Looking for high-mass young stellar objects: H$_{\sf 2}$O and OH masers in ammonia cores

Abstract

Context. The earliest stages of high-mass star formation have yet to be characterised well, because high-angular resolution observations are required to infer the properties of the molecular gas hosting the newly formed stars. Aims. We search for high-mass molecular cores in a large sample of 15 high-mass star-forming regions that are observed at high-angular resolution, extending a pilot survey based on a smaller number of objects. Methods. The sample was chosen from surveys of H 2 O and OH masers to favour the earliest phases of high-mass star formation. Each source was first observed with the 32-m single-dish Medicina antenna in the (1,1) and (2, 2) inversion transitions at 1.3 cm of ammonia, which is an excellent tracer of dense gas. High-resolution maps in the NH 3 (2, 2) and (3, 3) lines and the 1.3 cm continuum were obtained successively with the VLA interferometer. Results. We detect continuum emission in almost all the observed star-forming regions, which corresponds to extended and UCHII regions created by young stellar objects with typical luminosities of ~10 4 L ⊙ . However, only in three cases do we find a projected overlap between HII regions and H 2 O and OH maser spots. On the other hand, the VLA images detect eight ammonia cores closely associated with the maser sources. The ammonia cores have sizes of ~10 4 AU, and high masses (up to 10 4 M ⊙ ), and are very dense (from ~10 6 to a few x10 9 cm -3 ). The typical relative NH 3 abundance is ≤10 -7 , in agreement with previous measurements in high-mass star-forming regions. Conclusions. The statistical analysis of the distribution between H 2 O and OH masers, NH 3 cores, and HII regions confirms that the earliest stages of high-mass star formation are characterised by high-density molecular cores with temperatures of on average ≥30 K, either without a detectable ionised region or associated with a hypercompact HII region.