Detections of Compact Radio Continuum toward Methanol Maser Rings Using the VLA

TL;DR

This work uses sensitive JVLA imaging at C- and K-bands, plus maser observations at $6.7$ GHz and $22$ GHz, to probe the origins of ring-like methanol maser distributions around six HMYSOs. The data reveal cm-continuum emission in all targets, with five displaying thermal jets and one likely hosting an H II region, supporting a jet/ wind interpretation in most cases and highlighting the role of orientation in shaping observed maser morphologies. Across the sample, maser rings commonly coincide with continuum peaks, yet radial expansion and complex environments indicate a range of evolutionary states from jet-dominated to more ionized regimes. The results emphasize that ring-shaped masers do not map a single evolutionary stage and motivate multi-frequency, high-resolution studies to disentangle discs, jets, and envelopes in massive star formation.

Abstract

High-mass protostars are deeply embedded in dust inside their natal cores and are not easily detectable. However, maser emission at centimeter wavelengths, owing to its high brightness, enables us to study gas kinematics in protostars' circumstellar regions. We aim to understand the origin of the ring-like structures outlined by the 6.7 GHz methanol maser emission in six high-mass young stellar objects by performing a sensitive search of the associated radio-continuum emission and derive its properties. We used the Karl G. Jansky Very Large Array in the A configuration at C and K bands in order to image radio-continuum as well as 6.7 GHz methanol and 22 GHz water maser emission. We present the first images of the thermal jets towards four targets in our sample, G23.389+00.185, G23.657-00.127, G28.817+00.365, and G30.400-00.296. In a further target, G23.207-00.377, the complex K band continuum emission makes it unclear whether the detected peaks trace jet knots from a single young protostar or mark multiple compact young protostars. The remaining source G31.047+00.356 shows radio continuum emission associated with an evolved H II region.

Figures (10)

• Figure 1: G23.207$-$00.377: (a) The continuum emission detected using JVLA at the C band (colors) and K band (white contours). The contours correspond to 3$\sigma$, 7$\sigma$, 9$\sigma$, and 12$\sigma$ (where $\sigma$ is listed in Table \ref{['tab:obs']}) The synthesized beam sizes are presented at the bottom, left corner; the transparent ellipse with black outline and the white ellipse correspond to C- and K-band data, respectively. (b) The continuum emission overlaid with methanol and water maser spots detected using JVLA. Their sizes are proportional to the logarithm of S$_p$ of methanol and water peak fluxes, respectively. The colors of spots correspond to the LSR velocities as presented on wedges. (c) The spectra and the distributions of the maser transitions presented in this publication and from bartkiewicz2009bartkiewicz2011. The (0,0) point corresponds to the brightest methanol maser spot (Table \ref{['tab:resmaser']}). The vertical light gray lines in spectra represent the systemic velocities of sources as in szymczak2007. The sizes of the symbols are proportional to the logarithm of the peak flux density of a spot. The gray arrows at the left and right panels point out the variable (over 14 yr) methanol maser features. The thin and thick crosses corresponds to the peaks of C- and K-band radio continua with their positional uncertainties, respectively. The gray ellipse traces the best flux-weighted fit as in bartkiewicz2024.
• Figure 2: Similar as Figure\ref{['fig:G23p207']} but for G23.389$+$00.185. (a) The contours of the K-band emission correspond to 3$\sigma$, 7$\sigma$, 11$\sigma$, 16$\sigma$, and 20$\sigma$. (c) The red and orange crosses trace the NIR and MIR emission from debuizer2012, respectively. The crosses trace the likely spurious spots (see Sect. \ref{['subsec:targets']})
• Figure 3: Similar as Figure \ref{['fig:G23p207']} but for G23.657$-$00.127: (a) The contours of the K-band emission correspond to 1$\sigma$ and 3$\sigma$. (c) The red and orange crosses trace the NIR and MIR emission from debuizer2012, respectively. The crosses trace the likely spurious spots (see Sect. \ref{['subsec:targets']}).
• Figure 4: Similar as Figure \ref{['fig:G23p207']} but for G28.817$+$00.365: (a) The contours of the K-band emission correspond to 3$\sigma$, 10$\sigma$, 40$\sigma$, and 80$\sigma$.
• Figure 5: Similar as Figure \ref{['fig:G23p207']} but for G30.400$-$00.296: (a) The contours of the K-band emission correspond to 3$\sigma$, 7$\sigma$, 11$\sigma$, and 13$\sigma$.