Structure, star formation and magnetic fields in the OMC1 region

We present the results of a study of the submillimetre wavelength continuum emission, at 450 and 850 mu m, encompassing the OMC1 region in the norther n part of the Orion A cloud, and focusing on the structure and star formati on in an area of about similar to 70 arcmin(2) (similar to 1.2 pc(2)). Our observations are 3 times deeper in flux than previous submillimetre observa tions of this region and we have found a number of pre-stellar dust clumps in the region from which mass functions were determined. Our clump mass fun ctions include objects down to 0.1 M-circle dot and the power-law slope of -1.5 is similar to that generally found from spectral line observations of molecular gas clumps. The data do not show the steeper slope of -2.5 for ma sses above 0.5 M-circle dot identified by Motte et al. (1998) in dust maps of the rho Ophiuchus low-mass star-forming region, possibly indicating diff erent collapse processes. Polarimetry data for the north-east bar and KL re gions of OMC1 were also obtained, and it is apparent that the field orienta tions with respect to cloud elongation differ between star-forming and non- star-forming regions. The main OMC1 ridge is consistent with collapse down held lines while the north-east bar has a field structure roughly outlining the HII region. The dominant physical processes apear to be pressure from the expanding HII region and fragmentation on the Jeans scale, rather than magnetic effects.