Shocks in the warm neutral medium II -- Origin of neutral carbon at high pressure

Abstract

Aims: Ultraviolet (UV) lines of neutral carbon observed in absorption in the local diffuse interstellar medium (ISM) have long revealed that a substantial fraction of the mass of the gas lies at a thermal pressure one to three orders of magnitude above that of the bulk of the ISM. In this paper, we propose that this enigmatic component originates from shocks propagating at intermediate ($V_S>30$ km s${}^{-1}$) and high velocities ($V_S⩾100$ km s${}^{-1}$) in the Warm Neutral Medium (WNM). Methods: Shock waves irradiated by the standard interstellar radiation field (ISRF) are modelled using the Paris-Durham shock code designed to follow the dynamical, thermal, and chemical evolutions of shocks with velocities up to 500 km s${}^{-1}$. Each observed line of sight is decomposed into a high pressure and a low pressure components. The column density of carbon at high pressure is confronted to the model predictions to derive the number of shocks along the line of sight and their total dissipation rate. Results: Phase transition shocks spontaneously lead to the presence of high pressure gas in the diffuse ISM and are found to naturally produce neutral carbon with excitation conditions and linewidths in remarkable agreement with the observations. The amounts of neutral carbon at high pressure detected over a sample of 89 lines of sight imply a dissipation rate of mechanical energy with a median of $\sim 3x10^{-25}$ erg cm${}^{-3}$ s${}^{-1}$ and a dispersion of about a factor of three. This distribution of the dissipation rate weakly depends on the detailed characteristics of shocks as long as they propagate at velocities between 30 and 200 kms s${}^{-1}$ in a medium with a preshock density $n_H^0\ge 0.3$ cm s${}^{-3}$ and a transverse magnetic field $B_0⩽3$ $\mu$G. We not only show that this solution is consistent with a scenario of shocks driven by supernovae remnants (SNR) but also that this scenario is, in fact, unavoidable. Any line of sight in the observational sample is bound to intercept SNRs, mostly distributed in the spiral arms of the Milky Way, and expanding in the diffuse ionized and neutral phases of the Galaxy. Surprisingly, the range of dissipation rate derived here, in events that probably drive turbulence in the WNM, is found to be comparable to the distribution of the kinetic energy transfer rate of the turbulent cascade derived from the observations of CO in the Cold Neutral Medium (CNM). Conclusions: This work reveals a possible direct tracer of the mechanisms by which mechanical energy is injected in the ISM. It also suggests that a still unknown connection exists between the amount of energy dissipated during the injection process in the WNM and that used to feed interstellar turbulence and the turbulent cascade observed in the CNM.

Astronomy and Astrophysics - A&A, In press

References in corpus (29)

• The Statistics of Supersonic Isothermal Turbulence
• Momentum Injection by Supernovae in the Interstellar Medium
• Physical properties of molecular clouds for the entire Milky Way disk
• CHIANTI -- an atomic database for emission lines -- Paper XVI: Version 10, further extensions
• Ultraviolet Survey of CO and H_2 in Diffuse Molecular Clouds: The Reflection of Two Photochemistry Regimes in Abundance Relationships
• Herschel Survey of Galactic OH+, H2O+, and H3O+: Probing the Molecular Hydrogen Fraction and Cosmic-Ray Ionization Rate
• CHIANTI - an atomic database for emission lines - Paper XV: Version 9, improvements for the X-ray satellite lines
• SiO line emission from C-type shock waves : interstellar jets and outflows
• Radiative recombination data for modelling dynamic finite-density plasmas
• Models of turbulent dissipation regions in the diffuse interstellar medium
• Chemical probes of turbulence in the diffuse medium: the TDR model
• Models of irradiated molecular shocks
• Evolution of the local spiral structure of the Milky Way revealed by open clusters
• A Far Ultraviolet Spectroscopic Explorer Survey of Interstellar Molecular Hydrogen in the Galactic Disk
• 3D chemical structure of diffuse turbulent ISM. I. Statistics of the HI-to-H${}_2$ transition
• Thermal and turbulent properties of the Warm Neutral Medium in the solar neighborhood
• Large-scale turbulent driving regulates star formation in high-redshift gas-rich galaxies
• Turbulence and Particle Acceleration in Radiative Shock Waves in the Cygnus Loop II: Development of Postshock Turbulence
• Self-generated ultraviolet radiation in molecular shock waves II. CH+ and the interpretation of emission from shock ensembles
• Water emission from the high-mass star-forming region IRAS 17233-3606. High water abundances at high velocities
• Thermal instability revisited
• Physical Conditions in Shocked Interstellar Gas Interacting with the Supernova Remnant IC 443
• The evolution of HCO${}^{+}$ in molecular clouds using a novel chemical post-processing algorithm
• The Observed Age Gradient in the Milky Way -- as a Test for theories of spiral arm structure
• Radiative and mechanical energies in galaxies I. Contributions of molecular shocks and PDRs in 3C 326 N
• Interactions of a shock with a molecular cloud at various stages of its evolution due to thermal instability and gravity
• Cooling and Instabilities in Colliding Flows
• Shocks in the warm neutral medium I -- Theoretical model
• Thermal Pressures in the Interstellar Medium away from Stellar Environments

Cited by in corpus (2)

• Shocks in the warm neutral medium I -- Theoretical model
• Extended atomic carbon around molecular clouds