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Astron. Astrophys. 361, 877-887 (2000)

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5. Color-magnitude diagrams

We have derived color-magnitude diagrams for the two clusters and the field stars in N11C. Whilst these diagrams are solid enough for the conclusions reported in this paper, we caution that the data are not in the standard Strömgren system.

The C-M diagram of the Sk-66o41 cluster (Fig. 4) displays a rather well defined main sequence in the interval [FORMULA] and [FORMULA]. There is no evidence for stars evolved off the main sequence in Sk-66o41.

Assuming an average reddening of E(B-V) = 0.22 (Paper I) and adopting a distance modulus of 18.5, we can in principle estimate the age of the various stellar populations in N 11C by comparing the observed C-M diagrams with the isochrones derived from theoretical evolutionary tracks. However, fitting isochrones to very young stellar populations such as those of Sk-66o41 with high turn-off masses and no red supergiants is a quite difficult and rather uncertain procedure. In our case, this is even more uncertain since we are dealing with a non-standard C-M diagram. We have nevertheless attempted a comparison of our color-magnitude diagrams to the isochrones corresponding to the Z = 0.008 models of Schaerer et al. (1993). The isochrones were computed using a program kindly provided by Dr. G. Meynet. For the Sk-66o41 cluster we find that a reasonable upper limit to the age of the cluster is 5 Myr. This result is also in line with the spectral classification of the integrated spectrum of Sk-66o41 as O3V((f*)) + OB. In fact the most massive star of the cluster has not yet left the main sequence though it shows Of emission features characterizing pre-WR stages. Assuming a lowest possible ZAMS mass of 60 [FORMULA] for an O3 star, this spectral type puts an upper limit on the age of this star of 3.7 Myr (Schaerer et al. 1993), in agreement with the upper limit on the cluster age derived from the C-M diagram.

The C-M diagram of the HNT cluster is also shown in Fig. 4. The labels indicate those stars for which we have derived spectral types using our spectroscopic data. The main sequence of the HNT cluster is visible up to [FORMULA]. To the right of the main sequence, we find a couple of evolved stars with [FORMULA] between 0.2 and 0.6 that are brighter than [FORMULA]. Applying the same technique as above to the HNT cluster, we find from the main sequence turn-off and the red giant population that the age of this cluster is most probably [FORMULA] Myr, i.e. much older than the age derived for the Sk-66o41 cluster.

The C-M diagram of the field stars in N 11C is shown in Fig. 4. Inspection of this diagram reveals a mixture of stars of quite different ages. We find a main sequence that extends up to [FORMULA] as well as a couple of evolved stars to the right. Around [FORMULA] and [FORMULA], we notice the clump of older red giant field stars.

The difference in age between the two clusters is an interesting feature that raises some questions about the physical link between the HNT cluster and the N 11C complex. In fact, recent studies have revealed that massive stars tend to form in a rather coeval fashion. Massey et al. (2000) report results for a sample of 19 OB associations in the Magellanic Clouds. In about half of these associations, they find that most of the massive stars formed within a short time ([FORMULA] Myr). In the remaining associations, they found that star formation most probably occurred over a time-span of less than 10 Myr. Therefore the age difference between Sk-66o41 and the HNT cluster appears rather unusual if we assume that both clusters are part of the same association. The C-M diagram of the whole set of 464 stars observed towards LH 13 indicates that star formation is not coeval. This conclusion is in agreement with the results of UBV photometry obtained by DeGioa-Eastwood et al. (1993). A possible explanation could be that we are viewing different star formation regions lying at different distances along the same line of sight.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001