Magnetic fields and massive star formation

Magnetic fields may be observed via the Zeeman effect, linear polarization of dust emission, and linear polarization of spectral-line emission. Useful parameters that can be inferred from observations are the mass-to-flux ratio M/Φ and the scaling of field strength with density. The former tells us whether magnetic fields exert sufficient pressure to provide support against gravitational contraction; the latter tells whether or not magnetic fields are sufficiently strong to determine the nature (spherical or disk geometry) of the contraction. Examples of massive star formation regions for which detailed observations have been made of magnetic field strengths and morphologies include DR21OH, OMC1, and S106; observational results for these regions and relevant results for the diffuse ISM and masers will be reviewed. Results are that the strength of interstellar magnetic fields remains invariant at B ∼ 6μG between 0.1 cm^{-3} cm^{-3}, but increases as B ∝ ρ^{0.4-0.5} for 10^3 cm^{-3} cm^{-3}. Moreover, M/Φ is significantly subcritical (strong B with respect to gravity) in diffuse H I clouds that are not self-gravitating, but becomes approximately critical in high-density molecular cloud cores. This suggests that GMCs form primarily by accumulation of matter along magnetic field lines, a process that will increase density but not magnetic field strength. How clumps in GMCs evolve will then depend critically on the M/Φ ratio in each clump.