Since early studies (Gascoigne & Kron 1952; Hodge 1961), blue star clusters with globular cluster appearance have been known to exist in the Magellanic Clouds, where they are a common phenomenon. Very few young disk clusters in our Galaxy can be considered as such. An example is NGC 3603, which is populous and has a compact core also known as HD97950 (Moffat et al. 1985). It is located in a giant HII region/molecular cloud complex (e.g. Moffat 1983; Melnick et al. 1989). The identification of other open clusters with similar characteristics in the Milky Way disk is of great interest, since they may provide important clues on the environmental and internal conditions for enhanced star formation processes.
Westerlund1 was first detected by Westerlund (1961) using the
Uppsala Southern Station Schmidt telescope in Australia.
The cluster is heavily reddened and his early V, I photographic photometry
suggested AV = 12.0 mag, an age of 3 Myr and a distance of 1.4 kpc.
In fact, in the blue, the cluster is virtually
invisible, even in
the deep SRC J Sky Survey Schmidt plate, being prominent
in the ESO/SERC red Schmidt plate. Westerlund1 is located in the southern Milky Way in Ara,
at (B1950) 
,
. The
cluster, also known as OCl 985 in Alter et al. (1970), is
projected not far from the central regions of the Galaxy (
, 
).
It was included with nominal coordinates in the
ESO/Uppsala survey of the ESO (B) Atlas as ESO277-SC12 (Holmberg
et al. 1977), since it was of course invisible in the blue plate.
Westerlund1 is BH 197 in the Curtis-Schmidt star cluster survey of
the southern Milky Way
by van den Bergh & Hagen (1975). Westerlund (1968) pointed out that the
cluster could be identified as an extended source in the 
survey of
Price (1968).
Borgman et al. (1970) carried out photometry in the K band
of 12 stars in the region and concluded that several of them could be
supergiant members. They estimated a visual absorption of at least Av = 12.8 mag
for the cluster. More recently, Westerlund (1987, hereafter W87) using VRI
photographic
photometry for 258 stars together with near-infrared spectroscopy of the
brightest stars found Av = 10.0 mag and derived a significantly
larger distance of 5 kpc.
Westerlund2 was first reported by Westerlund (1960). It
is located in the southern Milky Way in Carina,
at (B1950) 
,
30
4,
which convert to 
, 
. The cluster
is also known as OCl807, BH95 and
ESO127-SC18. Westerlund2 is a tight open cluster,
which is the core of the HII region RCW49 (Belloni & Mereghetti 1994, and
references therein). Moffat et al. (1991, hereafter MSP91)
carried out UBV CCD photometry, estimating a distance of
7.9+1.2-1.0 kpc. They also identified spectroscopically a Wolf-Rayet
member of WN7 type. The earliest spectral type in the cluster is O7V.
In a comparison with NGC 3603, MSP91 concluded that Westerlund2 is only
slightly less massive and structurally
much less concentrated.
In this paper, a CCD colour-magnitude diagram of Westerlund1 is presented for the first time, in view of determining more accurately the cluster parameters. We also present CCD integrated spectroscopy for both Westerlund1 and 2, and derive cluster parameters by comparing their properties with those of Galactic and Large Magellanic Cloud clusters (Bica 1988; Bica et al. 1990; Santos Jr. & Bica 1993; Santos Jr. et al. 1995). In Sect. 2 (click here) we describe the photometric and spectroscopic observations and their calibrations. In Sect. 3 (click here) we present a V vs. (V-I) colour-magnitude diagram (CMD) of Westerlund1 and discuss its properties. In Sect. 4 (click here) we analyse the CCD integrated spectra of Westerlund1 and Westerlund2. In Sect. 5 (click here) we carry out a simple structural analysis of Westerlund1 in order to check its similarity to NGC 3603. Finally, in Sect. 6 (click here) we present the main conclusions of this work.