Disk Wind Feedback from High-mass Protostars. IV. Shock-Ionized Jets

TL;DR

This study models shock heating and free-free radio emission in outflows of massive protostars, revealing ionization levels, radio luminosities, and variability consistent with observations, and discusses the roles of shock ionization and photoionization.

Contribution

It provides detailed 3D MHD simulations of shock-ionized jets in massive star formation, linking shock physics to observable radio properties and variability.

Findings

Shock heating reaches ~10^7 K and causes near-complete ionization at outflow-envelope interfaces.

Model radio luminosities are similar to low/intermediate-mass protostars but 10-100 times less for massive ones.

Radio flux variability of ~5% over 10 years is consistent with observations of hyper-compact HII regions.

Abstract

Massive protostars launch accretion-powered, magnetically-collimated outflows, which play crucial roles in the dynamics and diagnostics of the star formation process. Here we calculate the shock heating and resulting free-free radio emission in numerical models of outflows of massive star formation within the framework of the Turbulent Core Accretion model. We post-process 3D magneto-hydrodynamic simulation snapshots of a protostellar disk wind interacts with an infalling core envelope, and calculate shock temperatures, ionization fractions, and radio free-free emission. We find heating up to ~10^7 K and near complete ionization in shocks at the interface between the outflow cavity and infalling envelope. However, line-of-sight averaged ionization fractions peak around ~10%, in agreement with values reported from observations of massive protostar G35.20-0.74N. By calculating radio continuum fluxes and spectra, we compare our models with observed samples of massive protostars. We find our fiducial models produce radio luminosities similar to those seen from low and intermediate-mass protostars that are thought to be powered by shock ionization. Comparing to more massive protostars, we find our model radio luminosities are ~10 to 100 times less luminous. We discuss how this apparent discrepancy either reflects aspects of our modeling related to the treatment of cooling of the post-shock gas or a dominant contribution in the observed systems from photoionization. Finally, our models exhibit 10-year radio flux variability of ~5%, especially in the inner 1000 au region, comparable to observed levels in some hyper-compact HII regions.