Measuring Vorticity in Astrophysical Turbulence

Turbulence is a general properties of fluids in nature, and is observed to be present in many space and astrophysical fluids . Vorticity is a crucial aspect of turbulence. Measurement of vorticity in a turbulent astrophysical fluid would be very important, but is difficult. Most astronomical measurements of flow velocity rely on Doppler-shifted spectral lines, which provide a path-integrated measurement of one component of the velocity. Raymond et al (ApJ 903,2,2020) report an estimate of vorticity from spatially-varying Doppler shifts in the Cygnus Loop supernova remnant. I further consider this matter by calculating the spectrum of an idealized vortex for which the vorticity is known. The estimate of the vorticity from the "observed" spectrum is a good estimate of the true value, but the inferred vorticity from the spectrum of a model irrotational flow is comparable. Position-dependent Doppler shifts may give accurate estimates of the vorticity in astrophysical turbulence, but caution with this conclusion would seem warranted.