Time-dependent modelling of the molecular line emission from shock waves in outflow sources

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.

SHOCK-ENHANCED C+EMISSION AND THE DETECTION OF H2O FROM THE STEPHAN'S QUINTET GROUP-WIDE SHOCK USINGHERSCHEL

“…We will argue later that this is consistent with a warm diffuse gas heated by a network of low-velocity magnetic shocks (C-shocks) and/or turbulence. The weakness of the [O i] 63 μm emission is not consistent with strongly dissipative J-shocks (Draine et al 1983;Hollenbach & McKee 1989;Flower & Pineau des Forêts 2012). 23 Region D in the smoothed IRAM spectrum contains a higher velocity component not seen in the [C ii] emission, but this may be partly baseline uncertainty in the IRAM data.…”

Chemical features in the circumnuclear disk of the Galactic center

“…Precise fractional abundances should be obtained with models, such as those by Gusdorf et al (2008), Jiménez-Serra et al (2008), Flower & Pineau des Forêts (2012), especially for SiO. In addition, physical parameters, such 5.4(-11) 3.0(-9) 9.8(-9) CN 3.5(-8) 4.4(-9) 9.6(-12) HCO + 3.3(-11)…”

“…For precise values of physical parameters, see Gusdorf et al (2008), Flower & Pineau des Forêts (2012). In order to study the effects of UV photons, we use the publicly available Meudon PDR code version 1.4.4 (Le Petit et al 2006), which we run separately from the shock model code.…”

Impacts of pure shocks in the BHR71 bipolar outflow

“…The most remarkable and well-studied pure shock region is the B1 knot of the L1157 bipolar outflow. Sufficiently distant from the central protostar, this region remains uncontaminated by infall or irradiation processes, and has been the subject of a number of studies dedicated to studying its energetics or chemical composition (Gusdorf et al 2008b;Codella et al 2010;Flower & Pineau des Forêts 2012;Benedettini et al 2013;Busquet et al 2014;Podio et al 2014). In particular, L1157 was mapped by the German Receiver for Astronomy at Terahertz Frequencies (GREAT) onboard the Stratospheric Observatory for Infrared Astronomy (SOFIA) in CO (11−10), but the low signal-to-noise ratio prevented Eislöffel et al (2012) from a thorough study of its energetics or chemistry.…”