Models of turbulent dissipation regions in the diffuse interstellar medium

Abstract

Aims. Supersonic turbulence is a large reservoir of suprathermal energy in the interstellar medium. Its dissipation, because it is intermittent in space and time, can deeply modify the chemistry of the gas. This is clearly seen in the framework of shock chemistry. Intense turbulent dissipation also occurs in regions of large velocity shears, sharing with shocks the property of intermittency. Whether these bursts of dissipation, short-lived and localized, have a measurable impact on molecular abundances in the diffuse medium, and how the chemical enrichment they drive compares to observations, are the questions we address here. Methods. We further explore a hybrid method to compute the chemical and thermal evolution of a magnetized dissipative structure, under the energetic constraints provided by the observed properties of turbulence in the cold neutral medium. For the first time, we model a random line of sight by taking into account the relative duration of the bursts with respect to the thermal and chemical relaxation timescales of the gas. The key parameter is the turbulent rate of strain a due to the ambient turbulence. With the gas density, it controls the size of the dissipative structures, therefore the strength of the burst. It also sets the relative importance of viscous dissipation and ion-neutral friction in the gas heating and chemical enrichment. Results. For a large range of rates of strain and densities, the models of turbulent dissipation regions (TDR) reproduce the CH+ column densities observed in the diffuse medium and their correlation with highly excited H 2 . They do so without producing an excess of CH. As a natural consequence, they reproduce the abundance ratios of HCO + /OH and HCO + /H 2 O, and their dynamic range of about one order of magnitude observed in diffuse gas. Large C 2 H and CO abundances, also related to those of HCO + , are another outcome of the TDR models that compare well with observed values. Neutral carbon exceeds the abundance expected at ionization equilibrium, in agreement with fine-structure line observations. The abundances and column densities computed for CN, HCN and HNC are one order of magnitude above PDR model predictions, although still significantly smaller than observed values. The dependence of our results on the rate of strain and density reveals that the chemical enhancements are in better agreement with observations if the dissipation is dominated by ion-neutral friction, involving shear structures of thickness ∼100 AU.

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).…”

The chemistry of ions in the Orion Bar I. – CH+, SH+, and CF+

“…Tielens et al (1993) have investigated that shocks do not contribute to the chemistry of the Orion Bar; therefore, we consider this scenario unlikely. Another scenario for CH + formation that has been successful in reproducing CH + abundances for the diffuse interstellar medium is the dissipation of turbulence (Godard et al 2009(Godard et al , 2012. Though the CH + 1-0 and 2−1 transitions have broader line widths than most dense gas tracers in the Orion Bar, most of the lines detected in the Orion Bar are narrow (2-3 km s −1 ); therefore, we find it unlikely that turbulence plays a role in the chemistry of species detected in the Orion Bar.…”

“…For low-density diffuse interstellar clouds, C-shocks (Pineau des Forêts et al 1986) and turbulent dissipation (Godard et al 2009) have been proposed and confirmed by Falgarone et al (2010a,b) and Godard et al (2012). Alternatively, in denser regions with strong far-ultraviolet (FUV) radiation fields, the internal energy available in the vibrationally excited H 2 molecules A&A 550, A96 (2013) has been proposed to help overcome the large activation barrier (Sternberg & Dalgarno 1995;Agundez et al 2010).…”

Intermittency of interstellar turbulence: parsec-scale coherent structure of intense, velocity shear

“…These pure velocity-structures do not follow those of dense gas: they tend to be parallel to the magnetic field orientation, they are associated with gas warmer (T kin > 25 K) than the bulk of the gas, and they bear chemical signatures of a warm chemistry that is not driven by UV photons (Falgarone et al 2006;Godard et al 2009). In one of these E-CVI structures, Plateau de Bure Interferometre (PdBI) observations disclose several substructures of intense velocity-shear at scales as small as 6 milliparsec (mpc) (Falgarone et al 2009, hereafter FPH09).…”