The SOFIA Massive (SOMA) Radio Survey. II. Radio Emission from High-Luminosity Protostars

TL;DR

This study uses VLA and ATCA centimeter-continuum data to probe ionized gas, multiplicity, and jet activity in nine protostars across seven high-luminosity regions from the SOMA survey. By constructing radio SEDs at multiple scales (SOMA, intermediate, and inner), the authors test ionization scenarios and compare with theoretical tracks for massive protostar evolution, including shock ionization and photoionization models. They find 37 radio sources with high multiplicity and several jet-like morphologies, including six well-supported ionized jets, and demonstrate a steep rise in radio luminosity at 5 GHz with increasing bolometric luminosity ($S_ u$ at 5 GHz scales as $L_{ m bol}$ across a large range), while acknowledging potential flux filtering and dust contamination issues. Overall, the results constrain how ionization processes evolve in massive protostars and highlight the role of jets and outflows in their early development, laying groundwork for future, more sensitive surveys at even higher resolution. The radio data imply that ionizing output becomes increasingly important at higher masses, though many sources lie below simple photoionization expectations, suggesting a combination of jet-driven (shock) and eventually photoionized emission, with some flux possibly missed by interferometric filtering. These findings enhance our understanding of the link between radio emission and protostellar structure/evolution and provide empirical benchmarks for models of massive-star formation. The work also demonstrates the value of multi-scale radio observations in disentangling clustered star formation environments.

Abstract

We present centimeter continuum observations of seven high luminosity massive protostars and their surrounding sources in regions with multiple targets, as part of the SOFIA Massive (SOMA) Star Formation Survey. With data from the Very Large Array and the Australia Telescope Compact Array, we analyze the spectral index, morphology and multiplicity of the detected radio sources. The high-sensitivity, high-resolution observations allow us to resolve many sources; 65$\%$ of the reported sources are resolved at least within the synthesized beam. We report thirteen new detections and two previously known detections that we observed for the first time in radio frequencies. We use the observations to build radio spectral energy distributions (SEDs) to calculate spectral indices. With radio morphologies and the spectral indices, we give assessments on the nature of the sources, highlighting six sources that display a radio jet-like morphology and a spectral index consistent with ionized jets. Combining with the SOMA Radio I sample, we present the radio - bolometric luminosity relation, especially probing the regime from $L_{\rm bol}\sim 10^4$ to $10^6\:L_\odot$. Here we find a steep rise in radio luminosity, which is expected by models that transition from shock ionization to photoionization.

Figures (10)

• Figure 1: Images are SOFIA-FORCAST 37 $\mu$m (from Liu_2019) with VLA contours $-$ red: lower frequency (C-band (6 cm) for the VLA observations and 5, 5.5 or 7.2 GHz for ATCA observations); cyan: (K-band (1.3 cm) for the VLA observations and 17, 19 or 19.9 GHz for ATCA observations) $-$ of the combined radio maps overlaid. The centimeter emission observations for regions IRAS 16562--3959 and G339.88--1.26 are originally presented in 2016ApJ...826..208G and 2016MNRAS.460.1039P, respectively. The cyan crosses in IRAS 16562--3959 denotes the position of detections only at the high frequency bands (cyan contours). The black $\times$ denotes the position of the mm core on the regions with millimeter observations (references as follows: G309.92+0.48: 2005MNRAS.363..405H, G35.58-0.03: 2014ApJ...784..107Z, IRAS 16562 3959: 2014ApJ...796..117G, G305A: 2006MNRAS.365..321W and G339.88-1.26: 2019ApJ...873...73Z). The blue dashed squares correspond to the area of the inset image showing a zoom-in of the central region, and the synthesized beams are shown in the lower corners of these insets. The blue circles are the SOMA apertures used by Telkamp_2025 and reported in Table C1. The aperture radius is defined using the optimal aperture size algorithm developed by Fedriani_2023. The blue and red arrows represent the direction of a molecular outflow detected toward the region, only in regions with known molecular outflows (more details in Section \ref{['Morph']}). A scale bar in units of au is shown in the upper left of the figures.
• Figure 2: (Continued)
• Figure 3: Left is L-band VLA (20 cm) continuum maps with VLA contours overlaid (1.3 cm--cyan, 6 cm--red, 20 cm--blue) of G45.12+0.13. The white $\times$ symbol are the location of the sources in the cluster reported by 2006ApJ...637..400V. Note the offset in the peak intensity position from the results of 2006ApJ...637..400V with respect to the VLA observations. Right is showing a zoom region of source S14 with the VLA contours of the three bands. The contour levels are at [-5, 5, 20, 100] $\times$ 35 $\mu$Jy/beam for C-band, at [-5, 5, 15] $\times$ 120 $\mu$Jy/beam for K-band and at [-3, 3, 10, 30, 55, 100, 300, 500] $\times$ 0.25 mJy/beam for the L-band. The synthesized beams are shown in the lower left corner.
• Figure 4: VLA continuum maps of the triple system towards G35.58-0.03 observed at 1.3 and 6 cm. The beam sizes are indicated in the lower left-hand corner. The contour levels are at $\sigma \times$(-10, 10, 100, 500, 1200, 1700) for C-band (red) and $\sigma \times$(-10, 10, 25, 100, 500) for K-band (cyan), with the $\sigma$ value reported in Table \ref{['tab:SOMA_Sources']}
• Figure 5: VLA continuum 1.3 and 6 cm maps showing the multiplicity of region G35.58-0.03 in the SW of the SOMA scale. The beam sizes are indicated in the lower left-hand corner. The contour levels are at $\sigma \times$(-8, 8, 11, 20, 35) for C-band and $\sigma \times$(-5, 5, 10) for K-band, with the $\sigma$ value reported in Table \ref{['tab:SOMA_Sources']}