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

TL;DR

This study uses high-resolution radio observations to analyze the properties of high-luminosity protostars, revealing their morphologies, spectral indices, and the transition from shock to photoionization in their radio emission.

Contribution

It provides new radio detections and detailed analysis of radio morphologies and spectral indices of high-luminosity protostars, expanding understanding of their ionization processes.

Findings

65% of sources are resolved at least within the synthesized beam

Identification of six sources with jet-like radio morphologies and ionized jets

Radio luminosity increases steeply with bolometric luminosity, indicating a transition in ionization mechanisms

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.