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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. [ABSTRACT FROM AUTHOR]

Context. Grain-grain processing has been shown to be an indispensable ingredient of shock modelling in high density environments. For densities higher than ∼105 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, in addition to sputtering, 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. Aims. We extend our study of the impact of grain-grain processing on C-type shocks in dense clouds. Various values of the magnetic field are explored. We study the corresponding consequences for molecular line emission and, in particular, investigate the influence of shattering and related vaporization on the rotational line emission of SiO. Methods. 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 ∼100 compared to a multi-fluid modelling approach. This implementation was combined with an LVG-based modelling of molecular line radiation transport. Using this combined model we calculated grids of shock models to explore the consequences of different dust-processing scenarios. Results. Grain-grain processing is shown to have a strong influence on C-type shocks for a broad range of magnetic fields: the shocks 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, although it does not yield an increase in width for the line profiles. To significantly increase the intensity and width of SiO rotational lines, SiO needs to be present in grain mantles. [ABSTRACT FROM AUTHOR]

ABSTRACT Cross-sections and rate coefficients have been calculated for the rovibrational excitation of CO by ortho- and para-H2 and by He. We adopted the quantal coupled channels method of solving the scattering equations, including CO rotational levels in the v = 0, 1 and 2 vibrational manifolds. Comparison is made with the results of previous calculations, which employed different interaction potentials and less accurate approximations to the collision dynamics. We consider that the present calculations yield reliable values of the rate coefficients for rovibrational excitation of CO at kinetic temperatures T ≈ 103 K. [ABSTRACT FROM AUTHOR]

ABSTRACT We have developed further the technique of time-dependent modelling of magnetohydrodynamic shock waves, with a view to interpreting the molecular line emission from outflow sources. The extensively observed source L1157 B1 was chosen as an exemplar of the application of this technique. The dynamical age of the shock wave model was varied in the range 500 ≤ t≤ 5000 yr, with the best fit to the observed line intensities being obtained for t= 1000 yr; this is of the same order as the dynamical age derived by Gueth, Guilloteau & Bachiller from their observations of L1157 B1. The emission line spectra of H2, CO, SiO, ortho- and para-H2O, ortho- and para-NH3, and A- and E-type CH3OH were calculated in parallel with the dynamical and chemical parameters of the model, using the 'large velocity gradient' (LVG) approximation to the line transfer problem. We compared the predictions of the models with the observed intensities of emission lines of H2, CO, SiO, ortho-H2O, ortho-NH3 and CH3OH, which include recent Herschel satellite measurements. In the case of SiO, we show (in Appendix A) that extrapolations of the collisional rate coefficients beyond the range of kinetic temperature for which they were originally calculated lead to spurious rotational line intensities and profiles. The computed emission-line spectra of SiO, NH3 and CH3OH are shown to depend on the assumed initial composition of the grain mantles, from whence they are released, by sputtering in the shock wave, into the gas phase. The dependence of the model predictions on the adopted form of the grain-size distribution is investigated in Appendix B; the corresponding integral line intensities are given in tabular form, for a range of C-type shock speeds, in the online Supporting Information. [ABSTRACT FROM AUTHOR]

We report the first calculations of the spectrum of methanol, arising in shock waves in molecular outflows. The small grid of shock wave models that we have computed incorporates the results of very recent computations of the rate coefficients for the collisional excitation of methanol by ortho- and para-H and by He. The two strongest transitions, one of A- and the other of E-type methanol, are masers that have been observed in a Class I methanol maser source, which is believed to be related to a molecular outflow. The same collisional propensities that give rise to population inversion and maser action can, in other transitions, lead to population anti-inversion and the lines appearing in absorption against the cosmic background radiation. We attempted to model specifically the outflow source L1157 B1, in which transitions of methanol have been observed recently by means of the Herschel satellite. Comparison with the predictions of the shock wave models is complicated by uncertainty in the value of the beam filling factor that should be adopted. [ABSTRACT FROM AUTHOR]

The dissipation of kinetic energy that occurs in interstellar shock waves is accompanied by the emission of radiation. In the case of shocks that are propagating into mainly molecular gas, the emission occurs principally in lines of the species H2, H, O, CO and H2O. The relative intensities of these emission lines are indicative of the type and speed of the shock wave and of the physical conditions in the ambient gas. We present the results of computations of the intensities of these lines, for small grids of models of C- and J-type shock waves, and compare with the results of previous calculations. Our results should serve to aid the interpretation of observations made with the Herschel and other satellites. [ABSTRACT FROM AUTHOR]

We have calculated emission by CO molecules from interstellar shock waves. Two approximations have been used to determine the population densities, , of the rotational levels, J: steady state and statistical equilibrium . A large-velocity-gradient approximation to the line-transfer problem was adopted in both cases. We find that there can be substantial differences between the values of the integrated rotational line intensities calculated in steady state and in the limit of statistical equilibrium. On the other hand, although CO can be the dominant coolant towards the rear of the cooling flow which follows the dynamical heating of the gas, the rate of cooling computed assuming statistical equilibrium is likely to be reasonably accurate, given that the limit of statistical equilibrium is approached in this region. [ABSTRACT FROM AUTHOR]

We present near-infrared (IR) spectra (0.98–2.5μm) from the group of Herbig–Haro (HH) objects comprising HH99: a series of knots forming a bow, HH99B, and a separate knot, HH99A. Observations of H2,[Fe ii] and[C i] are used to constrain shock-model parameters and determine the origin of the emission. Previous work has shown that it is likely that the atomic and ionic emission arises in regions of higher ionization than the molecular emission. On the basis of observations, it has often been suggested that the[Fe ii] and[C i] emission could arise, for example, at the apex of a bow shock, with the H2 emission produced in the wings. Accordingly, we have combined models of C-component and J-type shocks in order to reproduce the observed H2,[Fe ii] and[C i] line spectra. We can account for the H2 emission towards the HH99B complex by means of J-type shocks with magnetic precursors. We derive shock velocities of about 30 km s−1 and ages of order 100 yr; the pre-shock gas has a densityand the magnetic fieldG. The J-type shocks required to reproduce the intensities of the observed[Fe ii] and[C i] lines have velocities of 50 km s−1. It is necessary to assume that Fe has been previously eroded from grains, probably by the earlier passage of a C-type shock wave. Thus, our analysis supports the view that molecular outflows are episodic phenomena whose observed emission arises in shock waves. [ABSTRACT FROM AUTHOR]