Paper detail

On the Measurement of Vorticity in Astrophysical Fluids

Vorticity is central to the nature of, and dynamical processes in turbulence, including turbulence in astrophysical fluids. The results of \cite{Raymond20a,Raymond20b} on vorticity in the post-shock fluid of the Cygnus Loop supernova remnant are therefore of great interest. We consider the degree to which spectroscopic measurements of an optically-thin line, the most common type of astronomical velocimetry, can yield unambiguous measurements of the vorticity in a fluid. We consider an ideal case of observations in the plane of a flow which may or may not contain vorticity. In one case, the flow possesses vorticity in a direction perpendicular to the plane of observations. In the other case, the flow is irrotational (zero vorticity) by construction. The observationally-deduced vorticity (referred to as the {\em pseudovorticity}) is inferred from spatial differences in the line-of-sight component of velocity, and assumptions of symmetry. My principal result is that in the case of the vortical flow, the pseudovorticity is a reasonable match for the true vorticity. However, and importantly, the pseudovorticity in the case of the irrotational flow field is also nonzero, and comparable in magnitude to that for a vortical flow. The conclusion of this paper is that while astronomical spectroscopic observations may yield a good estimate of the vorticity in a remote fluid, the robustness of such an inference cannot be insured.