SIMBAD references

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.