The planar MHD shock code mhd_vode has been developed in order to simulate both continuous (C) type shock waves and jump (J) type shock waves in the interstellar medium. The physical and chemical ...state of the gas in steady-state may also be computed and used as input to a shock wave model. The code is written principally in FORTRAN 90, although some routines remain in FORTRAN 77. The documented program and its input data are described and provided as supplementary material, and the results of exemplary test runs are presented. Our intention is to enable the interested user to run the code for any sensible parameter set and to comprehend the results. With applications to molecular outflow sources in mind, we have computed, and are making available as supplementary material, integrated atomic and molecular line intensities for grids of C- and J-type models; these computations are summarized in the Appendices.
Aims.
The high abundances of CH+ in the diffuse interstellar medium (ISM) are a long-standing issue of our understanding of the thermodynamical and chemical states of the gas. We investigate here the ...formation of CH
+
in turbulent and multiphase environments, where the heating of the gas is almost solely driven by the photoelectric effect.
Methods.
The diffuse ISM is simulated using the magnetohydrodynamic (MHD) code RAMSES which self-consistently computes the dynamical and thermal evolution of the gas along with the time-dependent evolutions of the abundances of H
+
, H, and H
2
. The rest of the chemistry, including the abundance of CH
+
, is computed in post-processing, at equilibrium, under the constraint of out-of-equilibrium H
+
, H, and H
2
. The comparison with the observations is performed taking into account an often neglected yet paramount piece of information, namely the length of the intercepted diffuse matter along the observed lines of sight.
Results.
Almost all of the mass of CH
+
originates from unstable gas, in environments where the kinetic temperature is higher than 600 K, the density ranges between 0.6 and 10 cm
−3
, the electronic fraction ranges between 3 × 10
−4
and 6 × 10
−3
, and the molecular fraction is smaller than 0.4. Its formation is driven by warm and out-of-equilibrium H
2
initially formed in the cold neutral medium (CNM) and injected in more diffuse environments, and even the warm neutral medium (WNM) through a combination of advection and thermal instability. The simulation that displays the closest agreement with the HI-to-H
2
transition and the thermal pressure distribution observed in the solar neighborhood is found to naturally reproduce the observed abundances of CH
+
, the dispersion of observations, the probability of occurrence of most of the lines of sight, the fraction of nondetections of CH
+
, and the distribution of its line profiles. The amount of CH
+
and the statistical properties of the simulated lines of sight are set by the fraction of unstable gas rich in H
2
, which is controlled on Galactic scales by the mean density of the diffuse ISM (or, equivalently, its total mass), the amplitude of the mean UV radiation field, and the strength of the turbulent forcing.
Conclusions.
This work offers a new and natural solution to an 80-yr-old chemical riddle. The almost ubiquitous presence of CH
+
in the diffuse ISM likely results from the exchange of matter between the CNM and the WNM induced by the combination of turbulent advection and thermal instability, without the need to invoke ambipolar diffusion or regions of intermittent turbulent dissipation. Through two-phase turbulent mixing, CH
+
might thus be a tracer of the H
2
mass loss rate of CNM clouds.
Context. Tens of light hydrides and small molecules have now been detected over several hundreds sightlines 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 μm 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, ‾ε ≲ 10-23 erg cm-3 s-1 for nH = 20 cm-3, from the CH+ abundance; an upper limit to the ion-neutral velocity drift, υin ≲ 3.5 km s-1, from the SH+ to CH+ abundance ratio; and a range of dissipation timescales, 100 ≲ τV ≲ 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 × 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.
We have computed C- and J-type models of shock waves in molecular outflow sources. In addition to the (optically thin) emission line spectrum of molecular hydrogen, the spectra of CO, OH, SiO, H2O ...and NH3 were computed by means of the large velocity gradient approximation. We find that the intensities of the OH lines are particularly sensitive to the character (C- or J-type) of the shock wave. The results of these computations were used to guide the interpretation of the spectrum of the outflow source NGC 1333 IRAS 4B, recently observed by Herschel/PACS and the Spitzer satellites. We find that the best overall fit to the spectrum of this object is provided by quasi-time-dependent (CJ-type) models, which have both C- and J-type characteristics; the dynamical age of the emitting region is found to be of the order of 102 yr. The principal limitation to the robustness of the predictions of the current model relate to the possible effects of dust on the dynamical and thermal profiles of the gas. Specifically, the shattering and vaporization of grains, which can enhance the total grain cross-section, have not been taken into account. Furthermore, there remain significant uncertainties relating to the rate of reformation of H2 molecules, on dust grains, at the high gas kinetic temperatures at which this process occurs in the shock wave.
