatively old stars are single giants of the FK Comae type, which probably formed by coalescence. As shown by observations of field stars, these rapidly rotating objects are strong X-ray emitting sources as a consequence of the dependence of X-ray activity upon rotation in late-type stars (e.g. Fleming et al. 1989). Thus, the observation of relatively old open clusters allows to study homogeneous samples of such binaries. With the ROSAT satellite the first observations of old open clusters have been performed. A pointing to M 67 (age 5 Gyr) led to the detection of a number of sources, four of which have been identified with short-period binaries, one with a cataclysmic variable and one with a blue straggler (Belloni et al. 1993). Indication of chromospheric activity has been found from most of the optical candidates (Pasquini & Belloni 1994). Moreover, a DA white dwarf has been discovered during the optical follow ups (Pasquini et al. 1994). 49 X-ray sources have been detected in a ROSAT PSPC pointing to NGC 752 ( 2 Gyr). Seven of them are identified with optical cluster members, four of which are short period binaries, one is a rapid rotator and one is a blue straggler (Belloni & Verbunt 1996). The X-ray properties of these sources appear to be consistent with those of active binaries (see Dempsey et al. 1993a, 1993b).
In view of these results, we observed two intermediate age open clusters with the ROSAT PSPC detector, NGC 6940 and IC 4651, in the framework of a project to cover the X-ray observational gap between old and young clusters. Here we present the results obtained for NGC 6940, while the results for IC 4651 will be presented in a forthcoming paper.
NGC 6940 has an estimated age of 1 Gyr: its low distance ( pc) and angular size (between 27, Trumpler (1930), and , Vasilevskis & Rach (1957)) make it a good target for the PSPC detector. The reddening in the direction of the cluster is moderate, E(B-V) = 0.05-0.30 (variable across the field, Larsson-Leander 1960), low enough to allow the detection of possible soft X-ray emission from cluster members.
The paper is organized as follows: in Sect. 2 we present the PSPC observation and our data analysis, in Sect. 3 we present and discuss the results both for the cluster and non-cluster members, and in Sect. 4 we compare our results with those of other open clusters and discuss the implications.
© European Southern Observatory (ESO) 1997
Online publication: October 15, 1997