Free-Free Emission from Accretion Shocks

Abstract

Shocks associated with the supersonic infall of matter onto a protostellar disk are sources of potentially detectable radio free-free emission: a protostar of mass ≈ 1.5[D/(100 pc)] M☉ is sufficient to generate an 8.4 GHz flux of 100 μJy from its protostellar disk when viewed face-on at a distance D. The free-free emission from an accretion shock is a good diagnostic of the mass of the protostar, which determines the area of the optically thick free-free emission from the shocked surface of the inner disk. Accretion shocks are a possible explanation of recent observations of the source NGC 1333 IRAS 4, in which the measured fluxes at 4.8 and 8.4 GHz exceed the values expected for circumstellar dust emission alone. The accretion shock explanation requires the presence in this source of a protostar of mass 3.5-4 M☉, a value that is consistent with the observed bolometric luminosity only if accretion through the disk is nonsteady and only if the protostellar luminosity lies substantially below its main-sequence value, as indeed is predicted by theoretical models for the evolution of pre-main-sequence stars. We also discuss an alternative model in which the observed free-free emission arises in shocks produced by circumstellar winds impacting dense circumstellar material.