Tracing Water Masers at their Smallest Scale with VLBI

TL;DR

This study uses VLBI to observe water masers at very small scales in a star-forming region during an accretion burst, revealing structural changes and proposing radiative heating as a cause for maser flares.

Contribution

It provides the first detailed VLBI observations of water maser changes at AU scales during an accretion event, introducing a spectral method for proper motion analysis.

Findings

Maser features expanded from 0.6 to 1.4 AU post-burst

Brightness of a key maser feature increased fourfold in 0.2 years

Maser activity correlated with increased radiative heating from the protostar

Abstract

The high-mass star-forming region NGC6334I-MM1 underwent an energetic accretion event in January 2015. We report the large-scale ($10 - 100$ AU) and small-scale ($\sim 1$ AU) changes in spatial and velocity structures of 22 GHz water masers as observed with VERA before and during the accretion burst. The masers in the northern bow-shock CM2-W2 brightened, and better traced a bow structure during the burst. In the southern regions, there was both activation and disappearance of associations before and during the burst. We measured the amplitudes, central velocities and FWHMs of about 20 features in each epoch. We found that the linear scale of the brightest feature in CM2-W2 grew from 0.6 AU before the burst to 1.4 AU after the burst, possibly indicating that a larger volume of gas was able to sustain masing action as a consequence of the accretion burst. This feature also had a rapid (0.2 yr) brightness increase by a factor of four, which has been previously reported in long-term single-dish monitoring. We propose that the water maser flare could be explained by an increase of the collisional pump rate due to radiative heating of H$_2$ by increased high energy radiation (UV or X-ray) from the inner protostellar core. We also describe the spot and spectral method of maser proper motion calculations. We argue that for high spectral resolution observations the spectral method is more robust for calculating proper motions than the spot method.