Astron. Astrophys. 325, 282-294 (1997)

6. Conclusions

We have used the VLA for mapping the NH₃ (2,2) and (3,3) and the 1.3 cm continuum emission towards 5 sources selected from the Forster & Caswell (1989) sample of H₂ O and OH masers. We detect ammonia emission in all sources, but in one case the emission could not be mapped mostly because of the short integration time used. As for the 4 sources where the spatial distribution could be properly studied, we can draw the following conclusions:

1. NH₃ cores are seen toward the positions of the H₂ O and OH masers;
2. only in one case we detect an UC HII region coincident with the NH₃ core: such UC HII region is likely to be very young, still at the very beginning of its development;
3. the ammonia cores are small (), hot (30-90 K), and massive (), i.e. similar to the hot cores seen by other authors (e.g. Cesaroni et al.  1994) in regions of massive star formation;
4. in one case (G24.78+0.08) there seem to be indication of the fact that H₂ O masers appear in a stage of stellar evolution prior to that of OH masers and disappear when an UC HII region develops (see also e.g. Codella & Felli 1995 and Codella et al. 1996).

The most relevant result of our search is that H₂ O masers originate in hot massive cores, which are likely sites of very young massive (proto)stars prior to the development of an UC HII region.

© European Southern Observatory (ESO) 1997

Online publication: May 5, 1998

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