Chemical probes of turbulence in the diffuse medium: the TDR model

Context. Tens of light hydrides and small molecules have now been detected over several hundreds sight lines sampling the diffuse interstellar medium (ISM) in both the Solar neighbourhood and the inner Galactic disk. They provide unprecedented statistics on the first steps of chemistry in the diffuse gas. Aims. These new data confirm the limitations of the traditional chemical pathways driven by the UV photons and the cosmic rays (CR) and the need for additional energy sources, such as turbulent dissipation, to open highly endoenergetic formation routes. The goal of the present paper is to further investigate the link between specific species and the properties of the turbulent cascade in particular its space-time intermittency. Methods. We have analysed ten different atomic and molecular species in the framework of the updated model of turbulent dissipation regions (TDR). We study the influence on the abundances of these species of parameters specific to chemistry (density, UV field, and CR ionisation rate) and those linked to turbulence (the average turbulent dissipation rate, the dissipation timescale, and the ion neutral velocity drift in the regions of dissipation). Results. The most sensitive tracers of turbulent dissipation are the abundances of CH+ and SH+, and the column densities of the J = 3, 4, 5 rotational levels of H2 . The abundances of CO, HCO+, and the intensity of the 158 $\mu$m [CII] emission line are significantly enhanced by turbulent dissipation. The vast diversity of chemical pathways allows the independent determinations of free parameters never estimated before: an upper limit to the average turbulent dissipation rate, $\overline{\varepsilon}$ < 10$^{-23}$ erg cm$^{-3}$ s$^{-1}$ for $n_H$=20 cm$^{-3}$, from the CH+ abundance; an upper limit to the ion-neutral velocity drift, $u_{in}$ < 3.5 km s$^{-1}$, from the SH+ to CH+ abundance ratio; and a range of dissipation timescales, 100 < $\tau_V$ < 1000 yr, from the CO to HCO+ abundance ratio. For the first time, we reproduce the large abundances of CO observed on diffuse lines of sight, and we show that CO may be abundant even in regions with UV-shieldings as low as $5 \times 10^{-3}$ mag. The best range of parameters also reproduces the abundance ratios of OH, C2H, and H2O to HCO+ and are consistent with the known properties of the turbulent cascade in the Galactic diffuse ISM. Conclusions. Our results disclose an unexpected link between the dissipation of turbulence and the emergence of molecular richness in the diffuse ISM. Some species, such as CH+ or SH+, turn out to be unique tracers of the energy trail in the ISM. In spite of some degeneracy, the properties of the turbulent cascade, down to dissipation, can be captured through specific molecular abundances

Temporal evolution of magnetic molecular shocks I. Moving grid simulations

We present time-dependent 1D simulations of multifluid magnetic shocks with chemistry resolved down to the mean free path. They are obtained with an adaptive moving grid implemented with an implicit scheme. We examine a broad range of parameters relevant to conditions in dense molecular clouds, with preshock densities between 10^3 and 10^5 cm-3, velocities between 10 and 40 km/s, and three different scalings for the transverse magnetic field: B=0,0.1,1 \mu G \sqrt{n.cm3}. We first use this study to validate the results of Chi\`eze, Pineau des For\^ets & Flower (1998), in particular the long delays necessary to obtain steady C-type shocks, and we provide evolutionary time-scales for a much greater range of parameters. We also present the first time-dependent models of dissociative shocks with a magnetic precursor, including the first models of stationary CJ shocks in molecular conditions. We find that the maximum speed for steady C-type shocks is reached before the occurrence of a sonic point in the neutral fluid, unlike previously thought. As a result, the maximum speed for C-shocks is lower than previously believed. Finally, we find a large amplitude bouncing instability in J-type fronts near the H2 dissociation limit (u ~ 25-30 km/s), driven by H2 dissociation/reformation. At higher speeds, we find an oscillatory behaviour of short period and small amplitude linked to collisional ionisation of H. Both instabilities are suppressed after some time when a magnetic field is present. In a companion paper, we use the present simulations to validate a new semi-analytical construction method for young low-velocity magnetic shocks based on truncated steady-state models.Comment: A&A in pres

