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

The abundances of nitrogen-containing molecules during pre-protostellar collapse

We have studied the chemistry of nitrogen--bearing species during the initial stages of protostellar collapse, with a view to explaining the observed longevity of N2H+ and NH3 and the high levels of deuteration of these species. We followed the chemical evolution of a medium comprising gas and dust as it underwent free--fall gravitational collapse. Chemical processes which determine the relative populations of the nuclear spin states of molecules and molecular ions were included explicitly, as were reactions which lead ultimately to the deuteration of the nitrogen--containing species N2H+ and NH3. The freeze-out of `heavy' molecules onto grains was taken into account. We found that the timescale required for the nitrogen--containing species to attain their steady--state values was much larger than the free--fall time and even comparable with the probable lifetime of the precursor molecular cloud. However, it transpires that the chemical evolution of the gas during gravitational collapse is insensitive to its initial composition. If we suppose that the grain--sticking probabilities of atomic nitrogen and oxygen are both less than unity (S less than 0.3), we find that the observed differential freeze--out of nitrogen- and carbon--bearing species can be reproduced by the model of free--fall collapse when a sufficiently large grain radius (a_{g}= 0.5 micron) is adopted. Furthermore, the results of our collapse model are consistent with the high levels of deuteration of N2H+ and NH3 which have been observed in L1544 providing that 0.5<a_{g}<1 micron. We note that the o/p H2D+ ratio and fractional abundance of ortho-H2D+ should be largest when ND3 is most abundant

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

Collisional excitation of water by hydrogen atoms

We present quantum dynamical calculations that describe the rotational excitation of H$_2$O due to collisions with H atoms. We used a recent, high accuracy potential energy surface, and solved the collisional dynamics with the close-coupling formalism, for total energies up to 12 000 cm$^{-1}$. From these calculations, we obtained collisional rate coefficients for the first 45 energy levels of both ortho- and para-H$_2$O and for temperatures in the range T = 5-1500 K. These rate coefficients are subsequently compared to the values previously published for the H$_2$O / He and H$_2$O / H$_2$ collisional systems. It is shown that no simple relation exists between the three systems and that specific calculations are thus mandatory

Dense molecular globulettes and the dust arc towards the runaway O star AE Aur (HD 34078)

Some runaway stars are known to display IR arc-like structures around them, resulting from their interaction with surrounding interstellar material. The properties of these features as well as the processes involved in their formation are still poorly understood. We aim at understanding the physical mechanisms that shapes the dust arc observed near the runaway O star AEAur (HD34078). We obtained and analyzed a high spatial resolution map of the CO(1-0) emission that is centered on HD34078, and that combines data from both the IRAM interferometer and 30m single-dish antenna. The line of sight towards HD34078 intersects the outer part of one of the detected globulettes, which accounts for both the properties of diffuse UV light observed in the field and the numerous molecular absorption lines detected in HD34078's spectra, including those from highly excited H2 . Their modeled distance from the star is compatible with the fact that they lie on the 3D paraboloid which fits the arc detected in the 24 {\mu}m Spitzer image. Four other compact CO globulettes are detected in the mapped area. These globulettes have a high density and linewidth, and are strongly pressure-confined or transient. The good spatial correlation between the CO globulettes and the IR arc suggests that they result from the interaction of the radiation and wind emitted by HD 34078 with the ambient gas. However, the details of this interaction remain unclear. A wind mass loss rate significantly larger than the value inferred from UV lines is favored by the large IR arc size, but does not easily explain the low velocity of the CO globulettes. The effect of radiation pressure on dust grains also meets several issues in explaining the observations. Further observational and theoretical work is needed to fully elucidate the processes shaping the gas and dust in bow shocks around runaway O stars. (Abridged)Comment: Accepted for publication in Astronomy & Astrophysic

Modelling the molecular composition and nuclear-spin chemistryof collapsing pre-stellar sources★

We study the gravitational collapse of pre-stellar sources and the associated evolution of their chemical composition. We use the University of Grenoble Alpes Astrochemical Network (UGAN), which includes reactions involving the different nuclear-spin states of H2, H+3 , and of the hydrides of carbon, nitrogen, oxygen, and sulphur, for reactions involving up to seven protons. In addition, species-to-species rate coefficients are provided for the ortho/para interconversion of the H +3 + H2 system and isotopic variants. The composition of the medium is followed from an initial steady state through the early phase of isothermal gravitational collapse. Both the freeze-out of the molecules on to grains and the coagulation of the grains were incorporated in the model. The predicted abundances and column densities of the spin isomers of ammonia and its deuterated forms are compared with those measured recently towards the pre-stellar cores H-MM1, L16293E, and Barnard B1. We find that gas-phase processes alone account satisfactorily for the observations, without recourse to grain-surface reactions. In particular, our model reproduces both the isotopologue abundance ratios and the ortho:para ratios of NH2D and NHD2 within observational uncertainties. More accurate observations are necessary to distinguish between full scrambling processes – as assumed in our gas-phase network – and direct nucleus- or atom-exchange reactions

The depletion of NO in pre-protostellar cores

Understanding the depletion of heavy elements is a fundamental step towards determining the structure of pre-protostellar cores just prior to collapse. We study the dependence of the NO abundance on position in the pre-protostellar cores L1544 and L183. We observed the 150 GHz and 250~GHz transitions of NO and the 93 GHz transitions of \NTHP \ towards L1544 and L183 using the IRAM 30 m telescope. We compare the variation of the NO column density with position in these objects with the H column density derived from dust emission measurements. We find that NO behaves differently from \NTHP \ and appears to be partially depleted in the high density core of L1544. Other oxygen-containing compounds are also likely to be partially depleted in dense-core nuclei. The principal conclusions are that: the prestellar core L1544 is likely to be 'carbon-rich'; the nitrogen chemistry did not reach equilibrium prior to gravitational collapse, and nitrogen is initially (at densities of the order of $10^4$~cm$^{-3}$) mainly in atomic form; the grain sticking probabilities of atomic C, N and, probably, O are significantly smaller than unity