The distance and formation history of the very young open cluster NGC 6530
were investigated by studying a total of 132 probable member stars of this
cluster. It was concluded that the distance towards NGC 6530 is
, and its interstellar reddening E(B-V) is 030
and more or less uniform across the cluster. Furthermore, it was concluded
that the extinction law of the intracluster material is normal (i.e. 
), but anomalous extinction laws were found for several more embedded
stars in the cluster. No correlation between a star's position in the
cluster with its extinction law could be found, demonstrating that these
extinction laws are really due to circumstellar rather than intracluster
material. This also shows that in the study of such very young open clusters
the use of average extinction laws, as is done by some authors, is not
correct; each star has its own individual extinction characteristics.
Of the 132 stars included in this study, 11 are suspected to be variable,
five show H
in emission and nine show an infrared excess. Three member
stars of NGC 6530 (LkH
108, LkH
112 and LkH
115) were
found to have strong infrared excesses as well as strong H
emission.
Furthermore, they are photometrically variable. From this we conclude that
all three are most certainly members of the Herbig Ae/Be stellar class.
LkH
112 and LkH
115 were already included in the most recent
catalogue of Herbig Ae/Be stars (Thé et al. 1994), but
LkH
108 is a new Herbig Be star which deserves more attention. Two
other programme stars (NGC 6530-45 and NGC 6530-151) also show strong
infrared excesses and are photometrically variable. These two stars also
show some moderate H
emission, but the possibility that this is due
to the surrounding nebulosity cannot be completely excluded. Therefore, we
tentatively classify these stars as possible members of the Herbig Ae/Be
stellar group. More research is necessary to give a more definite answer on
the nature of these stars, however.
Herbig (1957) and Herbig & Bell (1988) list
several other emission line stars located in the field of NGC 6530 which
were not included in our present study. The brighter of these have low
membership probabilities in the proper motion study by van Altena &
Jones (1972) and are probably not physically associated with NGC
6530. Following the model for the M8 region by Lada et al.
(1976) and by Lightfoot et al. (1984) we propose that
these are young stellar objects located deeper in the molecular cloud, and
thus younger, than the stars in NGC 6530. No membership probability is known
for the fainter H
emitters in the field of NGC 6530 in the papers by
Herbig (1957) and Herbig & Bell (1988).
Therefore, it is conceivable that in these stars we are seeing the first
traces of a population of low mass pre-main sequence stars in NGC 6530. If
this is really the case, this would give further support our conclusion that
star formation in NGC 6530 has started longer ago than believed hitherto.
One probably cluster member (NGC 6530-338) was found to be located in a region of the HR-diagram located by post-AGB stars and is photometrically variable as well. Therefore we tentatively conclude that this star is a massive post-main sequence star that has just evolved off the Asymptotic Giant Branch. The same applies for another programme star (NGC 6530-304), but here no photometric variability was noted, making this very speculative. To confirm the post-AGB nature of both stars, new infrared data should be obtained to trace the dust shells these stars should have in this evolutionary stage.
We also constructed the luminosity function of NGC 6530, which was found to
agree well with the theoretical models for cluster luminosity functions by
Fletcher & Stahler (1994). A bigger sample extending to less
luminous stars is required to put these models through a more thorough test,
however. Finally, from the distribution of pre- and post-main sequence stars
in the cluster's HR-diagram, it was concluded that the process of star
formation in NGC 6530 must have started a few times 
tears ago. For
the low- and intermediate-mass stars this process is probably continuing up
to the present day, whereas it must have already stopped for the massive
stars. A further study of whether this process happened sequential or more
or less simultaneous could have important implications for the further
development of star formation theory.
Acknowledgements
The authors would like to thank E.A. Bibo for measuring some of the data in Table 2. This research has made use of the Simbad data base, operated at CDS, Strasbourg, France.