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J/A+A/606/A52         NGC7538-IRS1 THz water maser               (Herpin+, 2017)

First detection of THz water maser in NGC7538-IRS1 with SOFIA and new 22 GHz e-MERLIN maps. Herpin F., Baudry A., Richards A.M.S., Gray M.D., Schneider N., Menten K.M., Wyrowski F., Bontemps S., Simon R., Wiesemeyer H. <Astron. Astrophys. 606, A52 (2017)> =2017A&A...606A..52H (SIMBAD/NED BibCode)
ADC_Keywords: Infrared sources ; Masers Keywords: stars: formation - masers - stars: individual: NGC7538-IRS1 - ISM: molecules - stars: protostars - ISM: abundances Abstract: The formation of massive stars (M>10M, L>103L) 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. 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 therefore aim to detect for the first time the o-H2O 82,7-73,4 line in a star forming region, which model calculations predict to show maser action. We present SOFIA observations of the o-H2O 82,7-73,4 line at 1296.41106GHz and a 616-523 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-H2O 82,7-73,4 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 H2. Description: Observations in singlepoint chopping mode of the 82,7-73,4 line of ortho-H216O were carried out toward NGC7538-IRS1 (RA=23:13:45.3 (J2000) DE=+61:28:10.0) as part of SOFIA Cycle 3 project on 2015 December 9, using GREAT. At the systemic velocity of NGC7538-IRS1, the system temperature was typically 2000K (SSB) and signal-band zenith opacity 0.08. The on-source integration time was 20-min. The chop throw was 100" to either side of the on-source position (chop-nod method). One channel (L1) of GREAT was tuned to the 1296.41106GHz water line frequency (lower sideband LSB), the other channel, the LFA (Low Frequency Array) 7 pixel array, was tuned to the [CII] 158um line. In order to constrain the maser models, and because of the variability of the maser emission, nearly contemporaneous observations of the 616-523 transition of ortho-H2O (rest frequency 22.23508GHz) were carried out on 2015 December 11 (integration time on source of 15-min) with the MPIfR 100-m telescope at Elsberg, Germany. Interferometric observations of the same source of the ortho-H2O 616-523 line emission were performed with e-MERLIN (commissioning observations) on 2016 April 20. These were the first-ever 22GHz images made after the e-MERLIN upgrade. Objects: ----------------------------------------------------------------- RA (2000) DE Designation(s) ----------------------------------------------------------------- 23 13 45.32 +61 28 11.7 NGC7538-IRS1 = [WBN74] NGC 7538 IRS 1 ----------------------------------------------------------------- File Summary:
FileName Lrecl Records Explanations
ReadMe 80 . This file list.dat 183 3 List of fits files fits/* 0 3 Individual fits files
See also: J/ApJ/696/471 : MIR spectroscopy of NGC 7538 IRS 1 (Knez+, 2009) J/A+A/543/A88 : NGC7538IRS1 and NGC7538S 1.36mm maps (Beuther+, 2012) J/A+A/558/A81 : NGC 7538 IRS1 maps in CH3OH, HCN and HCO+ (Beuther+, 2013) Byte-by-byte Description of file: list.dat
Bytes Format Units Label Explanations
1- 9 F9.5 deg RAdeg Right Ascension of center (J2000) 10- 18 F9.5 deg DEdeg Declination of center (J2000) 20- 24 I5 --- Nx Number of pixels along X-axis 26- 29 I4 --- Ny ? Number of pixels along Y-axis 31- 33 I3 --- Nz ? Number of slices 35- 57 A23 --- Obs.Date Observation date (YYYY-MM-DDThh:mm:ss.sss) 59- 66 I8 Kibyte size Size of FITS file 68- 86 A19 --- FileName Name of FITS file, in subdirectory fits 88-113 A26 --- Title Title of the FITS file 114-125 E12.6 Hz bFreq ? Lower value of relative frequency interval (for the spectrum files) 127-138 E12.6 Hz BFreq ? Upper value of relative frequency interval (for the spectrum files) 140-148 E9.4 Hz dFreq ? Frequency resolution (for the spectrum files) 149-160 E12.6 m/s bVlsr ? Lower value of LSR velocity (for the datacube) 162-173 E12.6 m/s BVlsr ? Upper value of LSR velocity (for the datacube) 175-183 E9.4 m/s dVlsr ? LSR velocity resolution (for the datacube)
Acknowledgements: Fabrice Herpin, fabrice.herpin(at)
(End) Patricia Vannier [CDS] 20-Sep-2017
The document above follows the rules of the Standard Description for Astronomical Catalogues.From this documentation it is possible to generate f77 program to load files into arrays or line by line

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