Issue |
A&A
Volume 448, Number 3, March IV 2006
|
|
---|---|---|
Page(s) | 1083 - 1093 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20053510 | |
Published online | 03 March 2006 |
Influence of Alfvén waves on thermal instability in the interstellar medium
1
Laboratoire de Radioastronomie Millimétrique, UMR 8112 du CNRS, École Normale Supérieure et Observatoire de Paris, 24 rue Lhomond, 75231 Paris Cedex 05, France e-mail: patrick.hennebelle@ens.fr
2
CNRS, Observatoire de la Côte d'Azur, BP 4229, 06304 Nice Cedex 4, France e-mail: passot@obs-nice.fr
Received:
25
May
2005
Accepted:
14
November
2005
The effect of Alfvén waves on
the thermal instability of the Interstellar Medium (ISM) is
investigated both analytically and numerically. A
stability analysis of a finite amplitude circularly polarized Alfvén
wave propagating parallel to an ambient magnetic field in a
thermally unstable gas at thermal equilibrium is performed, leading
to a dispersion relation that depends on 3 parameters, namely the square
ratio of the sonic and Alfvén velocities (β), the
wave amplitude and the ratio between the wave temporal
period and the cooling time. Depending on the values of these 3 parameters,
the Alfvén waves can stabilize the large-scale perturbations, destabilize
those whose wavelength is a few times the Alfvén wavelength
, or leave the growth rate of the short scales unchanged.
To investigate the non-linear regime, two different
numerical experiments are performed in a slab geometry. The first
one deals with the development of an initial density perturbation in
a thermally unstable gas in the presence of Alfvén waves. The second one
addresses the influence of those waves on the thermal transition
induced by a converging flow. The numerical results confirm the
trends inferred from the analytic calculations, i.e. the waves
prevent the instability at scales larger than
and trigger the growth of wavelengths close to
,
therefore producing a very fragmented cold phase. The second
numerical experiments shows that i) the magnetic pressure
prevents the merging of the CNM fragments therefore maintaining the
complex structure of the flow and organizing it
into groups of clouds ii) these groups of CNM clouds have an
Alfvénic internal velocity dispersion iii) strong density
fluctuations (
) triggered by magnetic
compression occur. We note that during this event there is no stiff
variation of the longitudinal velocity field. This is unlike
the hydrodynamical case for which the clouds are uniform and
do not contain significant internal motions except after cloud
collisions. In this situation a strong density fluctuation
occurs, accompanied by a stationary velocity gradient through the cloud.
Key words: ISM: instabilities / magnetohydrodynamics / turbulence / ISM: clouds / ISM: magnetic fields
© ESO, 2006
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