Stellar activity is connected to stellar rotation, which decreases with age because of magnetic breaking, so no detectable coronal X-ray emission would be expected from old stars. However, there are stars older than 1 Gyr that are still fast rotators; those that are members of close binary systems where tidal interaction prevents the stars from losing angular momentum. It is now clear that stars in short period binaries show significant chromospheric and coronal X-ray emission even at very old age (Pasquini et al. 1991; Belloni et al. 1993; Fleming & Tagliaferri 1996; Cutispoto et al. 1997). This situation is shown by the study of the intermediate and old open clusters with ROSAT. As in the case of M 67 (Belloni et al. 1993) and NGC 752 (Belloni & Verbunt 1996), in NGC 6940 we find that the majority of detected members are binary systems. Three out of four, possibly five, are among the only six binaries known in NGC 6940. All five sources are extremely bright in X-ray, with values typically found in the RS CVn type of binaries (e.g. Dempsey et al. 1993b), confirming that 1 Gyr old binary stars can still be extremely active coronal sources.
In binary star systems, tidal interaction acts to synchronize the stars' rotation to the orbital revolution and to circularize the orbit. In order to achieve this the binary period must be short, of the order of P d for binaries with main-sequence stars, whereas binaries containing a giant are circularized at P d (Mayor & Mermilliod 1984; Mermilliod & Mayor 1992; but see also Verbunt & Phinney 1995). Note that, although the RS CVn systems were originally defined as a class of close binaries with orbital periods between 1 and 15 days (Hall 1976), nowadays this class includes binaries with periods up to days, in particular those containing a giant component, that are still very active in X-rays (e.g. Dempsey et al. 1993b). Two of the X-ray detected binaries in NGC 6940 conform to these expectations, but one does not (see Table 2). Star VR 111 has a remarkably long period ( 3600 days) and a relatively high eccentricity (0.3). Being the system not circularized, it is not expected to be in corotation. In any case, even if it was in corotation, its period is so long that the star rotation rate would not be high enough to sustain such a level of stellar activity as the one detected in our data. Its detection as an X-ray source is puzzling. An alternative possibility is that star VR 111 is not the counterpart of the X-ray source. Indeed there is another fainter object within the error box, although the chance probability of having a 11.5 magnitude star within the error box is rather small. Another possibility is that the system is a triple, with one of the two stars being itself a spectroscopic binary. A more detailed optical investigation of this star and of the field is needed in order to establish the nature of the X-ray emission. The same is true for the other X-ray source of the cluster that is not a known binary (#13). Since the search for binarity by Mermilliod & Mayor (1989) is complete for amplitudes km s-1 and periods days, if the star is a binary outside these values it is a rather extreme case that should not be relevant for the stellar activity. If this source is a single giant star, it is not at all clear why it shows such a strong X-ray emission (at an age of Gyr). The possibility that this star is a binary system missed in the analysis by Mermilliod & Mayor seems to be excluded for amplitudes 1.5 km/s (Mermilliod, private communication), in which case the period is probably not very short. Additional optical investigation is required.
We identify source #17 with star VR114, a G8 III star that we estimate to be more or less at the cluster distance, but that is not identified as a cluster member from the astrometric studies. As for source #13, if this star is a single giant it is not at all clear why it shows such a strong X-ray emission. However, since VR114 is not classified as a cluster member, it was not included in the sample studied by Mermilliod & Mayor, leaving the binary possibility open.
The remaining three binaries of the six discovered by Mermilliod & Mayor are not detected in the PSPC X-ray observation, but this is in line with the above picture. They have quite long periods (210.7, 281.7 and 549.2 days respectively) and eccentric orbits (0.3, 0.16, 0.45), so that they are not expected to be fast-rotator active stars. Moreover, the 3- upper limits that we can derive in the 0.1-2.4 keV X-ray energy band are not extremely stringent (7.6, 4.7 and erg s-1 respectively).
All the identified members are red giants. As pointed out by Belloni & Verbunt (1996), this is in agreement with the presence of a saturation level in the X-ray flux per unit area (see Vilhu 1987), the brightness per unit area being approximately constant and the total X-ray luminosity increasing with the bolometric luminosity (i.e. the radius). This has been found for different samples of active stars, e.g. for a sample of X-ray selected stars (Fleming et al. 1989), for a sample of flaring stars (Pallavicini et al. 1990) and for various young open clusters (Stauffer et al. 1994, Randich et al. 1995, 1996a). This means that the maximum X-ray luminosity possible for each star scales with the star's surface. In this scenario, the presence of a detection threshold could make all main-sequence stars unobservable, while bright giants are still detectable (see also Fig. 2 in Belloni & Verbunt 1996). To check this we assumed a saturation level of Lx /L (e.g. Stauffer et al. 1994), which implies a maximum X-ray luminosity for G-K main sequence stars of the order of erg s-1. These values are similar to those we determined for two of the four cluster members detected, so that in principle we should have been able to also detect main-sequence stars near their saturation level, but we found none. Of course to exist these stars must be in binary systems. This could mean either that the cluster distance is larger than the assumed value of 870 pc (which would imply that the luminosities reported in Table 2 are underestimate) or that in NGC 6940 the saturation level is below the canonical value of . Indeed, the four detected cluster members have (Lx /L in the range 4.6 to 4.0 (4.2 for source #17)
Eight sources lack optical counterparts. Although they can be all extragalactic in nature, some of them could still be a cluster member. The remaining three sources identified with field stars also deserve attention. Source #9 its probably an active K-M spectral type field star. Source #3, HD 334742 (F5V), and source #18, HD 340540 (K0V), have an X-ray luminosity that, although very high, is not unusual for X-ray selected active stars (Fleming et al. 1995). However, these values imply that these sources, HD 340540 in particular, are either very young objects or spectroscopic binaries (e.g. Favata et al. 1993, Tagliaferri et al. 1994). Source #4 (HD 196244), being of spectral type A2, is more peculiar. Stars of spectral type B8-A5 are known to be very weak X-ray emitters, if at all, with upper limits of the order of erg s-1 (e.g. Schmitt 1997). An exception seem to be the Ap-Am stars, for which X-ray emission has been observed with the Einstein observatory at a level of - erg s-1 for four out of nine stars studied (Cash & Snow 1982). However, a more complete study based on a sample of magnetic Bp-Ap stars observed with ROSAT finds only 3-5 cases where the X-ray emission could be attributed to the chemically peculiar star and not to a late type star companion (Drake et al. 1994). And even in these few cases it cannot be completely excluded that the chemically peculiar star has a late type companion responsible for the X-ray emission. Recently Simon et al. (1995) used PSPC data to search for X-ray emission in a sample of 58 early-A type stars, with 10 positive detections. Of these, 5 are known to be in binary systems, while the other 5 could be either single or binary and would require more optical follow-up observations to determine their nature. All in all, it seems unlikely that the A2 star is the real counterpart of the X-ray source. It is rather more probable that the X-ray emission arises from a low-mass binary companion (e.g. Stauffer et al. 1994; Stern et al. 1995).
In summary, we can say that the ROSAT data on NGC 6940 confirm the results found in other old open clusters, i.e. that binary stars are still very active coronal sources at an age Gyr, and that good spatial resolution images in the X-ray band are a powerful instrument to investigate their evolution in clusters of different age. However, it is clear that an optical follow-up is necessary in order to better establish the physical nature both of the detected members of the cluster and also of the objects still to be identified.
© European Southern Observatory (ESO) 1997
Online publication: October 15, 1997