A millimeter methanol maser ring tracing the deceleration of the heat wave powered by the massive protostellar accretion outburst in G358.93-0.03 MM1

TL;DR

This study investigates how accretion outbursts in massive protostars drive propagating heat waves through their envelopes, traced by newly discovered millimeter methanol masers around G358.93-0.03 MM1. Using multi-epoch ALMA and SMA data for transitions at 199 and 217 GHz, the authors map a near-complete ring of maser emission encircling MM1 and track its expansion over ~200 days. They find the ring diameter grows from about $1100$ AU to $1800$ AU, implying an average radial speed of $0.01c$ and a radius-time relation $R \,\propto\ $ $(t-t_0)^{0.39}$, closely matching the Taylor-von Neumann-Sedov self-similar explosion $R \propto t^{2/5}$. The results demonstrate the explosive energy transport in massive protostellar outbursts and establish millimeter methanol masers as tracers of heat waves across months in dense star-forming environments.

Abstract

We present multi-epoch, multi-band ALMA imaging of the new Class II millimeter methanol masers excited during the accretion outburst of the massive protostar G358.93-0.03 MM1. The highest angular resolution image (24 mas $\approx$ 160 au) reveals a nearly complete, circular ring of strong maser spots in the 217.2992 GHz ($v_t$=1) maser line that closely circumscribes the dust continuum emission from MM1. Weaker maser emission lies inside the eastern and southern halves of the maser ring, generally coincident with the centimeter masers excited during the outburst, but avoiding the densest parts of the hot core gas traced by high excitation lines of CH$_3$CN. Using a variety of fitting techniques on the image cubes of the two strongest maser lines, each observed over 3-4 epochs, we find the diameter of the ring increased by $\gtrsim$60% (from $\approx$1100 to $\approx$1800 au in the 217 GHz line) over 200 days, consistent with an average radial propagation rate of $\approx$0.01c, while the maser intensity declined exponentially. Fitting the angular extent of the millimeter masers versus time yields a power law of index 0.39$\pm$0.06, which also reproduces the observed extent of the 6.7 GHz masers in the first VLBI epoch of R. A. Burns et al. (2020). This exponent is consistent with the prediction of radius vs. time in the Taylor-von Neumann-Sedov self-similar solution for an intense spherical explosion from a point source ($R \propto t^{2/5}$). These results demonstrate the explosive nature of accretion outbursts in massive protostars and their ability to generate subluminal heat waves traceable by centimeter and millimeter masers for several months as the energy traverses the surrounding molecular material.

Figures (3)

• Figure 1: (a) Peak intensity image of the 217 GHz masers surrounding MM1 at Epoch 2019.451 with squares marking 6.7 GHz maser spots at Epoch 2019.429 from Burns2023 and $+$ signs marking 20.9706 GHz maser spots at Epoch 2019.421 from Bayandina2022. The blue dash-dotted circle is a fit to the Gaussian-fitted positions of the 8 strongest spatial components around the ring, while the dashed circles represent the 1$\sigma$ uncertainty on the fit. Panels (b-d) show in colorscale the peak intensity of the 199 GHz (b) and 217 GHz (c, d) CH$_3$OH transitions, smoothed to $0\farcs075$ resolution, from the 2021.6 epoch data when the emission is consistent with thermal excitation. White contours show the peak intensity of the 199 GHz (b) and 217 GHz (c, d) CH$_3$OH lines while they were still strongly masing circa 2019.5 (levels: 5, 10, 20, 40, 80% of the maser peak). Also overlaid on (b-d) are red contours of the CH$_3$CN integrated intensity, smoothed to $0\farcs075$ resolution, for the J=11-10 K=7, J=12-11 K=4, and J=12-11 K=7 transitions, respectively (levels: 25, 50, 75, 90% of the peak). Panels (e-f) show spectra of the central 10 km s$^{-1}$ spatially integrated over the region of significant emission at each of the epochs from Table \ref{['maserEpochs']}.
• Figure 2: Native resolution peak intensity images of the 217 GHz maser emission in greyscale, ordered by epoch, overlaid with 217 GHz continuum contours from epoch 2019.538 (0.05, 0.1, 0.2, 0.4, 0.8 Jy beam$^{-1}$), and marked with the 2-component fits as crosses colored by velocity and connected to their counterparts by dashed lines. The initial estimates are shown as magenta X's in panel b. Blue dashed lines denote channels fit successfully across all epochs, magenta dashed lines denote channels fit successfully across only the ALMA epochs, while white and black dashed lines denote the remaining channels fit successfully in two or fewer epochs. The blue circles in panels b and c are the circle fits (§ \ref{['technique']}) for their epochs, with the dashdot circle being the best fit and the dashed circles representing the 1$\sigma$ uncertainties.
• Figure 3: (a) Light curve of the 199 and 217 GHz maser lines on a semi-log scale overlaid by fitted models of exponential decay, with vertical dashed lines denoting the VLBI epochs Burns2020 and first SOFIA epoch Stecklum2021 for reference; (b) Ring diameter vs. time as inferred from the various maser tracers and methods described in § \ref{['technique']}. The solid curves are fractional power-law fits to the millimeter maser 2-component fit results from this work (blue) and independently to the 6.7 GHz VLBI measurements (red) from Burns2023. The best-fit blue solid curve is consistent with the Taylor-von Neumann-Sedov exponent (blue dashed curve) within the 90% confidence region indicated by the blue shaded area computed from bootstrap analysis (see Section \ref{['fitResults']}). The red spot marks the inception of the maser flare Sugiyama19. The gap in the red curve near the origin is where the fit would exceed the speed of light.