Dynamical condensation in a magnetized and thermally bistable flow Application to interstellar cirrus

We investigate the dynamical condensation process in a magnetized and thermally bistable flow, in one dimensional (slab) geometry. We find self-similar solutions able to describe the magneto-thermal process for weakly heterogeneous magnetic fields and perform a numerical simulation in the general case corresponding to the neutral interstellar medium. It is well known that a purely transverse magnetic field can prevent the condensation because magnetic pressure increases with density. In a converging flow however, if the field is oblique, the magnetic field lines are compressed and bent. The fluid is driven forward ahead of the original compression. The magnetic tension induces transverse velocities which tend to unbend the field lines. Two cases arise: in weak fields the transverse flow aligns the field lines parallel to the initial flow close to the convergence center, leading to an unimpeded central condensation along this direction. Stronger fields rapidly re-align the flow in the field direction, leading to condensation along the original field direction. The conditions for condensation are similar to the non magnetized case, with the additional constraint of a maximum angle between the initial fields. This constraint is most severe when kinetic and magnetic energies are comparable. At the end of the condensation, the magnetic field slowly relaxes back to its original direction, resulting in fields of identical intensity in the cloud and in the intercloud gas. For the neutral atomic interstellar medium, the maximum angle is in the range 20 to 40 degrees.