Submillimeter Imaging Polarimetry of the Ngc 7538 Region

Imaging polarimetry of the 850 km continuum emission in the NGC 7538 region, obtained with the SCUBA Polarimeter, is presented. The polarization map is interpreted in terms of thermal radiation by magnetically aligned dust grains. Two prominent cores associated with IRS 1 and IRS 11, IRS 1(SMM), and IRS 11(SMM), are found in the surface brightness map. Although these cores look similar in surface brightness, their polarization shows striking di†erences. In IRS 11(SMM), the polarization vectors are extremely well-ordered, and the degrees of polarization are quite high with an average of D3.9%. In IRS 1(SMM), on the other hand, the directions of polarization vectors are locally disturbed, and the degrees of polarization are much lower than those of IRS 11(SMM). These di†erences suggest that small scale Ñuctuations of the magnetic Ðeld are more prominent in IRS 1(SMM). This can be interpreted in terms of the di†erence in evolutionary stage of the cores. Inside IRS 1(SMM), which seems to be at a later evolutionary stage than IRS 11(SMM), substructures such as subclumps or a cluster of infrared sources have already formed. Small scale Ñuctuations in the magnetic Ðeld could have developed during the formation of these substructures. The distribution of magnetic Ðeld directions derived from our polarization map agrees well with those of molecular outÑows associated with IRS 1(SMM) and IRS 11(SMM). Comparisons of energy densities between the magnetic Ðeld and the outÑows show that the magnetic Ðeld probably plays an important role in guiding the directions of the outÑows. Subject headings : instrumentation : polarimeters È ISM: clouds È ISM: individual (NGC 7538) È ISM: magnetic Ðelds È stars : formation È submillimeter On-line material : machine-readable table