Using the data in Table 5, we constructed a histogram with the distribution
of the colour excess E(B-V), shown in Fig. 7 (click here). As can be seen from
this figure as well as directly from the B-V versus U-B two-colour
diagram (Fig. 3 (click here)), most stars are located in a strip with E(B-V)
from 034 to 040. Therefore, we conclude that the foreground
interstellar reddening is probably about 030, and stars with a smaller
colour excess must be foreground objects. Since the error in most E(B-V)'s
is about 002, all stars with 
were excluded from the
further analysis of cluster properties. Another interesting feature that can
be seen in Fig. 7 (click here) is that there also seem to be peaks around
and 
. This could indicate the existence
of clouds of obscuring material in NGC 6530. To test this hypothesis we
plotted the positions of the programme stars with 
, as well as the positions of stars with 
and with 
. In these three diagrams no
significant differences could be found in the distribution of the stellar
positions. Therefore we conclude that if these peaks around 
and 
are really due to clouds of obscuring material
in NGC 6530, these clouds must span the entire field of the cluster. It
seems more likely that these peaks really are stochastic noise, however.
Furthermore, we conclude that we could not reproduce the variable
interstellar reddening across NGC 6530 that was observed by Sagar & Joshi
(1978). We believe that the difference between our and their
conclusion is the result of a bigger sample and better selection criteria
for probable cluster members in our study.
Figure 7: Histogram of the distribution of colour excesses E(B-V) in
NGC 6530
Figure 8: Histogram of the distribution of 
-values in NGC 6530
Again using the data in Table 5, we also constructed a histogram with the
distribution of the ratios of total to selective extinction 
, shown in
Fig. 8 (click here). As can be seen from this figure, most stars in NGC 6530 do
not show anomalous extinction (i.e. 
), and none show extreme
values of 
, a situation similar to that found in very young open
clusters like NGC 6193 (Vázquez & Feinstein 1992). There
are clearly some cases of anomalous extinction with 
(i.e. the
average particle size of the material in our line of sight is larger than
that in the interstellar medium), but all of these also have values of
. However, most stars with these big values of E(B-V)
do not show anomalous extinction. From this we conclude that the matter
responsible for the anomalous extinction must be circumstellar rather than
intracluster. This also means that each star has its own individual
extinction law, and that the derivation of an average extinction law for a
very young open cluster, such as was done by McCall et al.
(1990) for NGC 6530, will not yield correct results.
Figure 9: The field of NGC 6530 with the position of our programme stars
indicated by circles. The size of the circle indicates the magnitude of the
visual extinction 
. Again stars with E(B-V) < 0.28 (i.e. probable
foreground objects) were omitted
Combining the 
and E(B-V) data from Table 5 we also constructed a
diagram with the visual extinction 
as a function
of the positions in the cluster, shown in Fig. 9 (click here). Also shown in
this figure is the cluster field as obtained from the digitized version of
the Palomar Observatory Sky Survey (POSS). From Fig. 9 (click here) we notice
that bigger values of 
(indicated by larger circles) seem to occur more
often in the outer parts of the cluster, whereas these large values of 
seem to be quite rare in the inner, more nebulous, part of NGC 6530. This
could indicate the presence of an obscuring dark cloud in the wider region
of the cluster. However, no obvious dark cloud region could be found by
visual inspection of POSS plates. Therefore we conclude that the larger
values in the outer parts are probably due to contamination of our
sample with background objects in the outer parts of NGC 6530, whereas in
the inner parts most background stars are obscured by the bright nebulosity.
Besides this effect, we do not note any particular correlation of the visual
extinction with the position in the cluster, again indicating that the
material responsible for the extra extinction must be circumstellar rather
than intracluster.
A histogram with the distribution of the distances of individual stars in
NGC 6530, obtained in the previous section by comparing the luminosity
computed from the SEDs with the intrinsic ones collected by Schmidt-Kaler
(1982), is shown in Fig. 10 (click here). A gaussian was fitted to this
distribution, yielding an average distance of 
towards
NGC 6530, in excellent agreement with the value of 
found by McCall et al. (1990).
Figure 10: Histogram of the distribution of the derived distances to
individual programme stars in NGC 6530
At this distance the cluster diameter of roughly 35 arcminutes corresponds to 18 pc, or two to three time bigger than very young open clusters like NGC 2244, NGC 2264 and NGC 6383 (Pérez et al. 1987; Thé et al. 1985). However, this 18 pc is comparable to the diameter of NGC 6611 (de Winter et al. 1996). Remarkable is that although we expect the total volume of NGC 6530 or NGC 6611 to be about 40 times bigger than NGC 2244, NGC 2264 or NGC 6383, the number of OB stars in NGC 6530 (90) is only a factor of three bigger and in NGC 6611 even comparable to that in these clusters. Whether this lower spatial density of hot stars in NGC 6530 and NGC 6611 is due to evolutionary effects or is the result of different initial conditions remains unclear at the moment.
