Inclination Effect on Observational Identification of Outflow Rotation in Magnetohydrodynamics Simulations

Abstract We investigate the observational signatures of outflow rotation in protostellar systems using magnetohydrodynamics simulations of protostellar evolution with radiative transfer and synthetic observation. The velocity gradient perpendicular to the outflow axis indicates outflow rotation. The rotation signature is clearly seen in the moment 1 map and a position–velocity (PV) diagram across an outflow lobe made from our model with an inclination angle of i ≳ 85°, as in observational studies of protostellar outflows. Velocity projection with lower inclinations distorts the moment 1 map because the outflow vertical (propagation) velocity contributes more to the line-of-sight velocity, leading to an incorrect outflow axis direction. The PV diagram adopting the incorrect outflow axis shows no clear velocity gradient. These effects may prevent us from identifying outflow rotation. Our analysis implies that rotational signatures can be obscured in ∼2/3 to ∼4/5 of the total outflow population ( i < 70°–80°), regardless of the evolutionary stage. Complicated structures in observed outflows make it difficult to determine the outflow axis, which may result in the apparent nondetection of outflow rotation.