J/A+A/550/A106 Low-velocity shocks models (Lesaffre+, 2013)
Low velocity shocks: signatures of turbulent-dissipation in diffuse
irradiated gas.
Lesaffre P., Pineau des Forets G., Godard B., Guillard P., Boulanger F.,
Falgarone E.
<Astron. Astrophys. 550, A106 (2013)>
=2013A&A...550A.106L 2013A&A...550A.106L
ADC_Keywords: Interstellar medium ; Magnetic fields ; Models
Keywords: shock waves - astrochemistry - ISM: molecules -
ISM: kinematics and dynamics - ISM: abundances
Abstract:
Large-scale motions in galaxies (supernovae explosions, galaxy
collisions, galactic shear etc.) generate turbulence, which allows a
fraction of the available kinetic energy to cascade down to small
scales before it is dissipated.
We establish and quantify the diagnostics of turbulence dissipation in
mildly irradiated diffuse gas in the specific context of shock
structures.
We incorporated the basic physics of photon-dominated regions into a
state-of-the-art steady-state shock code. We examined the chemical and
emission properties of mildly irradiated (G0=1) magnetised shocks
in diffuse media (nH=102 to 104cm-3) at low- to moderate
velocities (from 3 to 40km/s).
The formation of some molecules relies on endoergic reactions. Their
abundances in J shocks are enhanced by several orders of magnitude for
shock velocities as low as 7km/s. Otherwise most chemical
properties of J-type shocks vary over less than an order of magnitude
between velocities from about 7 to about 30km/s, where H2
dissociation sets in. C-type shocks display a more gradual molecular
enhancement with increasing shock velocity.
We quantified the energy flux budget (fluxes of kinetic, radiated and
magnetic energies) with emphasis on the main cooling lines of the cold
interstellar medium. Their sensitivity to shock velocity is such that
it allows observations to constrain statistical distributions of shock
velocities.
We fitted various probability distribution functions (PDFs) of shock
velocities to spectroscopic observations of the galaxy-wide shock in
Stephan's Quintet and of a Galactic line of sight which samples
diffuse molecular gas in Chamaeleon. In both cases, low velocities
bear the greatest statistical weight and the PDF is consistent with a
bimodal distribution. In the very low velocity shocks (below 5km/s),
dissipation is due to ion-neutral friction and it powers H2
low-energy transitions and atomic lines. In moderate velocity shocks
(20km/s and above), the dissipation is due to viscous heating and
accounts for most of the molecular emission. In our interpretation a
significant fraction of the gas in the line of sight is shocked (from
4% to 66%). For example, C+ emission may trace shocks in UV
irradiated gas where C+ is the dominant carbon species.
Low- and moderate velocity shocks are important in shaping the
chemical composition and excitation state of the interstellar gas.
This allows one to probe the statistical distribution of shock
velocities in interstellar turbulence.
Description:
Outputs from the grids of shock runs used in the paper.
* The first level of the directory tree decides the value for the
magnetic field (b=(B/Bo)/sqrt(nH), with Bo=1µGauss=10-10T):
b0.1/ for b=0.1
b1/ for b=1
* The second level decides the value for the density and the type of shock:
b0.1/J2g0/ has J-type shocks for nH=102cm-3
b0.1/J3g0/ has J-type shocks for nH=103cm-3
b0.1/J4g0/ has J-type shocks for nH=104cm-3
b1/J2g0/ has J-type shocks for nH=102cm-3
b1/J3g0/ has J-type shocks for nH=103cm-3
b1/J4g0/ has J-type shocks for nH=104cm-3
b1/C2g0/ has C-type shocks for nH=102cm-3
b1/C3g0/ has C-type shocks for nH=103cm-3
b1/C4g0/ has C-type shocks for nH=104cm-3
* The third level contains :
- static/ the run to get the pre-shock thermal and chemical equilibrium
- steady/ the 'PDR' run from these pre-shock conditions.
- u.o3/ each output from the run at u=N km/s.
- chemistry.in: the chemical network used
- species.in: the pre-shock chemical composition.
- inputmhdrun.u.template : the template with all physical parameters
used to sample the grid of velocities
* The fourth level contains for each run the following ascii files:
- cooling.out : local total emission from a number of species.
- energetics.out : various energy fluxes
- err_cool.out : a few error messages whichi have occurred during the run.
- excit.out : H2 excitation diagram throughout the shock
- fe_lines.out: zero (we discarded Fe emission)
- fe_pops.out: zero as well
- H2_lev.out: integrated column densities of each H2 level
- H2_line.out: integrated emission of 200 H2 lines
- info_mhd.out: an ascii file which describes the parameters used in
the run.
- intensity.out: integrated intensities of several atomic lines.
- jlb.out: a specific output file for J.L.B.
- mhd_coldens.out: column densities of all species along the shock
- mhd_phys.out: various physical quantities of interest along the shock
- mhd_speci.out: the chemical profile (abundances)
- populations.out: local populations of some atomic levels
- species.out: post-shock temperature and composition
Each ascii file has a self-explanatory first line which contains the
names of each quantities in the column below.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
b0.1/* . 4 Directory tree for magnetic field b=0.1
b1/* . 6 Directory tree for magnetic field b=1
lastgrid.tar 3727 2609666 All files (550Mb when compressed)
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Acknowledgements:
Pierre Lesaffre, pierre.lesaffre(at)lra.ens.fr
(End) Pierre Lesaffre [CNRS, LERMA, ENS/LRA, France] 28-Nov-2012