Dissipative structures of diffuse molecular gas: I - Broad HCO+^+(1-0) emission

Results: We report the detection of broad HCO+(1-0) lines (10 mK < T < 0.5 K). The interpretation of 10 of the HCO+ velocity components is conducted in conjunction with that of the associated optically thin 13CO emission. The derived HCO+ column densities span a broad range, $10^{11}< N(HCO+)/\Delta v <4 \times 10^{12} \rm cm^2/(km/s^{-1}$, and the inferred HCO+ abundances, $2 \times 10^{-10}<X(HCO+) < 10^{-8}$, are more than one order of magnitude above those produced by steady-state chemistry in gas weakly shielded from UV photons, even at large densities. We compare our results with the predictions of non-equilibrium chemistry, swiftly triggered in bursts of turbulence dissipation and followed by a slow thermal and chemical relaxation phase, assumed isobaric. The set of values derived from the observations, i.e. large HCO+ abundances, temperatures in the range of 100--200 K and densities in the range 100--1000 cm3, unambiguously belongs to the relaxation phase. The kinematic properties of the gas suggest in turn that the observed HCO+ line emission results from a space-time average in the beam of the whole cycle followed by the gas and that the chemical enrichment is made at the expense of the non-thermal energy. Last, we show that the "warm chemistry" signature (i.e large abundances of HCO+, CH+, H20 and OH) acquired by the gas within a few hundred years, the duration of the impulsive chemical enrichment, is kept over more than thousand years. During the relaxation phase, the \wat/OH abundance ratio stays close to the value measured in diffuse gas by the SWAS satellite, while the OH/HCO+ ratio increases by more than one order of magnitude.Comment: 14 page

H_2 formation and excitation in the Stephan's Quintet galaxy-wide collision

Context. The Spitzer Space Telescope has detected a powerful (L_(H_2) ~ 10^(41) erg s^(-1)) mid-infrared H_2 emission towards the galaxy-wide collision in the Stephan's Quintet (henceforth SQ) galaxy group. This discovery was followed by the detection of more distant H_2-luminous extragalactic sources, with almost no spectroscopic signatures of star formation. These observations place molecular gas in a new context where one has to describe its role as a cooling agent of energetic phases of galaxy evolution. Aims. The SQ postshock medium is observed to be multiphase, with H_2 gas coexisting with a hot (~5 × 10^6 K), X-ray emitting plasma. The surface brightness of H_2 lines exceeds that of the X-rays and the 0-0 S(1)H_2 linewidth is ~900 km  s^(-1), of the order of the collision velocity. These observations raise three questions we propose to answer: (i) why is H_2 present in the postshock gas? (ii) How can we account for the H_2 excitation? (iii) Why is H_2 a dominant coolant? Methods. We consider the collision of two flows of multiphase dusty gas. Our model quantifies the gas cooling, dust destruction, H_2 formation and excitation in the postshock medium. Results. (i) The shock velocity, the post-shock temperature and the gas cooling timescale depend on the preshock gas density. The collision velocity is the shock velocity in the low density volume-filling intercloud gas. This produces a ~5 × 10^6 K, dust-free, X-ray emitting plasma. The shock velocity is lower in clouds. We show that gas heated to temperatures of less than 10^6 K cools, keeps its dust content and becomes H_2 within the SQ collision age (~5 × 10^6 years). (ii) Since the bulk kinetic energy of the H_2 gas is the dominant energy reservoir, we consider that the H_2 emission is powered by the dissipation of kinetic turbulent energy. We model this dissipation with non-dissociative MHD shocks and show that the H_2 excitation can be reproduced by a combination of low velocities shocks (5-20 km s^(-1)) within dense (n_H > 10^3 cm^(-3)) H_2 gas. (iii) An efficient transfer of the bulk kinetic energy to turbulent motion of much lower velocities within molecular gas is required to make H_2 a dominant coolant of the postshock gas. We argue that this transfer is mediated by the dynamic interaction between gas phases and the thermal instability of the cooling gas. We quantify the mass and energy cycling between gas phases required to balance the dissipation of energy through the H_2 emission lines. Conclusions. This study provides a physical framework to interpret H_2 emission from H_2-luminous galaxies. It highlights the role that H_2 formation and cooling play in dissipating mechanical energy released in galaxy collisions. This physical framework is of general relevance for the interpretation of observational signatures, in particular H_2 emission, of mechanical energy dissipation in multiphase gas

