The young open cluster NGC 6913 (M29) (20h23m.9, +38.32(2000)) is located in the Cygnus OB1 association. In 1973, Sanders computed the probabilities of cluster membership for 228 stars in the field of NGC 6913, based on relative proper motions (Sanders 1973). He found there are 105 stars with probabilities greater than 50% out of 228 stars in the field. Before 1980, only a small part of the stars had UBV photoelectric values. Much more complete UBV photoelectric photometry was done by Joshi et al. (1983), using the 104-cm Sampurnanand reflector of the Uttar Pradesh State Observatory. The standard deviations of their observations were better than 0.025 mag. in (U-B), (B-V) and V magnitudes (Joshi et al. 1983). From their photometric study, the distance modulus to the cluster is obtained to be 10.85, which corresponds to a cluster distance 1.5 kpc. It also was found that the cluster stars have ages between 0.3 to 1.75 Myr with a few even younger than 0.3 Myr. This means that NGC 6913 is a very young open cluster.
Since the cluster is so young, it should have many stars in the pre-main-sequence phase. Many published and upcoming papers by different authors show galactic clusters containing stars young enough to be still surrounded by circumstellar disks. The presence of such disks has now been inferred around pre-main-sequence stars of masses ranging from 0.1 to 25. Circumstellar disks and mass loss of the stars are the two factors affecting the observational results. Another factor is the intracluster extinction due to the remaining star-forming molecular cloud. All of them contribute to the non-uniform extinction across a cluster, which has been studied by Sagar (1987).
The study of interstellar matter inside open clusters, especially very young open cluster, is quite important for a thorough understanding of the star formation processes (e.g. Burki 1975; Wallenquist 1975, 1979; Roth 1988). On the basis of the presence of non-uniform extinction in young open clusters and associations, Elmegreen & Lada (1977) proposed the sequential star formation process theory, which predicted that massive stars in open clusters should be younger than low-mass stars.
Although many people have done statistical research on this topic, little work has been done wholly based on precise spectroscopic data. Some studies of extinction of open clusters were done with photometric data, only partially calibrated with spectroscopy. The photometric data sometimes were not accurate. Actually, in the case of the variable extinction across a cluster, photometric observation is insufficient. It is too difficult to obtain accurate spectral type from photometric data. This will cause incorrect values for reddening and position in the H-R diagram. Besides, spectroscopy provides a powerful means for characterizing stars which appear to have unusual properties. A stellar spectrum is indicative of significant amounts of activity associated either with the star itself or with the presence of circumstellar material. Circumstellar material can be identified from the spectrum not only with emission lines but also without emission lines (Hillenbrand et al. 1993). Hence, spectroscopic observation is indispensable.
If one wants to study intracluster extinction, one should have a complete sample of cluster members, for which three sets of data must be available, i.e. (i) reliable cluster membership; (ii) precise photometric data; (iii) accurate spectral classification.
To study extinction inside the very young open cluster, we select those stars which have a membership probability greater than 50% in Sanders' star table of NGC 6913. We carry out spectroscopic classification for 100 stars of the NGC 6913 field. Even for those stars whose proper motions are in agreement with the average for the cluster, there could still be a non-negligible fraction of non-member stars (e.g. Crawford et al. 1977).
© European Southern Observatory (ESO) 2000
Online publication: March 28, 2000