Low-velocity shocks: signatures of turbulent dissipation in diffuse irradiated gas

Abstract

Context. Large-scale motions in galaxies (supernovae explosions, galaxy collisions, galactic shear etc.) generate turbulence, which allows a fraction of the available kinetic energy to cascade down to small scales before it is dissipated. Aims. We establish and quantify the diagnostics of turbulent dissipation in mildly irradiated diffuse gas in the specific context of shock structures. Methods. We incorporated the basic physics of photon-dominated regions into a state-of-the-art steady-state shock code. We examined the chemical and emission properties of mildly irradiated (G 0 = 1) magnetised shocks in diffuse media (n H = 10 2 to 10 4 cm −3 ) at lowto moderate velocities (from 3 to 40 km s −1 ). Results. The formation of some molecules relies on endoergic reactions. Their abundances in J-type shocks are enhanced by several orders of magnitude for shock velocities as low as 7 km s −1 . Otherwise most chemical properties of J-type shocks vary over less than an order of magnitude between velocities from about 7 to about 30 km s −1 , where H 2 dissociation sets in. C-type shocks display a more gradual molecular enhancement with increasing shock velocity. We quantified the energy flux budget (fluxes of kinetic, radiated and magnetic energies) with emphasis on the main cooling lines of the cold interstellar medium. Their sensitivity to shock velocity is such that it allows observations to constrain statistical distributions of shock velocities. We fitted various probability distribution functions (PDFs) of shock velocities to spectroscopic observations of the galaxy-wide shock in Stephan's Quintet and of a Galactic line of sight which samples diffuse molecular gas in Chamaeleon. In both cases, low velocities bear the greatest statistical weight and the PDF is consistent with a bimodal distribution. In the very low velocity shocks (below 5 km s −1 ), dissipation is due to ion-neutral friction and it powers H 2 low-energy transitions and atomic lines. In moderate velocity shocks (20 km s −1 and above), the dissipation is due to viscous heating and accounts for most of the molecular emission. In our interpretation a significant fraction of the gas in the line of sight is shocked (from 4% to 66%). For example, C + emission may trace shocks in UV irradiated gas where C + is the dominant carbon species. Conclusions. Low-and moderate velocity shocks are important in shaping the chemical composition and excitation state of the interstellar gas. This allows one to probe the statistical distribution of shock velocities in interstellar turbulence.

Mercapto radical (SH) in translucent interstellar clouds

“…In this letter, we present the first detection of SH absorption features in the near-UV in translucent interstellar clouds. The results presented here are well consistent with the predictions of SH production from the models for turbulent dissipation regions (TDRs) and C-type shocks that were recently introduced by Godard et al (2009Godard et al ( , 2014, Lesaffre et al (2013), Neufeld et al (2015).…”

“…We compared this value to the recent modeling study of SH production in TDRs and C-type shocks (Lesaffre et al 2013;Godard et al 2014;Neufeld et al 2015). Following the model predictions presented by Neufeld et al (2015), the SH column density N(SH) in translucent interstellar clouds with typical cloud conditions of 1.0 < A v < 5.0 mag and a total hydrogen volume density n H ∼ 100 cm −3 (see Zhao et al 2015 for the case of OH + ) was extrapolated in the range of 0.6-2.4 × 10 13 cm −2 from the standard TDR model 3 , and 0.8-1.8 × 10 13 cm −2 from the standard C-type shock model 4 .…”

Magnetic diffusivities in 3D radiative chemo-hydrodynamic simulations of protostellar collapse

“…We have performed 3D time-dependent chemical calculations of protostellar collapse, using the time-dependent chemistry code PDS code (Flower & Pineau des Forêts 2015Lesaffre et al 2005Lesaffre et al , 2013 coupled with the RAMSES code (Teyssier 2002). For the technical details, we refer to the Paper I.…”

Radiative and mechanical feedback into the molecular gas in the Large Magellanic Cloud

“…This code simulates one-dimensional stationary J-or C-type shocks and calculates the dynamical and chemical properties of a multi-fluid medium (neutrals and ions), including densities, temperatures, velocities, and chemical abundances. For our study, we use the modified version by Lesaffre et al (2013), which enables us to model A&A proofs: manuscript no. draft_061216 UV-irradiated shocks, and create a grid of models covering the following input parameters: pre-shock density n pre = 10 4 , 10 5 , and 10 6 cm −3 , magnetic field parameter b = 1 (defined as (B/µG)/(n pre /cm −3 ) 1/2 , where B is the strength of the magnetic field transverse to the direction of shock propagation), metallicity Z = 1 Z ⊙ , UV radiation field G ′ UV = 0 and 1 (defined as a scaling factor with respect to the interstellar radiation field by Draine 1978), and shock velocity v s from 4 km s −1 to 20 km s −1 with 2 km s −1 steps.…”