Shocks in dense clouds. IV. Effects of grain-grain processing on molecular line emission

Grain-grain processing has been shown to be an indispensable ingredient of shock modelling in high density environments. For densities higher than \sim10^5 cm-3, shattering becomes a self-enhanced process that imposes severe chemical and dynamical consequences on the shock characteristics. Shattering is accompanied by the vaporization of grains, which can directly release SiO to the gas phase. Given that SiO rotational line radiation is used as a major tracer of shocks in dense clouds, it is crucial to understand the influence of vaporization on SiO line emission. We have developed a recipe for implementing the effects of shattering and vaporization into a 2-fluid shock model, resulting in a reduction of computation time by a factor \sim100 compared to a multi-fluid modelling approach. This implementation was combined with an LVG-based modelling of molecular line radiation transport. Using this model we calculated grids of shock models to explore the consequences of different dust-processing scenarios. Grain-grain processing is shown to have a strong influence on C-type shocks for a broad range of magnetic fields: they become hotter and thinner. The reduction in column density of shocked gas lowers the intensity of molecular lines, at the same time as higher peak temperatures increase the intensity of highly excited transitions compared to shocks without grain-grain processing. For OH the net effect is an increase in line intensities, while for CO and H2O it is the contrary. The intensity of H2 emission is decreased in low transitions and increased for highly excited lines. For all molecules, the highly excited lines become sensitive to the value of the magnetic field. Although vaporization increases the intensity of SiO rotational lines, this effect is weakened by the reduced shock width. The release of SiO early in the hot shock changes the excitation characteristics of SiO radiation.Comment: Published in Astronomy and Astrophysics (2013). 26 pages, 16 figures, 14 table

UV-driven chemistry in simulations of the interstellar medium. I. Post-processed chemistry with the Meudon PDR code

Our main purpose is to estimate the effect of assuming uniform density on the line-of-sight in PDR chemistry models, compared to a more realistic distribution for which total gas densities may well vary by several orders of magnitude. A secondary goal of this paper is to estimate the amount of molecular hydrogen which is not properly traced by the CO (J = 1 -> 0) line, the so-called "dark molecular gas". We use results from a magnetohydrodynamical (MHD) simulation as a model for the density structures found in a turbulent diffuse ISM with no star-formation activity. The Meudon PDR code is then applied to a number of lines of sight through this model, to derive their chemical structures. It is found that, compared to the uniform density assumption, maximal chemical abundances for H2, CO, CH and CN are increased by a factor 2 to 4 when taking into account density fluctuations on the line of sight. The correlations between column densities of CO, CH and CN with respect to those of H2 are also found to be in better overall agreement with observations. For instance, at N(H2) > 2.10^{20} cm-2, while observations suggest that d[log N(CO)]=d[log N(H2)] = 3.07 +/- 0.73, we find d[log N(CO)]=d[log N(H2)] =14 when assuming uniform density, and d[log N(CO)]=d[log N(H2)] = 5.2 when including density fluctuations.Comment: 14 pages, 16 figures, accepted for publication in Astronomy & Astrophysic

