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

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.

Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with significant participation from NASA.Appendices are available in electronic form at http://www.aanda.org