2014A&A...567A..31J

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 620.701GHz can only be observed from above Earth's strongly absorbing atmosphere; its emission has recently been detected from space. We sought to further characterize the star-forming environment of Orion KL by investigating the linear polarisation of a source emitting a narrow 620.701GHz maser feature with the heterodyne spectrometer HIFI on board the Herschel Space Observatory. 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. 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 22.235GHz 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 22.235GHz maser at 7.2km/s, a weaker component at the same velocity as the 620.701GHz maser at 11.7km/s is much less polarised.