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 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:
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RA (2000) DE Designation(s)
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23 13 45.32 +61 28 11.7 NGC7538-IRS1 = [WBN74] NGC 7538 IRS 1
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
list.dat 183 3 List of fits files
fits/* . 3 Individual fits files
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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
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Bytes Format Units Label Explanations
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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)
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Acknowledgements:
Fabrice Herpin, fabrice.herpin(at)u-bordeaux.fr
(End) Patricia Vannier [CDS] 20-Sep-2017