Abstract
ABSTRACT The interstellar medium (ISM) is typically a hostile environment: cold, dilute and irradiated. Nevertheless, it appears very fertile for molecules. The localized heating resulting from turbulence dissipation is a possible channel to produce and excite molecules. However, large-scale simulations cannot resolve the dissipative scales of the ISM. Here, we present two-dimensional small-scale simulations of decaying hydrodynamic turbulence using the chemses code, with fully resolved viscous dissipation, time-dependent heating, cooling, chemistry and excitation of a few rotational levels of H2. We show that molecules are produced and excited in the wake of strong dissipation ridges. We carefully identify shocks and we assess their statistics and contribution to the molecular yields and excitation. We find that the formation of molecules is strongly linked to increased density as a result of shock compression and to the opening of endothermic chemical routes because of higher temperatures. We identify a new channel for molecule production via H2 excitation, illustrated by CH+ yields in our simulations. Despite low temperatures and the absence of magnetic fields (favouring CH+ production through ion-neutral velocity drifts), the excitation of the first few rotational levels of H2 shrinks the energy gap to form CH+. The present study demonstrates how dissipative chemistry can be modelled by statistical collections of one-dimensional steady-state shocks. Thus, the excitation of higher J levels of H2 is likely to be a direct signature of turbulence dissipation, and an indirect probe for molecule formation. We hope these results will help to bring new tools and ideas for the interpretation of current observations of H2 rotational lines carried out using the Stratospheric Observatory for Infrared Astronomy (SOFIA), and pave the way for a better understanding of the high-resolution mapping of H2 emission by future instruments, such as theJames Webb Space Telescope and the Space Infrared Telescope for Cosmology and Astrophysics.
Highlights
The diffuse interstellar medium (ISM) is cold and dilute, it appears to be quite fertile in the production of molecules, even when the formation of molecules needs adverse dissociating radiation orC The Author(s) 2020
Note that we do not fit for the viscous coefficient, as we keep it fixed to the input value used in the hydrodynamic computation; we have shown that the resolution is high enough so that the extra viscosity due to the numerical scheme is negligible
The present detailed simulations constitute a unique proof of concept to carefully examine both the chemistry and the dissipation at the same time
Summary
The diffuse interstellar medium (ISM) is cold and dilute, it appears to be quite fertile in the production of molecules, even when the formation of molecules needs adverse dissociating radiation orC The Author(s) 2020. Molecular excitation in decaying turbulence 817 hydrogenation of the O+ cation, provided the ultraviolet (UV) irradiation field is not too strong and that the H2 molecule remains unhindered. This was investigated in detail by Levrier et al (2012), who found that, for the standard irradiation field and for the densities around 100 cm−3 in the diffuse ISM, their models underpredict the observed line fluxes and column densities of molecules. Molecules are too fragile for the diffuse medium irradiation and additional physical processes are needed to increase their abundances, such as ion-neutral drift (as in C-type shocks; see Flower, Pineau des Forets & Hartquist 1985), turbulent diffusion (Lesaffre, Gerin & Hennebelle 2007) or turbulent dissipation (Godard, Falgarone & Pineau Des Forets 2009)
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