One of the critical issues of star formation is to understand how the evolution of molecular clouds is interrelated to the birth of new stars within them. The current theoretical picture of these early stages is affected by large uncertainties, but some of them can be effectively circumvented by means of dedicated observations aiming to separate the effects of the different variables as much as possible.
Obvious observational constraints have
biased so far the selection of targets towards clouds
of relatively low mass (M<105
) located
outside the plane of the Galaxy,
despite GMC's lying in the plane offer
the possibility of studying star formation where the bulk of
material actually lies.
Given these motivations, a few years ago we began
a multi-frequency study of the young stellar content in the VMR,
a complex composed of four different molecular clouds (with molecular masses
of 
) which
have been actively forming stars
(May et al. 1988;
Murphy & May 1991).
The Vela Molecular Ridge offers some advantages to the observer:
it is possible to study
4 different GMC's with the same methods, so as to derive properties
of YSO's which are affected by
the same environmental and observational effects;
it allows to investigate star formation in the outer
Galaxy minimizing the problems due to confusion; the region is expected
to be relatively nearby and it is adjacent to a sky area
of equal angular size, void of molecular gas, which constitutes a reference
field.
The selection criteria to identify the young objects, the assessment of their cloud membership, and the observational results obtained so far have been communicated in our previous papers (Liseau et al. 1992, and Lorenzetti et al. 1993, hereafter Paper I and Paper II respectively).
Throughout Papers I and II it was assumed that the infrared emission is dominated by a single object; here we report the first results of an imaging survey aimed to find the near infrared counterparts of the IRAS sources, to extend the spectral energy distributions (SED's) towards short wavelengths, to investigate the source multiplicity and extension. We are also performing a survey of the 1.3 mm continuum emission of IRAS objects in VMR, hence we can complement the near IR imaging data with mm-continuum photometry which allows us both to provide a more accurate determination of the bolometric luminosities and to evaluate the mass of the circumstellar environment around each source.
The details of the observations, data reduction and calibration procedures are described in Sect. 2, while the obtained results are reported in Sect. 3. A short discussion about the present observations is given in Sect. 4, but the reader should refer to a forthcoming paper for a more detailed analysis of the implications of such results.
Copyright The European Southern Observatory (ESO)