Interstellar chemistry of nitrogen hydrides in dark clouds

The aim of the present work is to perform a comprehensive analysis of the interstellar chemistry of nitrogen, focussing on the gas-phase formation of the smallest polyatomic species and in particular nitrogen hydrides. We present a new chemical network in which the kinetic rates of critical reactions have been updated based on recent experimental and theoretical studies, including nuclear spin branching ratios. Our network thus treats the different spin symmetries of the nitrogen hydrides self-consistently together with the ortho and para forms of molecular hydrogen. This new network is used to model the time evolution of the chemical abundances in dark cloud conditions. The steady-state results are analysed, with special emphasis on the influence of the overall amounts of carbon, oxygen, and sulphur. Our calculations are also compared with Herschel/HIFI observations of NH, NH$_2$, and NH$_3$ detected towards the external envelope of the protostar IRAS 16293-2422. The observed abundances and abundance ratios are reproduced for a C/O gas-phase elemental abundance ratio of $\sim0.8$, provided that the sulphur abundance is depleted by a factor larger than 2. The ortho-to-para ratio of H$_2$ in these models is $\sim10^{-3}$. Our models also provide predictions for the ortho-to-para ratios of NH$_2$ and NH$_3$ of $\sim2.3$ and $\sim0.7$ respectively. We conclude that the abundances of nitrogen hydrides in dark cloud conditions are consistent with the gas-phase synthesis predicted with our new chemical network.Comment: Accepted for publication in Astronomy & Astrophysics; 22 pages (9 in Appendix), 7 figures (2 in Appendix), 6 tables (3 in Appendix

Observations and modeling of the dust emission from the H_2-bright galaxy-wide shock in Stephan's Quintet

Context. Spitzer Space Telescope observations have detected powerful mid-infrared (mid-IR) H_2 rotational line emission from the X-ray emitting large-scale shock (~15 × 35 kpc^2) associated with a galaxy collision in Stephan's Quintet (SQ). Because H_2 forms on dust grains, the presence of H_2 is physically linked to the survival of dust, and we expect some dust emission to originate in the molecular gas. Aims. To test this interpretation, IR observations and dust modeling are used to identify and characterize the thermal dust emission from the shocked molecular gas. Methods. The spatial distribution of the IR emission allows us to isolate the faint PAH and dust continuum emission associated with the molecular gas in the SQ shock. We model the spectral energy distribution (SED) of this emission, and fit it to Spitzer observations. The radiation field is determined with GALEX UV, HST V-band, and ground-based near-IR observations. We consider two limiting cases for the structure of the H_2 gas: it is either diffuse and penetrated by UV radiation, or fragmented into clouds that are optically thick to UV. Results. Faint PAH and dust continuum emission are detected in the SQ shock, outside star-forming regions. The 12/24 μm flux ratio in the shock is remarkably close to that of the diffuse Galactic interstellar medium, leading to a Galactic PAH/VSG abundance ratio. However, the properties of the shock inferred from the PAH emission spectrum differ from those of the Galaxy, which may be indicative of an enhanced fraction of large and neutrals PAHs. In both models (diffuse or clumpy H_2 gas), the IR SED is consistent with the expected emission from dust associated with the warm (> 150 K) H_2 gas, heated by a UV radiation field of intensity comparable to that of the solar neighborhood. This is in agreement with GALEX UV observations that show that the intensity of the radiation field in the shock is GUV = 1.4±0.2 [Habing units]. Conclusions. The presence of PAHs and dust grains in the high-speed (~1000 km s^(-1)) galaxy collision suggests that dust survives. We propose that the dust that survived destruction was in pre-shock gas at densites higher than a few 0.1 cm^(-3), which was not shocked at velocities larger than ~200 km s^(-1). Our model assumes a Galactic dust-to-gas mass ratio and size distribution, and current data do not allow us to identify any significant deviations of the abundances and size distribution of dust grains from those of the Galaxy. Our model calculations show that far-IR Herschel observations will help in constraining the structure of the molecular gas, and the dust size distribution, and thereby to look for signatures of dust processing in the SQ shock