Submillimeter Imaging Polarimetry of the NGC 7538 Region

Abstract

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 diUerences. 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 diUerences suggest that small scale —uctuations of the magnetic —eld are more prominent in IRS 1(SMM). This can be interpreted in terms of the diUerence 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 —uctuations in the magnetic —eld could have developed during the formation of these substructures. The distribution of magnetic —eld directions derived from our polarization map agrees well with those of molecular out—ows associated with IRS 1(SMM) and IRS 11(SMM). Comparisons of energy densities between the magnetic —eld and the out—ows show that the magnetic —eld probably plays an important role in guiding the directions of the out—ows.