6. Summary and conclusion
Investigating the stellar density around the clusters (see Fig. 2, right) we see no signs of increased star density reaching from KMHK 1019 towards the cluster pair, but we see an enhanced star density between SL 538 and NGC 2006. This may indicate a stellar bridge and thus gravitational interaction between SL 538 and NGC 2006. The stars in the bridge are main sequence stars and thus they may either belong to the cluster "system" or to the young field star populations. However, artificial star experiments showed that a stellar bridge may also be explained (24% probability for a "small" but only 6% probability for a more pronounced bridge) by statistical fluctuations in the field star density.
Fitting Geneva isochrones (Schaerer et al. 1993) to the CMDs we find the following ages: SL 538: Myr, NGC 2006: Myr, KMHK 1019: 16 Myr, youngest field: 16 Myr. The three clusters might have formed sequentially as part of the same GMC that formed LH 77.
Be stars are concentrated in SL 538 and NGC 2006, and both clusters show the same ratio of . This is in agreement with Kontizas et al. (1993) who investigated integrated IUE spectra and the distribution of spectral types of stars and found that both clusters have similar stellar content. Since Be stars are usually rapid rotators this may indicate intrinsically higher rotational velocities in the components of the cluster pair. The amount of Be stars detected in the surrounding field is considerably lower.
An investigation of the IMF of the binary cluster candidate showed that the IMF slopes agree with each other within the errors and are compatible with the Salpeter value (, Salpeter 1955). Our results for the IMF slopes are for SL 538 and for NGC 2006. With these values we estimate the upper limits for the total cluster masses to be (SL 538) and (NGC 2006). The similarity of the cluster masses is in agreement with the findings from Kontizas et al. (1993). Let us assume that the cluster pair indeed is a binary system: Following Kepler's third law and assuming for the total mass of the binary system and 13.3 pc for the distance between the clusters we get an orbit period of Myr. This would mean that the clusters so far have moved less than half of an orbit since their formation.
The similarity of properties (ages, Be star content, slope of the IMF and masses) indicates possible joint formation and suggests small spatial separation. Our results suggest that SL 538 and NGC 2006 are a true binary cluster, but a final proof is still missing. Radial velocity measurements would help to understand the kinematics of the clusters and may prove or disprove its possible binary nature, but such data are not yet available. If the stellar bridge is real this may imply that the merger process has begun. However, our artificial star experiments showed that we cannot state whether this bridge is real or not.
© European Southern Observatory (ESO) 1998
Online publication: October 22, 1998