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 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.

The magnetic field structure in W51A

“…The apparant curvature of the field and the manner in which the field aligns with the velocity gradient in the CS bridge can be interpreted as evidence that the magnetic field is being shaped by the pressure and dynamics of the gas. Similar magnetic field behaviour has been seen in recent SCUBA polarimetry of NGC 7538 (Momose et al 2001) where the blueand red-shifted outflow lobes from IRS11 are observed to have quite a high degree of curvature. The magnetic field inferred from the polarimetry follows the same curvature.…”

“…We choose to briefly discuss these features here because simulations do show that such polarisation patterns are possible for pinched and twisted field morphologies (Aitken et al 2001) as well as for helical fields (Fiege & Pudritz 2000), although the latter deals with linear structures and is not really applicable to discrete sources. Furthermore, such "submillimetre polarisation nulls" either side of a bright core have been previously observed towards NGC7538 IRS1 (Momose et al 2001). In both of these cases, the field pattern appears very much like the familiar polarisation disks seen in scattered light, at nearinfrared and optical wavelengths, around sources with optically thick disks (e.g.…”

“…Zhang et al (1998) present evidence which suggest that cores within IRS1 are indeed collapsing and rotating. However, as a caveat to this it should be noted that this trend of low degrees of polarisation for high submillimetre intensities is not universal as can be seen in recent submillimetre polarimetric imaging of the NGC 7538 region (Momose et al 2001) where the degree of polarisation remains relatively high towards the cores.…”

The properties and polarization of the H2O and CH3OH maser environment of NGC 7538-IRS 1

“…Although the polarization vectors are locally disturbed, at an angular resolution of 14 arcsec the magnetic field directions agree with the direction of the outflow (Momose et al 2001), while at milliarcsecond (mas) resolution the OH maser observations indicate a magnetic field oriented orthogonal to the outflow (Hutawarakorn & Cohen 2003). Because H 2 O and CH 3 OH masers were detected close to the center of the continuum emission, i.e.…”

Magnetic alignment of nonmagnetic silicates caused by paramagnetic anisotropy: Origin of polarization observed in planetary formation region

“…In order to assess this hypothesis on planetary evolution, it is necessary to carry out high-spatial-resolution polarimetery mapping (about 1 AU). Near infrared (Lucas et al, 2004) and sub-millimeter (Tamura et al, 1999) imaging polarimetry was recently reported for YSO regions as well as for massive young objects (Momose et al, 2001). Spatial resolutions in these observations were about 100 AU although the data indicate that partial magnetic alignment of dust particles does take place in these systems.…”