ABSTRACT
The nuclear-spin chemistry of interstellar water is investigated using the University of Grenoble Alpes Astrochemical Network (UGAN). This network includes reactions involving the different ...nuclear-spin states of the hydrides of carbon, nitrogen, oxygen, and sulphur, as well as their deuterated forms. Nuclear-spin selection rules are implemented within the scrambling hypothesis for reactions involving up to seven protons. The abundances and ortho-to-para ratios (OPRs) of gas-phase water and water ions (H2O+ and H3O+) are computed under the steady-state conditions representative of a dark molecular cloud and during the early phase of gravitational collapse of a pre-stellar core. The model incorporates the freezing of the molecules on to grains, simple grain surface chemistry, and cosmic ray induced and direct desorption of ices. The predicted OPRs are found to deviate significantly from both thermal and statistical values and to be independent of temperature below ∼30 K. The OPR of H2O is shown to lie between 1.5 and 2.6, depending on the spin state of H2, in good agreement with values derived in translucent clouds with relatively high extinction. In the pre-stellar core-collapse calculations, the OPR of H2O is shown to reach the statistical value of 3 in regions with severe depletion (nH > 107 cm−3). We conclude that a low water OPR (≲ 2.5) is consistent with gas-phase ion-neutral chemistry and reflects a gas with OPR(H2) ≲ 1. Available OPR measurements in protoplanetary discs and comets are finally discussed.
The Stephan's Quintet (hereafter SQ) is a template source to study the impact of galaxies interaction on the physical state and energetics of their gas. We report on IRAM single-dish CO observations ...of the SQ compact group of galaxies. These observations follow up the Spitzer discovery of bright mid-IR H sub(2) rotational line emission (L(H sub(2)) approx = 10 super(35) W) from warm (10 super(2-3) K) molecular gas, associated with a 30 kpc long shock between a galaxy, NGC 7318b, and NGC 7319's tidal arm. We detect CO(1-0), (2-1) and (3-2) line emission in the inter-galactic medium (IGM) with complex profiles, spanning a velocity range of approx =1000 km s super(-1). The spectra exhibit the pre-shock recession velocities of the two colliding gas systems (5700 and 6700 km s super(-1)), but also intermediate velocities. This shows that much of the molecular gas has formed out of diffuse gas accelerated by the galaxy-tidal arm collision. CO emission is also detected in a bridge feature that connects the shock to the Seyfert member of the group, NGC 7319, and in the northern star forming region, SQ-A, where a new velocity component is identified at 6900 km s super(-1), in addition to the two velocity components already known. Assuming a Galactic CO(1-0) emission to H sub(2) mass conversion factor, a total H sub(2) mass of approx =5 x 10 super(9) M sub(middot in circle) is detected in the shock. The ratio between the warm H sub(2) mass derived from Spitzer spectroscopy, and the H sub(2) mass derived from CO fluxes is approx =0.3 in the IGM of SQ, which is 10--100 times higher than in star-forming galaxies. The molecular gas carries a large fraction of the gas kinetic energy involved in the collision, meaning that this energy has not been thermalized yet. The kinetic energy of the H sub(2) gas derived from CO observations is comparable to that of the warm H sub(2) gas from Spitzer spectroscopy, and a factor approx =5 greater than the thermal energy of the hot plasma heated by the collision. In the shock and bridge regions, the ratio of the PAH-to-CO surface luminosities, commonly used to measure the star formation efficiency of the H sub(2) gas, is lower (up to a factor 75) than the observed values in star-forming galaxies. We suggest that turbulence fed by the galaxy-tidal arm collision maintains a high heating rate within the H sub(2) gas. This interpretation implies that the velocity dispersion on the scale of giant molecular clouds in SQ is one order of magnitude larger than the Galactic value. The high amplitude of turbulence may explain why this gas is not forming stars efficiently.
We study the production of SiO in the gas phase of molecular outflows, through the sputtering of Si-bearing material in refractory grain cores, which are taken to be olivine. We calculate also the ...rotational line spectrum of the SiO. The sputtering is driven by neutral particle impact on charged grains, in steady-state C-type shock waves, at the speed of ambipolar diffusion. The emission of the SiO molecule is calculated by means of an LVG code. A grid of models, with shock speeds in the range 20 < vs < 50 km s-1 and preshock gas densities 104 < nH < 106 cm-3, has been generated. We compare our results with those of an earlier study (Schilke et al. 1997). Improvements in the treatment of the coupling between the charged grains and the neutral fluid lead to narrower shock waves and lower fractions of Si ($\la$10%) being released into the gas phase. Erosion of grain cores is significant ($\ga$1%) only for C-type shock speeds vs > 25 km s-1, given the adopted properties of olivine. More realistic assumptions concerning the initial fractional abundance of O2 lead to SiO formation being delayed, so that it occurs in the cool, dense postshock flow. Good agreement is obtained with recent observations of SiO line intensities in the L1157 and L1448 molecular outflows. The inferred temperature, opacity, and SiO column density in the emission region differ significantly from those estimated by means of LVG “slab” models. The fractional abundance of SiO is deduced and found to be in the range 4 $\times$ 10-8 $\la n({\rm SiO})/n_{\rm H} \la$ 3 $\times$ 10-7. Observed line profiles are wider than predicted and imply multiple, unresolved shock regions within the beam.
Context.
