Looking for high-mass young stellar objects: HO and OH masers in ammonia cores

¹ INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy e-mail: [codella;cesa;sepulcre;mbeltran;lt]@arcetri.astro.it
² Subaru Telescope, National Astronomical Observatory of Japan, 650 North A'ohoku Place, Hilo, HI 96720, USA e-mail: rsf@subaru.naoj.org
³ ESO, Karl Schwarzschild srt. 2, 85748 Garching, Germany e-mail: ltesti@eso.org

Received: 29 September 2009
Accepted: 5 November 2009

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 H₂O 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 NH₃(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 . However, only in three cases do we find a projected overlap between Hii regions and H₂O 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  AU, and high masses (up to 10⁴ ), and are very dense (from to a few  cm^-3). The typical relative NH₃ abundance is , in agreement with previous measurements in high-mass star-forming regions.

Conclusions. The statistical analysis of the distribution between H₂O and OH masers, NH₃ 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.