Multi-scale Dust Polarization and Spiral-like Stokes-I Residual in the Class I Protostellar System TMC-1A

We have observed the Class I protostar TMC-1A in the Taurus molecular cloud using the Submillimeter Array (SMA) and the Atacama Large Millimeter/submillimeter Array (ALMA) in the linearly polarized 1.3 mm continuum emission at angular resolutions of similar to 3 '' and similar to 0.3 '', respectively. The ALMA observations also include CO, (CO)-C-13, and (CO)-O-18 J = 2-1 spectral lines. The SMA observations trace magnetic fields on the 1000 au scale, the directions of which are neither parallel nor perpendicular to the outflow direction. Applying the Davis-Chandrasekhar-Fermi method to the SMA polarization angle dispersion, we estimate a field strength in the TMC-1A envelope of 1-5 mG. It is consistent with the field strength needed to reduce the radial infall velocity to the observed value, which is substantially less than the local freefall velocity. The ALMA polarization observations consist of two distinct components-a central component and a north/south component. The central component shows polarization directions in the disk minor axis to be azimuthal, suggesting dust self-scattering in the TMC-1A disk. The north/south component is located along the outflow axis and the polarization directions are aligned with the outflow direction. We discuss possible origins of this polarization structure, including grain alignment by a toroidal magnetic field and mechanical alignment by the gaseous outflow. In addition, we discover a spiral-like residual in the total intensity (Stokes I) for the first time. The (CO)-O-18 emission suggests that material in the spiral-like structure is infalling at a speed that is 20% of the local Keplerian speed.

Automated summary

Observations of the Class I protostar TMC-1A in the Taurus molecular cloud reveal magnetic fields on the 1000 au scale that are neither parallel nor perpendicular to the outflow direction. ALMA polarization observations show two distinct components, with consistent polarization directions indicating possible alignment by a toroidal magnetic field or gaseous outflow. The overall polarization structure suggests systematic organization possibly due to these different mechanisms.