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Manuscript Title: A computer program for calculating the structure of magnetohydrodynamical shocks in interstellar clouds.
Authors: L. Heck, D.R. Flower, G. Pineau Des Forets
Program title: MHD
Catalogue identifier: ABLS_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 58(1990)169
Programming language: Fortran.
Computer: SUN 3/160 WORKSTATION.
Operating system: UNIX.
RAM: 180K words
Word size: 32
Keywords: Astrophysics, Interstellar medium, Shocks, Interstellar clouds, Molecules, Magnetic fields, Polycyclic aromatic Hydrocarbons.
Classification: 1.8.

Nature of problem:
Shocks are known to be generated in the interstellar medium, for example, in regions of star formation. The structure of the shocks is modified by the interstellar magnetic field. The continuous or C-type shocks which are anticipated in such regions change the physical and chemical state of the gas. Chemical changes to the degree of ionization impinge, in turn, on the shock structure, and so the magnetohydrodynamical (MHD) and chemical rate equations need to be solved simultaneously. The column densities of atomic and molecular species may be calculated and compared with observations, yielding information on the physical and chemical conditions in the interstellar gas.

Solution method:
The MHD and chemical rate equations may be expressed as a set of coupled, first order, non-linear, ordinary differential equations. The independent variable is a distance parameter, specifying the position in the shock, and the dependent variables are physical parameters such as velocity and temperature, and also the number densities of the specified atomic and molecular species. These differential equations are 'stiff' owing to the many and varied distance-scales which characterize the dependent variables, particularly the atomic and molecular abundances. Accordingly, they are integrated using the Hindmarsh version of the Gear stiff differential equation solver.

A stationary state is assumed, yielding a 'snap-shot' of the shock structure. The shock is taken to be plane-parallel, reducing the problem to a single dimension. The shock is assumed to be continuous (C-type) and the differential equations are solved as an initial value problem. The initial values (of the dependent variables) are given as input and may correspond, for example, to the equilibrium state of the gas.