NGC7538-IRS1 THz water maser

We present SOFIA observations of the o-H_2_O 8_2,7_-7_3,4_ line at 1296.41106GHz and a 6_16_-5_23_ 22GHz e-MERLIN map of the region (first-ever 22GHz images made after the e-MERLIN upgrade). In order to be able to constrain the nature of the emission - thermal or maser - we use near-simultaneous observations of the 22GHz water maser performed with the Elsberg radiotelescope and e-MERLIN. A thermal water model using the RATRAN radiative transfer code is presented based on HIFI pointed observations.

Molecular water abundances are derived for the hot core. The of the o-H_2_O 8_2,7_-7_3,4_ line is detected toward NGC7538-IRS1 with one feature at the source velocity (-57.7km/s) and another one at -48.4km/s. We propose that the emission at the source velocity is consistent with thermal excitation and is excited in the innermost part of the IRS1a massive protostellar object's closest circumstellar environment.

The other emission is very likely the first detection of a water THz maser line, pumped by shocks due to IRS1b outflow, in a star-forming region. Assuming thermal excitation of the THz line, the water abundance in NGC7538-IRS1's hot core is estimated to be 5.2x10^-5^ with respect to H_2_.