J/A+A/691/A157 Water maser variability in NGC6334I (Vorster+, 2024)
Identifying the mechanisms of water maser variability during the
accretion burst in NGC6334I.
Vorster J.M., Chibueze J.O., Hirota T., MacLeod G.C., van der Walt D.J.,
Vorobyov E. I., Sobolev A.M., Juvela M.
<Astron. Astrophys. 691, A157 (2024)>
=2024A&A...691A.157V 2024A&A...691A.157V (SIMBAD/NED BibCode)
ADC_Keywords: Masers ; VLBI
Keywords: masers - techniques: high angular resolution - stars: formation -
ISM: jets and outflows - ISM: individual objects: NGC6334I
Abstract:
High-mass young stellar objects gain most of their mass in short
intense bursts of accretion. Maser emission is an invaluable tool in
discovering and probing these accretion bursts.
Our aim was to observe the 22GHz water maser response induced by the
accretion burst in NGC6334I-MM1B and to identify the underlying maser
variability mechanisms.
We report seven epochs of very long baseline interferometry (VLBI)
observations of 22GHz water masers in NGC6334I with the VLBI
Exploration of Radio Astrometry (VERA) array, from 2014 to 2016,
spanning the onset of the accretion burst in 2015.1. We also report
2019 Atacama Large Millimeter/submillimeter Array (ALMA) observations
of 321GHz water masers and 22 GHz single-dish maser monitoring by the
Hartebeesthoek Radio Astronomical Observatory (HartRAO). We analysed
long-term variability patterns and used proper motions with the 22GHz
to 321GHz line ratio to distinguish between masers in
non-dissociative C-shocks and dissociative J-shocks. We also
calculated the burst-to-quiescent variance ratio of the single-dish
time series.
We detected a water maser distribution resembling a bipolar outflow
morphology. The constant mean proper motion before and after the burst
indicates that maser variability is due to excitation effects from
variable radiation rather than jet ejecta. For the whole region, we
find that the flux density variance ratio in the single-dish time
series can identify maser efficiency variations in 22GHz masers. The
northern region, CM2-W2, is excited in C-shocks and showed long-term
flaring with velocity-dependent excitation of new maser features after
the onset of the burst. We propose that radiative heating of H2 due
to high-energy radiation from the accretion burst be the main
mechanism for the flaring in CM2-W2. The southern regions are excited
by J-shocks, which have shown short-term flaring and dampening of
water masers. We attribute the diverse variability patterns in the
southern regions to the radiative transfer of the burst energy in the
complex source geometry.
Our results indicate that the effects of source geometry, shock type,
and incident radiation spectrum are fundamental factors affecting
22GHz maser variability. Investigating water masers in irradiated
shocks will improve their use as a diagnostic in time-variable
radiation environments, such as accretion bursting sources.
Description:
Positions of 22 GHz maser features in NGC6334I observed with the VLBI
Exploration for Radio Astrometry (VERA) array on 19 Sep 2014,
25 Nov 2014, 31 Jan 2015, 14 Apr 2015 17, 09 Feb 2016 and 12 Mar 2016.
Objects:
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RA (2000) DE Designation(s)
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17 20 53.35 -35 47 01.4 NGC6334I = NAME NGC 6334-I
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table2.dat 81 199 Positions of 22 GHz maser features in NGC6334I
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Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
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1- 4 A4 --- ID ID the form EP-XX, EP: epoch number,XX: ID
6- 13 A8 --- Assoc Association name (see paper for
association description)
15- 22 F8.5 arcsec oRA Right Ascension (J2000) offset to 17:20:52.600
24- 30 F7.5 arcsec e_oRA Uncertainty on RA
32- 39 F8.5 arcsec oDE Declination (J2000) offset to -35:46:50.508
41- 47 F7.5 arcsec e_oDE Uncertainty on DE
49- 53 F5.1 Jy/beam Ipeak Peak Intensity at 22GHz
55- 59 F5.2 Jy/beam e_Ipeak Uncertainty on Ipeak
61- 66 F6.2 km/s VLSR Centre Velocity
68- 72 F5.3 km/s e_VLSR Uncertainty on VLSR
74- 76 F3.1 km/s FWHM Full Width at Half Maximum
78- 81 F4.2 km/s e_FWHM Uncertainty on FWHM
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Acknowledgements:
Jakobus Vorster, jakobus.vorster(at)helsinki.fi
(End) Patricia Vannier [CDS] 28-Aug-2024