Identifying the Mechanisms of Water Maser Variability During the Accretion Burst in NGC6334I

Abstract

HMYSOs gain most of their mass in short bursts of accretion. Maser emission is an invaluable tool in discovering and probing accretion bursts. We observed the 22 GHz water maser response induced by the accretion burst in NGC6334I-MM1B and identified the underlying maser variability mechanisms. We report seven epochs of VLBI observations of 22 GHz water masers in NGC6334I with the VERA array, from 2014 to 2016, spanning the onset of the accretion burst in 2015.1. We also report 2019 ALMA observations of 321 GHz water masers and 22 GHz maser monitoring by HartRAO. We analyze variability patterns and use proper motions with the 22 GHz to 321 GHz line ratio to distinguish between masers in C-shocks and J-shocks. We also calculated the burst-to-quiescent variance ratio of the single-dish time series. 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. We find that the flux density variance ratio in the single-dish time series can identify maser efficiency variations in 22 GHz masers. The northern region, CM2-W2, is excited in C-shocks and showed long-term flaring with velocity-dependent excitation of new maser features. We propose that radiative heating of H2 due to high-energy radiation from the accretion burst be the mechanism for the flaring in CM2-W2. The southern regions are excited by J-shocks and have short-term flaring and dampening of water masers. We attributed the diverse variability patterns in the southern regions to the radiative transfer of the burst energy in the source. Our results indicate that the effects of source geometry, shock type, and incident radiation spectrum are fundamental factors affecting 22 GHz maser variability. Investigating water masers in irradiated shocks will improve their use as a diagnostic in time-variable radiation environments.

21 pages, 12 figures, Table 2 in electronic publication. Contact: [email protected]

References in corpus (18)

• An extraordinary outburst in the massive protostellar system NGC6334I-MM1: quadrupling of the millimeter continuum
• Trigonometric parallaxes of star forming regions in the Sagittarius spiral arm
• {Millimeter Multiplicity in NGC 6334 I and I(N)
• A heat-wave of accretion energy traced by masers in the G358-MM1 high-mass protostar
• The Role of Outflows, Radiation Pressure, and Magnetic Fields in Massive Star Formation
• The Massive Protostellar Cluster NGC6334I at 220 AU Resolution: Discovery of Further Multiplicity, Diversity and a Hot Multi-Core
• Evolution of dust in protoplanetary disks of eruptive stars
• Time-Resolved AU-Scale Jets Traced by Masers in the IRAS 4A/B Regions of NGC1333
• Parameter study for the burst mode of accretion in massive star formation
• On the episodic excursions of massive protostars in the Hertzsprung-Russell diagram
• VERA and ALMA Observations of the H2O Supermaser Burst in Orion KL
• Submillimeter Array Observations of 321 GHz Water Maser Emission in Cepheus A
• Accretion bursts in high-mass protostars: a new testbed for models of episodic accretion
• Water maser variability in a high-mass YSO outburst -- VERA and ALMA observations of S255 NIRS 3
• A multi-transition methanol maser study of the accretion burst source G358.93-0.03-MM1
• Episodic accretion and mergers during growth of massive protostars
• Maser Flare Simulations from Oblate and Prolate Clouds
• The Extraordinary Outburst in the Massive Protostellar System NGC6334I-MM1: Spatio-kinematics of Water Masers during a Contemporaneous Flare Event