Molecular hydrogen, H
2
, is the most abundant molecule in the Universe. Thanks to its widely spaced energy levels, it predominantly lights up in warm gas,
T
≳ 10
2
K, such as shocked ...regions externally irradiated or not by interstellar UV photons, and it is one of the prime targets of
James Webb
Space Telescope (JWST) observations. These may include shocks from protostellar outflows, supernova remnants impinging on molecular clouds, all the way up to starburst galaxies and active galactic nuclei.
Aims.
Sophisticated shock models are able to simulate H
2
emission from such shocked regions. We aim to explore H
2
excitation using shock models, and to test over which parameter space distinct signatures are produced in H
2
emission.
Methods.
We here present simulated H
2
emission using the Paris-Durham shock code over an extensive grid of ~14 000 plane-parallel stationary shock models, a large subset of which are exposed to a semi-isotropic external UV radiation field. The grid samples six input parameters: the preshock density, shock velocity, transverse magnetic field strength, UV radiation field strength, the cosmic-ray-ionization rate, and the abundance of polycyclic aromatic hydrocarbons, PAHs. Physical quantities resulting from our self-consistent calculations, such as temperature, density, and width, have been extracted along with H
2
integrated line intensities. These simulations and results are publicly available on the Interstellar Medium Services platform.
Results.
The strength of the transverse magnetic field, as quantified by the magnetic scaling factor,
b
, plays a key role in the excitation of H
2
. At low values of
b
(≲0.3, J-type shocks), H
2
excitation is dominated by vibrationally excited lines; whereas, at higher values (
b
≳ 1, C-type shocks), rotational lines dominate the spectrum for shocks with an external radiation field comparable to (or lower than) the solar neighborhood. Shocks with
b
≥ 1 can potentially be spatially resolved with JWST for nearby objects. H
2
is typically the dominant coolant at lower densities (≲10
4
cm
−3
); at higher densities, other molecules such as CO, OH, and H
2
O take over at velocities ≲20 km s
−1
and atoms, for example, H, O, and S, dominate at higher velocities. Together, the velocity and density set the input kinetic energy flux. When this increases, the excitation and integrated intensity of H
2
increases similarly. An external UV field mainly serves to increase the excitation, particularly for shocks where the input radiation energy is comparable to the input kinetic energy flux. These results provide an overview of the energetic reprocessing of input kinetic energy flux and the resulting H
2
line emission.
Context.
The energetics and physical conditions of the interstellar medium and feedback processes remain challenging to probe.
Aims.
Shocks, modelled over a broad range of parameters, are used to ...construct a new tool to deduce the mechanical energy and physical conditions from observed atomic or molecular emission lines.
Methods.
We compute magnetised, molecular shock models with velocities
V
s
= 5–80 km s
−1
, pre-shock proton densities
n
H
= 10
2
–10
6
cm
−3
, weak or moderate magnetic field strengths, and in the absence or presence of an external UV radiation field. These parameters represent the broadest published range of physical conditions for molecular shocks. As a key shock tracer, we focus on the production of CH
+
and post-process the radiative transfer of its rovibrational lines. We develop a simple emission model of an ensemble of shocks for connecting any observed emission lines to the mechanical energy and physical conditions of the system.
Results.
For this range of parameters, we find the full diversity (C-, C
*
-, CJ-, and J-type) of magnetohydrodynamic shocks. H
2
and H are dominant coolants, with up to 30% of the shock kinetic flux escaping in Ly
α
photons. The reformation of molecules in the cooling tail means H
2
is even a good tracer of dissociative shocks and shocks that were initially fully atomic. The known shock tracer CH
+
can also be a significant coolant, reprocessing up to 1% of the kinetic flux. Its production and excitation is intimately linked to the presence of H
2
and C
+
. For each shock model we provide integrated intensities of rovibrational lines of H
2
, CO, and CH
+
, and atomic H lines, and atomic fine-structure and metastable lines. We demonstrate how to use these shock models to deduce the mechanical energy and physical conditions of extragalactic environments. As a template example, we interpret the CH
+
(1−0) emission from the Eyelash starburst galaxy. A mechanical energy injection rate of at least 10
11
L
⊙
into molecular shocks is required to reproduce the observed line. We find that shocks with velocities as low as 5 km s
−1
irradiated by a strong UV field are compatible with the available energy budget. The low-velocity, externally irradiated shocks are at least an order magnitude more efficient than the most efficient shocks with no external irradiation in terms of the total mechanical energy required. We predict differences of more than two orders of magnitude in the intensities of the pure rotational lines of CO, Ly
α
, and the metastable lines of O, S
+
, and N between representative models of low-velocity (
V
s
~ 10 km s
−1
) externally irradiated shocks and higher-velocity shocks (
V
s
≥ 50 km s
−1
) with no external irradiation.
Conclusions.
Shock modelling over an extensive range of physical conditions allows for the interpretation of challenging observations of broad line emission from distant galaxies. Our new method opens up a promising avenue to quantitatively probe the physical conditions and mechanical energy of galaxy-scale gas flows.
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