Using the distance obtained in the last § and the computed values for
, we constructed the cluster's Hertzsprung-Russell
diagram, shown in Fig. 11 (click here). For most points in this plot the error
in 
will be about 0.05 (or one subclass in spectral type),
but individual data points may have larger errors. The error in
is dominated by the error in the distance and is about
0.1. The relative error with respect to other data points will be much
smaller, though.
Also shown in Fig. 11 (click here) are the pre-main sequence evolutionary tracks
and the birthlines (i.e. the line where a star first becomes optically
visible on its evolution to the main-sequence) for accretion rates of
10
and 
computed by Palla & Stahler
(1993). Furthermore, we also plotted a line indicating the
completeness limit of our study. This was computed by fitting reddened (with
a colour excess E(B-V) of 030) Kurucz (1991) models to
the limiting visual magnitude of 136 from the proper motion study by van
Altena & Jones (1972), after which their luminosity was
computed using formula (6). The stars which are located far below this line
are probably foreground stars.
Figure 11: Hertzsprung-Russell diagram of our programme stars in
NGC 6530. Probable members (
) are indicated by circles.
Possible members (
) are indicated by squares. The
diamonds indicate programme stars not included in the proper motion survey
by van Altena & Jones (1972). Filled plot symbols indicate
stars with infrared excess.
Also shown are the theoretical pre-main sequence evolutionary tracks (solid
lines and dashed lines) and the birthlines for 
(upper dotted
line) and 
(lower dotted line) by Palla &
Stahler (1993). The dashed-dotted line shows the completeness limit
of our study
Remarkable is that we don't see any evidence for a horizontal branch of
pre-main sequence stars in the HR-diagram like the one observed by Walker
(1957). Therefore, his age determination of a few million years
might very well be incorrect. Furthermore, we note that several stars in our
HR-diagram are located to the right of the birthline. This situation is very
similar to the one in the very young open cluster NGC 6611 (de Winter et al.
1996), which was explained by demonstrating that this cluster
contains a mixture of normal main-sequence stars, young stars still
contracting towards the main sequence as well as older post-main sequence
stars evolving off the main sequence. In the next section we will
demonstrate that this is also the case for NGC 6530. Since the oldest stars
which are without any doubt associated with NGC 6530 are about 15 million
years old and the youngest stars must be younger than 100 000 years (see
next section) we conclude that star formation in this cluster must have
started a few times 
years ago and probably is continuing up to the
present day. In view of the small amount of massive (heavier than 8 solar
masses) stars located near the zero-age main-sequence in Fig. 11 (click here),
we conclude that the formation of such stars must have stopped already,
while the formation of lighter stars is still going on. There is no evidence
for a conclusion that the massive stars were the first to form, however:
older low-mass stars may also be present.
In Fig. 11 (click here) we also notice that the stars with infrared excesses (filled plot symbols) are all located close to the main sequence, whereas those without are scattered throughout the diagram. If these infrared excesses are due to the remnants of dust shells or circumstellar disks left over from star formation, we would expect that all of our infrared excess stars would also be the youngest and lie more towards the birthlines in our HR-diagram. As we observe quite the opposite, we conclude that besides age several other factors must determine the magnitude of the infrared excess, presumably corresponding to the survival time of a circumstellar disk or dust shell, in this very young open cluster. However, we only looked at the infrared excess at near-IR wavelengths. It is quite possible that many of the stars we classified as not having an infrared excess do show such an excess at mid- and far-IR wavelengths. This will not affect our conclusions, as the magnitude of these mid- and far-IR excesses will always be smaller than the infrared excesses we have found here.
Another interesting thing to notice is that all five stars in NGC 6530
showing intrinsic H
emission in their spectra all have strong
near-IR excesses, and are thus good candidates for members of the Herbig
Ae/Be stellar group, and are located close to the main sequence. The stars
right of the main sequence in this diagram, with 
,
are most probably background giants. This will be discussed further in the
next section.
Again using the data from Table 5 we also constructed the luminosity function
of NGC 6530 by binning over 
steps, shown in
Fig. 12 (click here)a. In order to get an idea of the effects of the rebinning
of our (small) sample we also constructed the incremental luminosity
function, shown in Fig. 12 (click here)b. Also shown in Fig. 12 (click here) is the
theoretical luminosity function for a cluster with an age of 
years by Fletcher & Stahler (1994). Although perhaps some
depletion of massive stars in NGC 6530 with respect to these theoretical
models can be seen, we conclude that in general the two curves match
adequately. The agreement between the theoretical and the observed curve is
69%, according to the Kolmogorov-Smirnov test. In this computation data
with 
(the point where our completeness limit in
the HR diagram intersects the birthline for an accretion rate of 
) were omitted because of the incompleteness of our
sample in that region. Also note that for luminosity functions with other
ages we can also obtain satisfactory fits, demonstrating the difficulty in
the observational tests of such models.