VLA Observations of DG Tau's Radio Jet: A highly collimated thermal outflow

TL;DR

This study presents high-resolution VLA observations of DG Tau's jet, confirming its thermal bremsstrahlung emission, measuring its properties, and detecting a new radio knot likely caused by post-shock gas expansion.

Contribution

First high-resolution VLA polarization and spectral observations of DG Tau's jet confirming thermal emission and identifying a new radio knot.

Findings

Jet is unpolarized with a spectral index of +0.46, consistent with free-free emission.

Radio jet size is 42 AU, matching optical and X-ray observations.

Detected a new radio knot 7" from the jet base, likely due to shock expansion.

Abstract

The active young protostar DG Tau has an extended jet that has been well studied at radio, optical, and X-ray wavelengths. We report sensitive new VLA full-polarization observations of the core and jet between 5 GHz and 8 GHz. Our high angular resolution observation at 8 GHz clearly shows an unpolarized inner jet with a size 42 AU (0.35") extending along a position angle similar to the optical-X ray outer jet. Using our nearly coeval 2012 VLA observations, we find a spectral-index=+0.46+/-0.05, which combined with the lack of polarization, is consistent with bremsstrahlung (free-free) emission, with no evidence for a non-thermal coronal component. By identifying the end of the radio jet as the optical depth unity surface, and calculating the resulting emission measure, we find our radio results are in agreement with previous optical line studies of electron density and consequent mass-loss rate. We also detect a weak radio knot at 5 GHz located 7" from the base of the jet, coincident with the inner radio knot detected by Rodriguez et al. (2012) in 2009 but at lower surface brightness. We interpret this as due to expansion of post-shock ionized gas in the three years between observations.