The SOFIA Massive(SOMA)Radio Survey. III. Radio Emission from Intermediate-Mass Protostars

TL;DR

This work extends the SOMA program by presenting high-sensitivity VLA observations at 6 cm and 1.3 cm for seven regions containing twelve intermediate-mass protostars, enabling precise localization, multiplicity assessment, and construction of radio SEDs across multiple spatial scales. By combining radio data with updated infrared-derived bolometric properties, the study identifies jet-like ionized-outflow structures in several sources and resolves complex multiplicity in NGC 2071 and IRAS 21391+5802. A key finding is that IM protostars are radio-faint relative to a naive low-mass extrapolation, consistent with theories predicting lower photoionization and reduced shock emission due to expanded protostellar radii, while shock ionization models can account for much of the SOMA-scale emission. Overall, the expanded SOMA Radio sample (29 sources) provides important constraints on massive protostellar evolution and the connection between ionized gas structures and protostellar properties across $L_{ m bol}$ from $10^{2}$ to $10^{6}\,L_{\odot}$. $S_{ u}$ follows $S_{ u} \propto \nu^{\alpha}$ with scale-dependent spectral indices, reflecting diverse emission mechanisms from ionized jets to HII-region-like components.

Abstract

We present results from Very Large Array (VLA) radio continuum observations of twelve intermediate-mass (IM) protostars, as part of the \textit{SOFIA} Massive Star Formation Survey. Using these observations, we studied their morphology, multiplicity and radio spectral energy distributions (SEDs). Across our target regions, we resolve multiple compact sources and report eight new detections, four of which are entirely new and four that have counterparts at other wavelengths, but are detected here for the first time at radio frequencies. Based on radio morphologies and spectral indices, we assess the nature of the detected sources, highlighting seven that display jet-like structures and spectral indices consistent with ionized jets. Combining our results with the SOMA Radio I and II results, we expand the overall sample to 29 protostars, covering a bolometric luminosity range from $L_{\rm bol}\sim 10^2$ to $10^6\:L_\odot$. These sources help define a potential evolutionary sequence in the radio versus bolometric luminosity diagram. IM protostars have radio luminosities that are lower than expected from a simple power law extrapolation from low-mass protostars. However, this result is consistent with theoretical expectations from protostellar evolution models, which show low levels of photoionization and reduced shock ionization emission due to expanded stellar radii during this phase. Overall our expanded SOMA Radio sample provides new constraints on theoretical models of massive protostellar evolution, especially the connection to ionized gas structures.

Figures (12)

• Figure 1: Images are SOFIA-FORCAST 37 $\mu$m with VLA contours $-$ red: C-band (6 cm); cyan: K-band (1.3 cm) $-$ of the combined radio maps overlaid. The blue dashed squares correspond to the area and the location 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 cyan crosses in NGC 2071 denotes the position of detections only at K-band (cyan contours). The black $\times$ denotes the position of the mm core on the regions with millimeter observations, references as follows: IRAS 22198+6336: Sanchez-Monge2010IRASCore, NGC 2071: Cheng2022, Cepheus E: 2018AA...618A.145O, L1206 A: Beltran2006, IRAS 21391+5802 BIMA2 and BIMA3: 2007AA...468L..33N. The blue circles are the SOMA apertures used by Telkamp_2025 and reported in Table C1. The blue and red arrows represent the direction of a molecular outflow detected toward the region, only for regions with know association with molecular outflows (references are given 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: VLA 3.6 continuum map and contours (white) from Fig. 1 in Carrasco2012 (see more details on their paper regarding the radio maps), of the radio sources IRS 1 and IRS 1Eb in the NGC 2071 region with VLA contours overlaid ( 1.3--cyan, 3.6--white, and 6 cm--red). The contour levels are at [-3, 3, 5, 10, 50, 100] $\times$ 13.7 $\mu$Jy/beam for 1.3 cm (K-band), at [-3, 3, 5, 7, 10, 15, 20, 40, 60] $\times$ 30 $\mu$Jy/beam for 3.6 cm, and at [-3, 3, 5, 15, 60, 120] $\times$ 7.6 $\mu$Jy/beam for 6 cm (C-band). The beam sizes of the 1.3 and 6 cm observations are indicated in the lower left-hand corner.
• Figure 4: VLA 3.6 continuum map and contours (white) from Fig. 1 in Carrasco2012 (see more details on their paper regarding the radio maps), of the radio source IRS 3 in the NGC 2071 region with VLA contours overlaid ( 1.3--cyan, 3.6--white, and 6 cm--red). The contour levels are at [-3, 3, 5, 10, 50, 100] $\times$ 13.7 $\mu$Jy/beam for 1.3 cm (K-band), at [-3, 3, 5, 7, 10, 15, 20, 40, 60] $\times$ 30 $\mu$Jy/beam for 3.6 cm, and at [-3, 3, 5, 15, 60, 120] $\times$ 7.6 $\mu$Jy/beam for 6 cm (C-band). The beam sizes are indicated in the lower left-hand corner.
• Figure 5: VLA 3.6 continuum map and contours (white) from Fig. 1 in Carrasco2012 (see more details on their paper regarding the radio maps), of the radio source VLA 1 in the NGC 2071 region with VLA contours overlaid ( 1.3--cyan, 3.6--white, and 6 cm--red). The contour levels are at [-3, 3, 5, 10, 50, 100] $\times$ 13.7 $\mu$Jy/beam for 1.3 cm (K-band), at [-3, 3, 5, 7, 10, 15, 20, 40, 60] $\times$ 30 $\mu$Jy/beam for 3.6 cm, and at [-3, 3, 5, 15, 60, 120] $\times$ 7.6 $\mu$Jy/beam for 6 cm (C-band). The beam sizes are indicated in the lower left-hand corner.