5 Conclusions

We have presented JHK images along with NIR and 1.3 mm photometry of 11 Class I sources and a possible Herbig Ae/Be star in the Vela Molecular Ridge D cloud, previously identified by Liseau et al. (1992) and Lorenzetti et al. (1993). Whereas clustering of YSO's will be discussed in a forthcoming paper, we have established the following points:

1

Through NIR- and mm-photometry, we have determined the SED's for the 11 newly discovered Class I sources (and the Herbig Ae/Be star). Bolometric luminosities range between 120 and 5600 $L_{\hbox{$\odot$}}$, indicating protostars with masses between 3.5-10 $M_{\hbox{$\odot$}}$.

2

The mm-photometry allows also to derive envelope masses ranging between 0.2-15 $M_{\hbox{$\odot$}}$, hence of the same order of the protostar masses deduced above.

3

Although clustering is well evident, the NIR counterparts of the IRAS sources seem to be single objects (or, possibly, close binaries), though quite often there is more than one source per field with JHK colours typical of Class I.

4

The NIR images confirm that 11 IRAS sources are real Class I objects; in the case of IRS 14, we found more appropriate to identify it as a Herbig Ae/Be star.

5

Only 4 of the previously identified counterparts through single channel photometry, have been confirmed by the present observations. The NIR contribution to the SED's of the IRAS Class I source candidates must be furtherly revised downward with respect to previous determinations and appears to be < 10%.

6

Many sources are associated to diffuse emission; 5 NIR counterparts appear embedded in reflection nebulae, whereas 2 NIR counterparts are close to nebulosities (and are probably illuminating them). A bright nebula has been found near the IRAS uncertainty ellipse in one field (IRS 62), possibly unrelated to the NIR counterpart. This frequent association with nebulosities confirms the youthness of the targeted objects.

7

In IRS 19, IRS 20 and IRS 21, the identified Class I sources may represent the illuminating sources for the observed nebulae. This occurrence implies high values of the extinction towards them (more than A_V $\sim 30$ mag in one case).