Implications of the low frequency turn-over in the spectrum of radio knot C in DG Tau

TL;DR

This paper investigates the low frequency turn-over in the radio spectrum of knot C in DG Tau, revealing a stronger magnetic field and shock-related phenomena, providing insights into pre-acceleration electron properties and shock-induced radio emission.

Contribution

It demonstrates that the magnetic field in knot C is at least 10 mG, significantly higher than previous estimates, and links the spectral features to shock processes and pre-acceleration conditions.

Findings

Magnetic field strength is at least 10 mG.

The source has a low volume filling factor for synchrotron emission.

Spectral index suggests electrons are below Fermi acceleration energies.

Abstract

The synchrotron spectrum of radio knot C in the protostellar object DG Tau has a low frequency turn-over. This is used to show that its magnetic field strength is likely to be at least 10 mG, which is roughly two orders of magnitude larger than previously estimated. The earlier, lower value is due to an overestimate of the emission volume together with an omission of the dependence of the minimum magnetic field on the synchrotron spectral index. Since the source is partially resolved, this implies a low volume filling factor for the synchrotron emission. It is argued that the high pressure needed to account for the observations is due to shocks. In addition, cooling of the thermal gas is probably necessary in order to further enhance the magnetic field strength as well as the density of relativistic electrons. It is suggested that the observed spectral index implies that the energy of the radio emitting electrons is below that needed to take part in first order Fermi acceleration. Hence, the radio emission gives insights to the properties of its pre-acceleration phase. Attention is also drawn to the similarities between the properties of radio knot C and the shock induced radio emission in supernovae.