Models of irradiated molecular shocks

Abstract

Context. The recent discovery of excited molecules in starburst galaxies observed with ALMA and the Herschel space telescope has highlighted the necessity to understand the relative contributions of radiative and mechanical energies in the formation of molecular lines and explore the conundrum of turbulent gas bred in the wake of galactic outflows. Aims. The goal of the paper is to present a detailed study of the propagation of low velocity (5 to 25 km s −1 ) stationary molecular shocks in environments illuminated by an external ultraviolet (UV) radiation field. In particular, we intend to show how the structure, dynamics, energetics, and chemical properties of shocks are modified by UV photons and to estimate how efficiently shocks can produce line emission. Methods. We implemented several key physico-chemical processes in the Paris-Durham shock code to improve the treatment of the radiative transfer and its impact on dust and gas particles. We propose a new integration algorithm to find the steady-state solutions of magnetohydrodynamics equations in a range of parameters in which the fluid evolves from a supersonic to a subsonic regime. We explored the resulting code over a wide range of physical conditions, which encompass diffuse interstellar clouds and hot and dense photon-dominated regions (PDR). Results. We find that C-type shock conditions cease to exist as soon as G 0 > 0.2 (n H / cm −3 ) 1/2 . Such conditions trigger the emergence of another category of stationary solutions, called C*-type and CJ-type shocks, in which the shocked gas is momentarily subsonic along its trajectory. These solutions are shown to be unique for a given set of physical conditions and correspond to dissipative structures in which the gas is heated up to temperatures comprised between those found in C-type and adiabatic J-type shocks. High temperatures combined with the ambient UV field favour the production or excitation of a few molecular species to the detriment of others, hence leading to specific spectroscopic tracers such as rovibrational lines of H 2 and rotational lines of CH + . Unexpectedly, the rotational lines of CH + may carry as much as several percent of the shock kinetic energy. Conclusions. Ultraviolet photons are found to strongly modify the way the mechanical energy of interstellar shocks is processed and radiated away. In spite of what intuition dictates, a strong external UV radiation field boosts the efficiency of low velocity interstellar shocks in the production of several molecular lines which become evident tracers of turbulent dissipation.

Molecule formation in dust-poor irradiated jets

“…0.01 µm and 0.3 µm, respectively (Mathis et al 1977). Following Godard et al (2019) (see their Appendix B) and for simplicity, absorption of UV photons by grains is calculated assuming the absorption coefficient of single size spherical graphite grains of radius a g = 0.02 µm derived by Draine & Lee (1984) and Laor & Draine (1993), where a g ≡ < r 2 c > is calculated from the mean square radius of the grains.…”

“…Time-dependent populations of the first 50 ro-vibrational levels of H 2 in the electronic ground state are computed, including de-excitation by collision with H, He, H 2 and H + , UV radiative pumping of electronic lines followed by fluorescence (Godard et al 2019), and excitation of H 2 due to formation on grain surfaces. For H 2 formation on grain surface, we assume that the levels are populated following a Boltzmann distribution at a temperature of 1/3 of the dissociation energy of Table 1.…”

“…The chemical network is constructed from Flower & Pineau des Forêts (2015) and Godard et al (2019). It includes 140 species and about a thousand gas-phase reactions.…”

“…Other reactions related to H 2 formation and added to the original network are reported in Table B.2. Godard et al (2019). Photodetachement rate of H − is given for a 4000 K black-body radiation field diluted by a factor W. Fig.…”

“…Red lines and red dots are computed or measured rate coefficients taken from references indicated in Table B.1, and blue are the Arrhenius fits used in this work. Godard et al (2019). All reaction but photoionization are from KIDA database.…”