Projection-angle effects when “observing” a turbulent magnetized collapsing molecular cloud

Context . Interstellar magnetic fields are thought to play a fundamental role in the evolution of star-forming regions. Polarized thermal dust emission serves as a key probe for understanding the structure of the plane-of-the-sky component of the magnetic field in such regions. However, inclination effects can potentially significantly influence the apparent morphology of the magnetic field and lead to erroneous conclusions regarding its dynamical importance. Aims . Our aim is to investigate how projection-angle effects impact dust polarization maps and to explore new ways for accessing the inclination angle of the mean component of the magnetic field with respect to the plane of the sky. Methods . We post-processed a 3D ideal magnetohydrodynamic simulation of a turbulent collapsing molecular cloud at a central density of 10 5 cm −3 , when the cloud has flattened perpendicular to the mean magnetic field. We produced synthetic dust polarization measurements under various projection angles, ranging from “face-on” (i.e., viewed along the mean magnetic field direction) to “edgeon” (perpendicular to the mean magnetic field direction). Additionally, we used synthetic position-position-velocity (PPV) data cubes from the CO ( J = 1 → 0) transition, presented in a companion paper. Results . The projected magnetic-field morphology is found to be highly affected by the projection angle with the hourglass morphology being clearly visible only for projection angles close to “edge-on”. We find that the direction of the apparent “flow” between successive velocity channels in the simulated PPV data cubes shows an increasing correlation with the synthetic dust polarization observations, as the cloud is observed closer to an “edge-on” orientation. Based on this property, we have developed a new method to probe the inclination angle of the magnetic field relative to the plane of the sky. We validated our approach by generating additional synthetic data (PPV cubes and polarization maps) at an earlier stage of the cloud’s evolution. We demonstrate an excellent quantitative agreement between the derived inclination angle and the true observational angle. We note that our method is relevant only for collapsing clouds.