Using HST NICMOS Polarization to Characterize the Magnetic Field Structure of AFGL 2591

AFGL 2591 is a massive young stellar object (YSO) with a luminosity of 2-3 times 10^5 solar luminosities at a distance of 3.33 kpc. It has a single lobe of scattered light to the west, with its east lobe appearing only as a bow shock in molecular hydrogen. The west lobe almost certainly defines the outflow axis, also seen in blue-shifted CO, but the east lobe is completely obscured by the extended dense envelope and massive accretion toroid or disk. We are studying the massive YSOs in the nearest star-forming regions using 2 micron polarimetry with the 0.2" resolution of the Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) on Hubble Space Telescope (HST). Visible and near-infrared polarization of extended sources is caused by light scattered from dust grains, which can be either spherical or non-spherical. Polarization of point sources is caused by differential absorption of the components of light perpendicular and parallel to the major axis of any elongated grains in the light path. It is generally believed that the spin axis of the elongated grains is aligned parallel to the local magnetic field. In AFGL 2591 the extended emission of the outflow more than a few arcsec from the YSO is highly polarized with the polarization position angles indicating that the dust is illuminated by the YSO. The polarization of many of the stars in the field of view and the YSO itself indicates polarization due to absorption by aligned grains oriented in various directions. In particular, the polarization of the extended emission in the central few arcsec is not in agreement with that expected by scattering but is heavily affected by contributions from absorptive polarization. From the absorptive polarization we infer that the grains are aligned in the AFGL 2591 toroid and in the outflow; this is consistent with both a toroidal magnetic field and a field parallel to the outflow. We will discuss additional sources with similar polarization morphology. Support for program 10519 was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.