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Magnetic fields, stellar feedback, and the geometry of H II regions

Published online by Cambridge University Press:  01 November 2008

Gary J. Ferland
Gary J. Ferland*
Affiliation:
Department of Physics & Astronomy, University of Kentucky, Lexington, KY 40506, USA email: [email protected]
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Abstract

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Magnetic pressure has long been known to dominate over gas pressure in atomic and molecular regions of the interstellar medium. Here I review several recent observational studies of the relationships between the H+, H0 and H2 regions in M42 (the Orion complex) and M17. A simple picture results. When stars form they push back surrounding material, mainly through the outward momentum of starlight acting on grains, and field lines are dragged with the gas due to flux freezing. The magnetic field is compressed and the magnetic pressure increases until it is able to resist further expansion and the system comes into approximate magnetostatic equilibrium. Magnetic field lines can be preferentially aligned perpendicular to the long axis of quiescent cloud before stars form. After star formation and pushback occurs ionized gas will be constrained to flow along field lines and escape from the system along directions perpendicular to the long axis. The magnetic field may play other roles in the physics of the H II region and associated PDR. Cosmic rays may be enhanced along with the field and provide additional heating of atomic and molecular material. Wave motions may be associated with the field and contribute a component of turbulence to observed line profiles.

Keywords

ISM: magnetic fields – HII regions – ISM: individual (M17,M42) – cosmic rays – ISM: molecules
Type
Contributed Papers
Information
Proceedings of the International Astronomical Union , Volume 4 , Symposium S259: Cosmic Magnetic Fields: From Planets, to Stars and Galaxies , November 2008 , pp. 25 - 34
Copyright
Copyright © International Astronomical Union 2009

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