2012A&A...542A..14A

Shock-induced H₂O masers are important magnetic field tracers of very high density gas. Water masers are found in both high- and low-mass star-forming regions, and are a powerful tool for comparing magnetic field morphologies in both mass regimes. We present one of the first magnetic field determinations for the low-mass protostellar core IRAS 16293-2422 at volume densities as high as 10^8–10cm^–3. Our goal is to determine wether the collapsing regime of this source is controlled by magnetic fields or other factors such as turbulence. We used the Very Large Array (VLA) to carry out spectropolarimetric observations of the 22GHz Zeeman emission from H₂O masers. From the Stokes V line profile, we are then able to estimate the magnetic field strength in the dense regions around the protostar. A blend of at least three maser features can be inferred from our relatively high spatial resolution data set (∼0.1''), which is reproduced as a clear non-Gaussian line profile. The emission is very stable in terms of polarization fraction and position angle across the channels. The maser spots are aligned with some components of the complex outflow configuration of IRAS 16293-2422, and are excited in zones of compressed gas produced by shocks. The post-shock particle density is in the range of 1-3x10⁹cm^–3, consistent with typical water-maser pumping densities. Zeeman emission is produced by a very strong line-of-sight magnetic field (B∼113mG). The magnetic field pressure derived from our data is comparable to the ram pressure of the outflow dynamics. This indicates that the magnetic field is energetically important to the dynamical evolution of IRAS 16293-2422.