Massive open star clusters using the VVV survey I. Presentation of the
  data and description of the approach
  Context: The ESO Public Survey "VISTA Variables in the V\'ia L\'actea" (VVV)
provides deep multi-epoch infrared observations for an unprecedented 562 sq.
degrees of the Galactic bulge, and adjacent regions of the disk. Aims: The VVV
observations will foster the construction of a sample of Galactic star clusters
with reliable and homogeneously derived physical parameters (e.g., age,
distance, and mass, etc.). In this first paper in a series, the methodology
employed to establish cluster parameters for the envisioned database are
elaborated upon by analyzing a subsample of 4 known young open clusters: Danks
1, Danks 2, RCW 79, and DBS 132. The analysis offers a first glimpse of the
information that can be gleaned for the final cluster database from the VVV
observations. Methods: Wide-field, deep JHKs VVV observations, combined with
new infrared spectroscopy, are employed to constrain fundamental parameters for
a subset of clusters. Results: Results inferred from the deep near-infrared
photometry which features mitigated uncertainties (e.g. the accuracy of the
photometry is better than 0.1mag for Ks<18mag), the wide field-of-view of the
VVV survey, and numerous high quality low resolution spectra (typically more
than 10 per cluster), are used to establish independent cluster parameters
which enable existing determinations to be assessed. An anomalous reddening law
in the direction toward the Danks' clusters is found, i.e.
E(J-H)/E(H-Ks)=2.20+/-0.06, which exceeds published values for the inner
Galaxy. The G305 star forming complex, which includes the Danks' clusters, lies
beyond the Sagittarius-Carina spiral arm and occupies the Centaurus arm.
Finally, the first deep infrared color-magnitude diagram of RCW 79 is presented
which reveal a sizable pre-main sequence population. A list of candidate
variable stars in G305 region is reported.
