Free access article

A&A 414, 235-243 (2004)
DOI: 10.1051/0004-6361:20031637

Observations of hydroxyl ground state transitions in a complete sample of methanol sources

M. Szymczak¹ and E. Gérard<sup>2

1</sup>  Torun Centre for Astronomy, Nicolaus Copernicus University, Gagarina 11, 87-100 Torun, Poland
²  GEPI, UMR 8111, Observatoire de Paris, 5 place J. Janssen, 92195 Meudon Cedex, France

(Received 6 June 2003 / Accepted 1 October 2003)

Abstract
High sensitivity observations of all four transitions of the ground state , J=3/2 of OH in both senses of circular polarization have been carried out with the Nançay radio telescope. The sample was a set of 100 star forming regions detected in a recent unbiased survey of 6668 MHz CH ₃OH masers. OH maser emission was found in 55 objects of which 31 were not previously catalogued. The 1665 MHz line was seen in almost all OH maser sources and was accompanied by the 1667 MHz line in about 75% of cases. Respectively 7% and 11% of OH 1665 MHz masers were accompanied by maser lines at 1612 and 1720 MHz. These two satellite line masers never occurred simultaneously in the same source nor at the same radial velocity, suggesting mutually exclusive physical conditions as predicted by models. OH maser emission usually shared the same velocity range as the 6668 MHz CH ₃OH maser. The intensity ratio of the 6668 MHz and 1665 MHz lines clearly divides the sample into methanol- and hydroxyl-rich sources and could be controlled by the abundance of maser molecules and the kinetic temperature. The OH maser emission was substantially polarized with a mean fractional circular polarization of 0.30 and Zeeman pair candidates were found in 15 targets. 36 sources were found in OH absorption at the main lines and 24 of them were also accompanied by OH maser emission. OH absorption features were blueshifted with regard to the related OH masers, indicating that they were formed in front of the central continuum sources. Absorption at 1720 MHz was always accompanied by emission at 1612 MHz and vice versa. The behaviour of stimulated emission and absorption in both satellite lines was thus conjugated and the 1720 MHz emission features seem to be signatures of regions of low hydrogen density and OH column density. The correlation of OH and CH ₃OH flux densities with the IRAS flux densities found for our sample appears to support pumping schemes of both molecules by infrared photons. Statistics of masers in the sample appear to be consistent with the scenario that the CH ₃OH masers appeared earlier than the OH masers.

Key words: masers - surveys - stars: formation - ISM: molecules - radio lines: ISM - HII regions

Offprint request: M. Szymczak, msz@astro.uni.torun.pl

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