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Session 51 - Interstellar Medium II.
Display session, Thursday, January 08
Exhibit Hall,

[51.17] Magnetohydrodynamics of HI Clouds: A New Scenario in the Dynamics of Galactic Cloud Collisions

F. Miniati (U. Minnesota), D. Ryu (Chungnam National U.), T. Jones (U. Minnesota), A. Ferrara (Arcetri)

We have investigated the role of magnetic fields in supersonic interstellar cloud collisions through 2D MHD numerical simulations of head-on collisions between clouds of equal mass. We consider two limiting field geometries; parallel (aligned) and perpendicular (transverse) to the motion of the colliding clouds. We also compare with analogous hydrodynamic (HD) calculations of Miniati et al (1997). We explore both adiabatic and radiative cases as well as clouds colliding promptly and after significant individual evolution. For some collisions the clouds are evolved to different ages to break the interaction symmetry.

The presence of the magnetic field significantly alters the collision outcomes. However, results vary strongly with the field geometry. Adiabatic collisions are very disruptive when the field is aligned or absent, independent of the collision symmetry. Radiative cooling leads to partial coalescence, producing a new, bigger cloud mass about 10% greater than either initial cloud mass. No such coalescence was observed in our HD simulations. The role of the magnetic field is most dramatic for a transverse geometry. Also, then the initial conditions play a much bigger role. For this geometry the outcomes are remarkably different for the unevolved and evolved cloud collisions. The former case is almost unaffected by the the magnetic field. However, prior propagation through the ISM generates an important new field structure; that is, field lines in front of the cloud are strongly stretched, creating a region of very high magnetic energy. This region acts like a bumper preventing direct contact between the clouds. During the virtual impact, cloud kinetic energy is temporarily stored in the bumper to stop the cloud motions. Afterwards the field relaxes and returns some of that energy to the clouds, whose motion has been reversed. The ``elasticity'', defined as the ratio of the final to the initial kinetic energy of each cloud, is about 0.6 during this collision.

This work is supported at the University of Minnesota by the NSF and by the University of Minnesota Supercomputing Institute.

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