2018A&A...617A...3L

Aims. The aims of this work are to test the feasibility of observing dust polarization at frequencies lower than 50GHz, which is the optically thinner part of the modified black body spectrum, and to clarify whether or not the polarization mechanism is identical or similar to that for (sub)millimeter observations.
Methods. We performed the new Karl G. Jansky Very Large Array (JVLA) full polarization observations at 40-48GHz (6.3-7.5mm) towards the nearby (d=147±3.4pc) Class 0 young stellar object (YSO) IRAS 16293-2422, and compared these with the previous Submillimeter Array (SMA) observations. We observed the quasar J1407+2827, which is weakly polarized and can be used as a leakage term calibrator for <9GHz observations, to gauge the potential residual polarization leakage after calibration.
Results. We did not detect Stokes Q, U, and V intensities from the observations of J1407+2827, and constrain (3σ) the residual polarization leakage after calibration to be ≤0.3%. Limited by thermal noise, we only detected linear polarization from one of the two binary components of our target source, IRAS 16293-2422 B. The measured polarization percentages range from less than one percent to a few tens of percent. The derived polarization position angles from our observations are in excellent agreement with those detected from the previous observations of the SMA, implying that on the spatial scale we are probing (∼50-1000au), the physical mechanisms for polarizing the continuum emission do not vary significantly over the wavelength range of ∼0.88-7.5mm.
Conclusions. We hypothesize that the observed polarization position angles trace the magnetic field, which converges from large scale to an approximately face-on rotating accretion flow. In this scenario, magnetic field is predominantly poloidal on >100 au scales, and becomes toroidal on smaller scales. However, this interpretation remains uncertain due to the high dust optical depths at the central region of IRAS 16293-2422 B and the uncertain temperature profile. We suggest that dust polarization at wavelengths comparable or longer than 7mm may still trace interstellar magnetic field. Future sensitive observations of dust polarization in the fully optically thin regime will have paramount importance for unambiguously resolving the magnetic field configuration.