Table 1: Properties of programme stars in NGC 6530 from literature
Table 1: continued
Table 1: continued
NGC 6530 (C1801-243, in the central part of the HII region M 8, the
Lagoon nebula) is one of the most studied open clusters in our galaxy.
Together with many other clusters and nebulae like NGC 6531, 6611, 6613,
M 17 and M 20 it forms the star forming region SFR Sgr I (Stal'bovskii &
Shevchenko 1981). Walker (1957) concluded, from UBV
photoelectric observations of 118 stars in NGC 6530, that the
colour-magnitude diagram of this cluster consists of a normal main sequence
extending from O5 to about A0, with stars of later spectral type lying above
the main sequence. From this he concluded that the cluster age must be about
years. Walker also suggested that the reddening in NGC 6530 might
be anomalous, which was confirmed for a small number of stars by the results
of Johnson (1967).
Thé (1960) argued, on the basis of star counts, that a
significant fraction of Walker's pre-main sequence stars must be foreground
or background stars. This was later confirmed by a proper motion study of
363 stars (complete up to 
) in the vicinity of NGC 6530 by
van Altena & Jones (1972). Later Chini & Neckel (1981) demonstrated
that the bulk of the probable members from these proper motion studies are
also located on the main sequence.
A model for the structure of the M 8 region was given by Lada et al. (1976), who proposed that NGC 6530 was formed from a pre-existent massive molecular cloud, in which it produced a "blister'' due to the ionizing radiation of the newly formed hot stars. Because of this cavity we can look further into the molecular cloud and we can also see the more recently formed embedded stars. Lightfoot et al. (1984) used this model to explain the star formation in M 8 as occurring in three generations: first to form were the hot stars in NGC 6530. These triggered the formation of the O4f star 9 Sgr, located deeper into the molecular cloud, which in turn has caused the formation of the even younger protostar Herschel 36, located the furthest away from the sun.
There seems to be consensus between the different authors on the distance and
interstellar reddening towards NGC 6530. Walker (1957) already
obtained a distance between 1.4 and 2.0 kpc and an E(B-V) between 033
and 037 for NGC 6530. These results were confirmed by most subsequent
authors, although Sagar & Joshi (1978) found E(B-V) to vary
from 025 to 048 across the cluster. The most recent values for
E(B-V) and the distance are 
by Chini & Neckel
(1981) and 
by McCall et al. (1990).
Because Chini & Neckel (1981) neglected the effects of
anomalous extinction and used only very rough spectral classifications from
UBV photometry we concluded that a new study of this cluster, taking into
account these effects, might yield different results. Therefore, we selected
all stars from the proper motion study by van Altena & Jones
(1972) with membership probabilities 
10%, together with five
additional stars from the original paper by Walker (1957),
which were not included in their study, to investigate this interesting
cluster further using new photometric and spectroscopic data. Properties of
all programme stars, obtained from literature, are listed in
Table 1 (click here). In the remainder of this paper these stars will be
indicated by their number in the paper by van Altena & Jones
(1972), also listed in the first column of Table 1 (click here). The five
programme stars which were not included in this paper will be indicated by a
W followed by their number in the paper by Walker (1957) which
is listed in the second column of Table 1 (click here). The star numbers higher
than 118 in this column are from the extension of this numbering system by
Kilambi (1977). The third column in Table 1 (click here) lists the
corresponding stellar numbers from the paper by Sagar & Joshi
(1978).
