4.1. Morphology of the S 235 A-B complex
The new picture of this region that comes out of our data is that of a progressive star formation activity from the outside to the inner regions of a molecular cloud. In fact, the peak of the molecular emission coincide with a cluster of deeply embedded stellar objects. One of the objects detected (M1, in Table 3), is believed to be one of the earliest in the cluster, with the (proto-)star itself embedded in its parental dust cloud. The star may not be directly visible, and the observed K-band source may be the emission of the hot dust envelope around the YSO. At the edge of the core is located S 235 B , probably an intermediate mass object, which is blowing away the gas and dust material still around it, as witnessed by the high value of the extinction toward the source, and, at the same time, the H nebulosity on the side of the star. Just outside the molecular core is S 235 A , which is a more evolved massive star with a well developed HII region arount it.
The important aspect that comes out of the present work is that the H2 O maser and its associated exciting star represent the youngest sources to be found in the cluster (see also Testi et al. 1994; Hunter et al. 1995; Palla et al. 1995; Persi et al. 1996). While originally H2 O masers had been found close to HII regions, and associated genetically to them, it is now becoming clear that this association is fortuitous, or more clearly, derived from the fact that other stars in the same cluster may have already formed their own HII region, but are not necessarely connected to the masers. Instead, the maser seems to be present only in the earliest phases, when no thermal continuum radio emission is detectable. The study of the regions around the H2 O masers is thus of fundamental importance in the understanding of the earliest (proto-)stars and their interaction with the ambient medium. Due to the complexity of the morphology of the regions in which these objects are forming, high resolution observations are essential to separate the youngest objects from more evolved HII regions and to avoid the confusion introduced by the presence of nearby more evolved stars. In fact in all the regions that we have investigated in detail (this one as well as the ones discussed in: Hunter et al. 1995; Palla et al. 1995; Persi et al. 1996), the previous low resolution (and low sensitivity) observations had not been able to reveal the true connection between H2 O masers and the earliest stellar evolutionary phases.
4.2. Star forming efficiency in the cloud core
From our molecular observations is it also possible to derive an estimate of the star forming efficiency in the molecular cloud. If we assume a spherical uniform cloud with a density of , we obtain a total mass , while the virial mass is . This two values are in good agreement, if we take into account the uncertainties in the density and distance determinations.
The total mass of the stars within the cloud core is more difficult to estimate, since we do not know the spectral types of each source. From our K-band image and from the K' image of Hodapp (1994) we can estimate that the number of stars within the half power level in the C34 S (3-2) map is of the order of . Assuming a mean mass of 1 per star, we find that the mass already converted into stars is of the molecular mass.
© European Southern Observatory (ESO) 1997
Online publication: June 30, 1998