SUBMILLIMETER POLARIMETRIC MAPPING OF DR-21 AND NGC-7538-IRS-11 - TRACING THE CIRCUMSTELLAR MAGNETIC-FIELD

We have detected 800mum polarization at the flux peak and positions ar ound the outflow sources DR 21 and NGC 7538-IRS 11. For both sources t he direction of the polarization vectors is extremely uniform and the dispersion is small. This implies that, within the reslution of our ob servations, the magnetic field is both uniform in direction and strong . The direction of the magnetic field around DR 21 does not appear to be aligned with either the direction of the outflow axis or the major axis of the submm circumstellar dust structure. For NGC 7538-IRS 11 th e magnetic field is aligned with the outflow axis, implying that withi n the resolution of our observations (14 arcsec FWHM) the circumstella r magnetic field is poloidal. The direction of the molecular outflows, magnetic fields and dust ridges for both IRS 11 and its neighbour IRS 1 (60 arcsec/0.9 parsec to the north) are in identical directions. As IRS 1 and IRS 11 are linked by an arm of submm emission and form an e longated dust ridge that is orthogonal to the ambient magnetic field o f the NGC 7538 region, part of the cloud may have collapsed along the magnetic field lines to produce the core from which IRS1 and IRS 11 ha ve formed. For both DR 21 and NGC 7538-IRS 11 the percentage polarizat ion at the position of the flux peak is low compared to the surroundin g points. This has been noted for several other outflow sources and is commonly referred to as a polarization 'hole'. The most plausible exp lanation is a change in the magnetic field alignment in the vicinity o f the outflow source. There is a marked increase in the observed perce ntage polarization from DR 21 at higher wavelengths. This implies the grain composition cannot be predominantly silicate, but instead is mai nly graphite/metallic, and may require grains of different magnetic su sceptibility or varying elongation to also be at different temperature s along the observed line of sight.