Research Note

Polarisation observations of H₂O J_K-1 K₁ = 5₃₂ − 4₄₁ 620.701 GHz maser emission with Herschel/HIFI in Orion KL ^⋆,⋆⋆

¹ University of Western Ontario, Department of Physics and Astronomy, London, Ontario, N6A 3K7, Canada
e-mail: sjone7@uwo.ca
² Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
³ Cornell University, Center for Radiophysics & Space Research, 511 H Street, SW, Washington, DC 20024-2725, USA
⁴ Herschel Science Centre, ESAC, European Space Agency, 28691 Villanueva de la Cañada, Madrid, Spain
⁵ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁶ University of Waterloo, Department of Physics and Astronomy, Waterloo, Ontario, N2L 3G1, Canada
⁷ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS 66, Cambridge, MA 02138, USA
⁸ Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, California, USA

Received: 17 December 2013
Accepted: 4 June 2014

Abstract

Context. The high intensities and narrow bandwidths exhibited by some astronomical masers make them ideal tools for studying star-forming giant molecular clouds. The water maser transition J_K-1K1 = 5₃₂−4₄₁ at 620.701 GHz can only be observed from above Earth’s strongly absorbing atmosphere; its emission has recently been detected from space.

Aims. We sought to further characterize the star-forming environment of Orion KL by investigating the linear polarisation of a source emitting a narrow 620.701 GHz maser feature with the heterodyne spectrometer HIFI on board the Herschel Space Observatory.

Methods. High-resolution spectral datasets were collected over a thirteen month period beginning in 2011 March, to establish not only the linear polarisation but also the temporal variability of the source.

Results. Within a 3σ uncertainty, no polarisation was detected to an upper limit of approximately 2%. These results are compared with coeval linear polarisation measurements of the 22.235 GHz J_K-1K1 = 6₁₆ − 5₂₃ maser line from the Effelsberg 100-m radio telescope, typically a much stronger maser transition. Although strongly polarised emission is observed for one component of the 22.235 GHz maser at 7.2 km s^-1, a weaker component at the same velocity as the 620.701 GHz maser at 11.7 km s^-1 is much less polarised.