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Session 117 - Galactic ISM.
Oral session, Thursday, January 16
Harbour B,

[117.02] Superbubbles as Magnetic Phenomena

B. D. Frei, R. N. Henriksen, J. A. Irwin, D. L. Giguere (Queen's U.)

The largest superbubbles observed have estimated input energies on the order of 10 000 SNe. Current superbubble models cannot adequately explain the formation of these shells. Alternative energy sources must be considered. This paper presents the results of a feasibility study on the formation of superbubbles via magnetic fields. A possible time independent field is derived analytically from Maxwell's equations and geometric constraints. A coupling between the magnetic field and galactic rotation is revealed. By fitting this field to the two extreme superbubbles observed in M108 by Giguere and Irwin (1996), we are able to compare our model results with observational evidence.

We calculate the energy transfer between the disk and bubble expansion by assuming that the magnetic and velocity fields are in a state of energy equipartition. In general, the modelled energies are too low to produce the observed expansion. However, adequate energy can be supplied if either a global magnetic field or a wide-scale velocity disturbance is included. The resulting magnetic field is twisted in a manner resembling the field produced by the Parker Instability and the field required to produce the \alpha-effect in galactic dynamo theory.

We conclude that the magnetic field is influential in forming superbubbles and as such should not be modelled merely as a secondary effect. Future models must consider the effects of both the global magnetic field as well as a locally perturbed field, perhaps due to the Parker Instability.

Program listing for Thursday