1996A&A...313..255V

Extreme-infrared (800µm) linear polarization observations have been obtained at the JCMT on Mauna Kea in Hawaii, toward the elongated molecular cloud M17-SW located at a distance of 2.2kpc. The six strongest dust peaks in total continuum intensity were observed. From the total intensity continuum observations, we find that the typical dust peaks in M17-SW have a mean size of 0.24pc, a mean density of 3.7x10⁵cm^–3, and a mean mass of 245M_☉. 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 wavelengths is at PA=169±19deg. This mean E-vector PA is mostly parallel to the cloud elongation, which is at PA=158±10deg follows that the mean magnetic field vector is at PA=79±19deg, implying that the cloud's magnetic field is mostly perpendicular to the cloud's elongation. We have grouped the theoretical models for the magnetic fields in molecular clouds into eleven `magnetic classes', according to the shape and the scale of the magnetic field involved. Comparisons are made between the predictions from these 11 classes and the JCMT observations for the B-vectors in the 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-dimensional 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 10pc to 100pc outside the cloud (larger than the cloud size), and of the order of 1pc to 10pc inside the cloud M17-SW (larger than the clump sizes of 0.2pc). Two scenarios for the origin of the dust peaks in M17-SW are examined. The `sequential star formation' scenario is supported, and our polarization data and other data cast doubts on the `radially expanding shell' scenario for M17-SW.