A large Scale Helical Magnetic Field in OMC-1

9 December 2011

Frédérick Poidevin, P. Bastien, T. J. Jones, ApJ, 741, 112 (2011)

The role that magnetic fields play in star formation is still a matter of debate. The observed star formation rate is slower than expected from free-fall gravitational collapse. Supersonic turbulence and strong magnetic fields are the two main candidates proposed to regulate star formation, but it remains unclear from observations which mechanism dominates over the other one in supporting the cloud against collapse. 

The Orion Molecular Cloud 1 (OMC-1) is an example of a well studied region of high mass star formation. It is located behind the well-known Orion Nebula. Observations suggest that the effects of turbulence do not dominate over those of the magnetic field on a large spatial scale. On the other hand, inferring the three-dimensional structure of the uniform magnetic field from observations is, in general, a challenging problem in astronomy. In order to address this problem, we have combined several polarimetry data sets with other sources of information. Our analysis shows that a large scale magnetic field, possibly helical, could be wrapping the OMC-1 star-forming region. 

The schematic model that emerges from our analysis is shown in the figure. The 850 microns map of Johnstone & Bally (1999) is used to show the cold dust emission in OMC-1. The morphology of the large-scale magnetic field pervading OMC-1 is shown by the large-scale two-component structure. The foreground component of the magnetic field is shown by a full line, while the background component is shown by a dashed line.

The symmetry axis of the two large-scale magnetic field components is suggested by the vertical translucent line. The green-blue vectors drawn by eye show the mean orientation of the magnetic field probed in several subregions with 350 microns data. The black circles indicate regions where the foreground and background components probably cancel each other within OMC-1. The crosses (right) and head-on arrows (left) indicate the direction of the magnetic field with respect to the line-of-sight as suggested by Zeeman measurements. Crosses indicate magnetic fields pointing away from the observer and head-on arrows magnetic fields pointing toward the observer. The rotation that is produced when moving from the far region, i.e., OMC-1, toward us on the line-of-sight is illustrated by the blue arrows. 

Such results will be very useful for further modeling and to better understand the energy budget in the region.

A large Scale Helical Magnetic Field in OMC-1

 Figure courtesy of the ApJ