2025ApJ...987....2R

We assess the diagnostic for shock wave ram pressure, $\rho {V}_{s}^{2}\,\propto \,{I}_{[{\rm{O}}\,\mathrm{III}]}\,{V}_{\mathrm{Dopp}}$, and apply it to high-resolution multifiber spectra of the Cygnus Loop. We find that, while there are many caveats, and while high-resolution profiles are important for controlling some sources of error, the diagnostic is reliable if one selects positions with simple line profiles that are offset away from zero velocity. The Cygnus Loop shows a range of conditions from ≈400 km s^–1 nonradiative shocks that produce X-rays to ≈100 km s^–1 radiative shocks that produce bright optical emission. We find a range of ram pressures in agreement with previous estimates from [O III] and from X-ray measurements. The ram pressure is also consistent with recent analysis of the evolutionary stage of the Cygnus Loop.