Mol 112 , the SIMBAD biblio

We present observations of NH₃ (1,1) and (2,2) lines in two flux-limited samples of IRAS sources selected according to colour criteria which should result in a high fraction of Young Stellar Objects. The first sample contains sources (named 'Low') whose evolutionary status is essentially unknown, while the second sample contains sources (named 'High') possibly associated with ultracompact HII regions, the distinction being based on the IRAS [25-12] colour. Indications from a previous study of H₂O maser emission suggest that the sources in the first group may be in an evolutionary phase prior to the appearance of an HII region, thus being among the youngest known high-mass forming objects. Low sources were detected in ammonia with a lower rate than High sources (45% and 80% respectively); the only difference between the two groups is in the linewidths: the (1,1) lines are generally narrower than (2,2) lines in High sources (FWHMs median values are 1.81km/s and 2.00km/s respectively), while the opposite is true in Low sources (FWHMs median values are 1.72km/s and 1.33km/s for the (1,1) and (2,2) lines respectively). We propose that the Low group consists of two distinct populations of evolutionary different objects, based on the (non-) association with ammonia emission. The Low sources showing ammonia emission are characterized by more quiescent envelopes than those surrounding High sources, and the relationships between relevant physical quantities derived from our observations and the IRAS colours suggest that High sources, contrary to Low, may harbour objects which dominate the physical and dynamical properties of the clump, thus possibly implying the relative youth of this subgroup of the Low sources.