Stellar clustering as induced by a supernova: the non-linear evolution

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Abstract
When the shock wave from a supernova expands, it sweeps up not only interstellar matter but also magnetic field. The field is greatly amplified by compression and will provide the dominant pressure during the cool radiative phase of an expanding supernova shell. A previous paper examined a hydromagnetic instability in this system (a form of the Parker instability) and found that it will concentrate gas at intervals of the order of parsecs. The length and time-scales make the instability promising as an explanation of the fragmentation and stellar clustering that is seen in Canis Major R1. The present paper goes beyond the perturbation analysis to study the nonlinear evolution. The major findings are the following. (1) The magnetic field changes little in configuration and strength. (2) The magnetic forces will compress the gas by the factor 2π times the ratio (Alfvén speed)/(sound speed), a factor that can easily lie in the range 100–1000. (3) As the gas moves toward the valleys, its motion becomes supersonic but not super-Alfvénic. (4) A polarization map of the Canis Major R1 region should find a magnetic field directed from one stellar cluster to the next, parallel to the shock front. The quantitative estimates support the earlier, perturbative reasoning about supernova-induced stellar clustering. Moreover, the non-linear calculations contribute to our general understanding of the Parker instability, this time in the domain of magnetic dominance: the magnetic pressure dominates thoroughly over the gas pressure, but the latter's influence may not be ignored.

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