Cornell University, Center for Radiophysics & Space Research,
511 H street, SW, Washington, DC 20024-2725, USA e-mail: firstname.lastname@example.org
2 University of Western Ontario, Department of Physics and Astronomy, London, Ontario, N6A 3K7, Canada e-mail: email@example.com
3 Johns Hopkins University, Department of Physics and Astronomy, Baltimore, MD 21218, USA
4 IPAC, Caltech, Pasadena, CA 91925, USA
5 Consejo Superior de Investigaciones Cientificas, 28006 Madrid, Spain
6 Katolieke Universiteit Leuven, Institut voor Sterrenkunde, Heverlee 3001, Belgium
7 Sterrenkundig Instituut Anton Pannekoek, University of Amsterdam, Science Park 904, 1098 Amsterdam, The Netherlands
8 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
9 National University of Ireland, Maynooth, Department of Experimental Physics, County Kildare, Ireland
10 Space Research Organization of the Netherlands (SRON), 9700 AV Groningen, The Netherlands
11 University of Waterloo, Department of Physics and Astronomy, Waterloo, Ontario, N2L 3G1, Canada
12 Harvard-Smithsonian Center for Astrophysics Cambridge, MA 02138, USA
13 European Space Astronomy Centre, Urb. Villafranca del Castillo, PO Box 50727, 28080 Madrid, Spain
14 SOFIA Science Center, Universities Space Research Association, NASA Ames Research Center, Moffett Field, CA 94035-0001, USA
15 Observatorio Astronómico Nacional (IGN), Alfonso XII N3, 28014 Madrid, Spain
16 Observatorio Astronómico Nacional (IGN), Ap 112, 28803 Alcalá de Henares, Spain
17 Department of Astrophysics/IMAPP, Radboud University Nijmegen, Nijmegen, The Netherlands
18 Onsala Space Observatory, Dept. of Radio and Space Science, Chalmers University of Technology, 43992 Onsala, Sweden
19 The Netherlands and Astronomical Institute, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands
20 Department of Astronomy, AlbaNova University Center, Stockholm University, 10691 Stockholm, Sweden
21 Joint ALMA Observatory, El Golf 40, Las Condes, Santiago, Chile
22 N. Copernicus Astronomical Center, Rabiańska 8, 87–100 Toruń, Poland
Accepted: 3 September 2010
Context. Water vapour maser emission from evolved oxygen-rich stars remains poorly understood. Additional observations, including polarisation studies and simultaneous observation of different maser transitions may ultimately lead to greater insight.
Aims. We have aimed to elucidate the nature and structure of the VY CMa water vapour masers in part by observationally testing a theoretical prediction of the relative strengths of the 620.701 GHz and the 22.235 GHz maser components of ortho H2O.
Methods. In its high-resolution mode (HRS) the Herschel Heterodyne Instrument for the Far Infrared (HIFI) offers a frequency resolution of 0.125 MHz, corresponding to a line-of-sight velocity of 0.06 km s-1, which we employed to obtain the strength and linear polarisation of maser spikes in the spectrum of VY CMa at 620.701 GHz. Simultaneous ground based observations of the 22.235 GHz maser with the Max-Planck-Institut für Radioastronomie 100-m telescope at Effelsberg, provided a ratio of 620.701 GHz to 22.235 GHz emission.
Results. We report the first astronomical detection to date of H2O maser emission at 620.701 GHz. In VY CMa both the 620.701 and the 22.235 GHz polarisation are weak. At 620.701 GHz the maser peaks are superposed on what appears to be a broad emission component, jointly ejected from the star. We observed the 620.701 GHz emission at two epochs 21 days apart, both to measure the potential direction of linearly polarised maser components and to obtain a measure of the longevity of these components. Although we do not detect significant polarisation levels in the core of the line, they rise up to approximately 6% in its wings.
Key words: stars: AGB and post-AGB / stars: winds, outflows / supergiants / circumstellar matter / masers / reference systems
Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
Appendix (page 5) is only available in electronic form at http://www.aanda.org
© ESO, 2010