SOMA: The SOFIA Massive Radio Survey. III. Radio Emission from Intermediate-mass Protostars

Journal article, 2026

We present results from Karl G. Jansky Very Large Array radio-continuum observations of 12 intermediate-mass (IM) protostars, as part of the 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 jetlike 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 Lbol ∼ 10² to 10⁶ L⊙. 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.