Dynamical condensation in a magnetized and thermally bistable flow. Application to interstellar cirrus
P. Hennebelle, M. Pérault;
AaA, 2000, 359, 1124
ABSTRACT: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.
KEYWORDS: magnetic fields, magnetohydrodynamics (mhd), instabilities, book reviews, ism: structure, ism: clouds
PERSOKEY:h_i, cirrus, general ism, ,
CODE: hennebelle2000