EPoS Contribution
EPoS Contribution
Magnetic fields in an Orion PDR and the earliest phases of star formation

Thomas Troland
U Kentucky, Lexington KY, US
New images (Fig. left) of the line-of-sight magnetic field in the Orion Veil, a PDR in front of the Orion Nebula, reveal aspects of the earliest phases of star formation. One of the HI velocity components in which the 21 cm Zeeman effect has been detected appears to be undisturbed by the star formation process. Hence, its present magnetic field strength (50 microgauss) may reflect conditions in the diffuse gas that eventually formed OMC-1. In particular, the ratio B/N(H) for the HI gas of the Veil (Fig. right, red and blue dots) is about 4 times higher than it is in other gas sampled by the Zeeman effect at similar N(H). See Fig right, black dots. We argue that the high ratios B/N(H) in the Veil (equivalently, the low mass-to-flux ratios) can be ascribed to the effects of magnetic reconnection diffusion in the diffuse gas that led to OMC-1. Specifically, conditions of lower than normal turbulence in the diffuse gas could have led to the onset of gravitational contraction at lower values of n(H) and N(H). If so, higher ratios B/n(H) and B/N(H) are expected in OMC-1 than in other GMC environments. This difference results from the lower efficiency of reconnection diffusion in a low turbulence environment. Lower turbulence in the diffuse gas that gave rise to OMC-1 may result from its location 150 pc above the galactic midplane. At such a position, the injection of mechanical energy into the diffuse gas from adjacent star forming regions may be less. In short, the earliest epochs of star formation, in which diffuse gas becomes self-gravitating, may be sensitive to the level of turbulence in the local diffuse gas. The level of turbulence is likely to vary with position in the Galaxy, and it may be a function of distance from the galactic midplane.
Caption: (left) - Image of line-of-sight magnetic field strengths (colors and contours) in HI gas of the Orion Veil, a PDR directly in front of the Orion Nebula. Field strengths are based upon the 21 cm Zeeman effect in an HI component centered near 5 km/s. Contours are for -45 (white), -75 (red), -100 (green) & -150 microgauss (blue). The image extends only over the bright Huygens Region of the nebula. The image has been masked to include only those pixels for which the ratio of field strength to error is at least 3. The Trapezium stars are denoted by black symbols. (right) - Line-of-sight magnetic fields from Zeeman effect measurements (HI, OH, CN) plotted against H column densities. Original plot (black dots) taken from Crutcher (2012) who lists references. Red and blue dots indicate field strengths at various positions in HI gas of the Veil via the 21 cm Zeeman effect. The green dot represents the field strength in a molecular region of the Veil via the 18 cm OH Zeeman effect.
C. Brogan, NRAO, US
R. Crutcher, U Illinois, US
W. M.Goss, NRAO, US
Suggested Session: Magnetic Fields