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Signatures of fast and slow magnetohydrodynamic shocks in turbulent molecular clouds

The character of star formation is intimately related to the supersonic magnetohydrodynamic (MHD) turbulent dynamics of the molecular clouds in which stars form. A significant amount of the turbulent energy dissipates in low velocity shocks. Fast and slow MHD shocks differ in how they compress and heat the molecular gas, and so their radiative signatures reveal distinct physical conditions. We use a two-fluid model to compare one-dimensional fast and slow MHD shocks propagating at low speeds (a few km/s). Fast shocks are magnetically driven, forcing ion species to stream through the neutral gas ahead of the shock front. This magnetic precursor heats the gas sufficiently to create a large, warm transition zone where all the fluid variables smoothly change in the shock front. In contrast, slow shocks are driven by gas pressure, and neutral species collide with ion species in a thin hot slab that closely resembles an ordinary gas dynamic shock. We consider shocks at velocities $v_s = 2$-$4$ km/s and preshock Hydrogen nuclei densities $n_\mathrm{H} = 10^2$-$10^4$ cm$^{-3}$. We include a simple oxygen chemistry and cooling by CO, H$_2$ and H$_2$O. CO rotational lines above $J = 6\to 5$ are more strongly excited in slow shocks. These slow shock signatures may have already been observed in infrared dark clouds in the Milky Way.