EXTREME-INFRARED (800 MU-M) POLARIMETRY OF THE M17-SW MOLECULAR CLOUDWITH THE JCMT

Extreme-infrared (800 mu m) linear polarization observations have been obtained at the JCMT on Mauna Kea in Hawaii, toward the elongated mol ecular cloud M17-SW located at a distance of 2.2 kpc. The six stronges t dust peaks in total continuum intensity were observed. From the tota l intensity continuum observations, we find that the typical dust peak s in M17-SW have a mean size of 0.24 pc, a mean density of 3.7 x 10(5) cm(-3), and a mean mass of 245 M.. From the polarimetric observations , we measure a mean polarized E-vector amplitude of 2.0% +/- 0.3%, and we find that the mean E-vector position angle at extreme IR wavelengt hs is at PA = 169 degrees +/- 19 degrees. This mean E-vector PA is mos tly parallel to the cloud elongation, which is at PA = 158 degrees +/- 10 degrees. It follows that the mean magnetic field vector is at PA = 79 degrees +/- 19 degrees, implying that the cloud's magnetic field i s mostly perpendicular to the cloud's elongation. We have grouped the theoretical models for the magnetic fields in molecular clouds into el even 'magnetic classes', according to the shape and the scale of the m agnetic field involved. Comparisons are made between the predictions f rom these 11 classes and the JCMT observations for the B-vectors in th e cloud M17-SW. Nine of the 11 magnetic classes are unlikey to pertain to M17-SW. The other two magnetic classes are: the class with a 1-dim ensional cloud collapse along the localized magnetic field lines; and the class with the magnetic field vectors in the cloud being parallel to the surrounding magnetic field. These results suggest that a proper distance scale for the magnetic field involved is of the order of 10 pc to 100 pc outside the cloud (larger than the cloud size), and of th e order of 1 pc to 10 pc inside the cloud M17-SW (larger than the chim p sizes of 0.2 pc). Two scenarios for the origin of the dust peaks in M17-SW are examined. The 'sequential star formation' scenario is suppo rted, and our polarization data and other data cast doubts on the 'rad ially expanding shell' scenario for M17-SW.