VLA observations of water masers towards 6.7 GHz methanol maser sources

¹ Toruń Centre for Astronomy, Nicolaus Copernicus University, Gagarina 11, 87-100 Toruń, Poland
e-mail: annan@astro.uni.torun.pl
² Department of Physics and Astronomy, MSC07 4220, University of New Mexico, Albuquerque, NM 87131, USA
³ National Radio Astronomy Observatory, 1003 Lopezville Road, Socorro, NM 87801, USA
⁴ Joint Institute for VLBI in Europe, Postbus 2, 7990 AA Dwingeloo, The Netherlands
⁵ Sterrewacht Leiden, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands
⁶ Max-Planck-Insitut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁷ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA

Received: 18 June 2010
Accepted: 9 September 2010

Abstract

Context. Both 22 GHz water and 6.7 GHz methanol masers are usually interpreted as signposts of early stages of high-mass star formation but little is known about their associations and the physical environments in which they occur.

Aims. We attempt to derive accurate positions and morphologies of the water maser emission and relate them to the methanol maser emission mapped earlier with Very Long Baseline Interferometry.

Methods. We searched for 22 GHz water masers in 31 methanol maser sources was searched for 22 GHz water masers using the VLA and observed in the 6.7 GHz methanol maser line with the 32 m Torun dish simultaneously.

Results. Water maser clusters are detected towards 27 sites leading to the identification of 15 new sources. The detection rate of water maser emission associated with methanol sources is as high as 71%. In a large number of objects (18/21), the structure of water maser is well aligned with that of the extended emission at 4.5 μm confirming the origin of water emission in outflows. The sources of methanol emission with ring-like morphologies, which likely trace a circumstellar disc/torus, either do not show associated water masers or the distribution of water maser spots is orthogonal to the major axis of the ring.

Conclusions. The two maser species are generally powered by the same high-mass young stellar object but probe different parts of its environment. The morphology of water and methanol maser emission in a minority of sources is consistent with a scenario where 6.7 GHz methanol masers trace a disc/torus around a protostar while the associated 22 GHz water masers arise in outflows. The majority of sources in which methanol maser emission is associated with the water maser appear to trace outflows. The two types of associations may be related to different evolutionary phases.