Origin of Molecular Outflow Determined from Thermal Dust Polarization

Abstract

Abstract The observational expectation of polarization measurements of thermal dust radiation is investigated to find information on molecular outflows based on magnetohydrodynamical (MHD) and radiation-transfer simulations. There are two major proposed models for driving of molecular outflows: (1) molecular gas is accelerated by magnetic pressure gradient or magnetocentrifugal wind mechanism before the magnetic field and molecular gas are decoupled, (2) the linear momentum of a highly collimated jet is transferred to the ambient molecular gas. In order to distinguish between these two models, it is crucial to observe the configuration of the magnetic field. An observation of a toroidal magnetic field would be strong evidence that the first model is appropriate. We calculated the polarization distribution of thermal dust radiation due to the alignment of dust grains along the magnetic field using molecular outflow data obtained from two-dimensional axisymmetric MHD simulations. An asymmetric distribution around the $z$-axis is characteristic for magnetic fields composed of both poloidal and toroidal components. We found that the outflow has a low polarization degree compared with the envelope and that the envelope and outflow have different polarization directions (B-vector); i.e., the magnetic field within the envelope is parallel to the global magnetic field lines while the magnetic field of the outflow is perpendicular to it. We, then, demonstrated that the point-symmetric (rather than axisymmetric) distributions of low polarization regions indicate that molecular outflows are likely to be magnetically driven. Observations of this polarization distribution with tools such as ALMA would confirm the origin of the molecular outflow.