First detection of THz water maser in NGC7538-IRS1 with SOFIA and new 22 GHz e-MERLIN maps

F Herpin, A Baudry, Anita Richards, Malcolm Gray, N. Schneider, K. M. Menten, F Wyrowski, R Bontemps, R Simon, H. Wiesemeyer

    Research output: Contribution to journalArticlepeer-review

    75 Downloads (Pure)


    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 languageEnglish
    Pages (from-to)52-
    Number of pages15
    JournalAstronomy and Astrophysics
    Early online date9 Oct 2017
    Publication statusPublished - 2017


    • Stars: formation – ISM: UCHii regions –masers– molecules–individual objects: NGC7538


    Dive into the research topics of 'First detection of THz water maser in NGC7538-IRS1 with SOFIA and new 22 GHz e-MERLIN maps'. Together they form a unique fingerprint.

    Cite this