Abstract
Context. The formation of massive stars (M>10 M , L> 103 L ) 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, over a broad range of energies, 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.
Aims. 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 H2 O line. Water maser models predict that several THz water masers should be detectable in these objects. We
therefore aim to detect for the first time the o-H2 O 82,7 − 73,4 line in a star forming region, which model calculations predict to show
maser action.
Methods. We present SOFIA observations of the o-H2 O 82,7 − 73,4 line at 1296.41106 GHz and a 616 − 523 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.
Results. The o-H2 O 82,7 − 73,4 line is detected toward NGC7538-IRS1 with one feature at the source velocity (-57.7 km s−1 ) and
another one at -48.4 km s−1 . 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 5.2 × 10−5 with respect to H2 .
Original language | English |
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Pages (from-to) | 52- |
Number of pages | 15 |
Journal | Astronomy and Astrophysics |
Volume | 606 |
Early online date | 9 Oct 2017 |
DOIs | |
Publication status | Published - 2017 |
Keywords
- Stars: formation – ISM: UCHii regions –masers– molecules–individual objects: NGC7538