On the Measurement of Vorticity in Astrophysical Fluids

Abstract

Vorticity is central to the nature of, and dynamical processes in turbulence, including turbulence in astrophysical fluids. The results of Raymond et al. on vorticity in the post-shock fluid of the Cygnus Loop supernova remnant are therefore of great interest. I 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. I consider an ideal case of observations in the plane of a flow which possesses vorticity in a direction perpendicular to the plane of observations. The observationally deduced vorticity (referred to as the pseudovorticity) is a reasonable match for the true vorticity. However, the pseudovorticity of an irrotational model flow is also nonzero, and comparable in magnitude to that for a vortical flow. Astronomical spectroscopic observations may yield a good estimate of the vorticity, but its robustness cannot be insured.