2010A&A...521L..51H

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. 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 620.701GHz and the 22.235GHz maser components of ortho H₂O.In its high-resolution mode (HRS) the Herschel Heterodyne Instrument for the Far Infrared (HIFI) offers a frequency resolution of 0.125MHz, corresponding to a line-of-sight velocity of 0.06km/s, which we employed to obtain the strength and linear polarisation of maser spikes in the spectrum of VY CMa at 620.701GHz. Simultaneous ground based observations of the 22.235GHz maser with the Max-Planck-Institut fuer Radioastronomie 100-m telescope at Effelsberg, provided a ratio of 620.701GHz to 22.235GHz emission.We report the first astronomical detection to date of H₂O maser emission at 620.701GHz. In VY CMa both the 620.701 and the 22.235GHz polarisation are weak. At 620.701GHz the maser peaks are superposed on what appears to be a broad emission component, jointly ejected from the star. We observed the 620.701GHz 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.