Influence of Alfvén waves on thermal instability in the interstellar medium

Abstract

The effect of Alfven 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 Alfven 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 Alfven 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 Alfven waves can stabilize the large-scale perturbations, destabilize those whose wavelength is a few times the Alfven wavelength λ AW , 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 Alfven 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 λ AW and trigger the growth of wavelengths close to λ AW , 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 Alfvenic internal velocity dispersion iii) strong density fluctuations (≃10ρ cnm ) 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.