First detection of THz water maser in NGC7538-IRS1 with SOFIA and new 22
  GHz e-MERLIN maps
  The formation of massive stars is still not well understood. Accumulating a
large amount of mass infalling within a single entity in spite of radiation
pressure is possible if, among several other conditions, enough thermal energy
is released. Despite numerous water line observations, with the Herschel Space
Observatory, in most of the sources observations were not able to trace the
emission from the hot core around the newly forming protostellar object. We
want to probe the physical conditions and water abundance in the inner layers
of the host protostellar object NGC7538-IRS1 using a highly excited H2O line.
Water maser models predict that several THz water masers should be detectable
in these objects. We present SOFIA observations of the o-H2O 8(2,7)-7(3,4) line
at 1296.41106 GHz and a 6(1,6)-5(2,3) 22 GHz e-MERLIN map of the region
(first-ever 22 GHz 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 22 GHz water maser performed with the
Effelsberg 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 H2O 8(2,7)- 7(3,4)
line is detected toward NGC7538-IRS1 with one feature at the source velocity
(-57.7 km/s) and another one at -48.4 km/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 H2.
