[105.13] OMC-1 as Revealed by HST NICMOS Polarization Measurements

AAS 205th Meeting, 9-13 January 2005
Session 105 Triggered Star Formation and the Orion Protostellar Population
Poster, Wednesday, January 12, 2005, 9:20am-6:30pm, Exhibit Hall

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[105.13] OMC-1 as Revealed by HST NICMOS Polarization Measurements

J. P. Simpson (SETI Inst.), M. G. Burton (UNSW), S. W. J. Colgan, E. F. Erickson (NASA ARC), A. S. B. Schultz (SETI Inst.), E. Simpson (SRI Int.)

The Orion Molecular Cloud (OMC-1) harbors the nearest and most studied massive star-forming region. Signs of the formation of multiple stars in this optically obscured region include powerful CO outflows, H₂O and SiO maser emission, remarkable H₂ "bullets", "fingers", and "streamers", and X-rays from pre-main-sequence stars. Highly polarized clouds indicate that the illuminating sources lie in the directions of the Becklin-Neugebauer object (BN), and stars in the vicinity of IRc2, radio source I, NIR source n, and others. Here we present 2 \mum polarization measurements of positions north and south of BN made with NICMOS Camera 2 on the Hubble Space Telescope. Near-infrared starlight can be polarized by scattering from nearby dust grains and by dichroic absorption by non-spherical dust grains aligned by a magnetic field. Within the 19'' field of view of Camera 2, BN appears to be the illuminating source of most of the nebulosity to its north; however, the material to the south is illuminated either by a star near I (IRc4) or by source n (IRc2B). Source n also illuminates material 1'' - 2'' to its northeast and southwest, at the same position angles as the extended radio source at the same location. We discuss possible interpretations of the strong polarization of IRc7, which is not illuminated by source I. We also display several stars (NICMOS point sources) that are the source of their own polarization, which ranges up to 40% and occurs at distinctly different angles from the polarization of the immediately surrounding diffuse emission. This may be caused by dichroic absorption and scattering in edge-on circumstellar disks. At least two faint stars are variable.

Support for proposal 9752 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 NAS5-26555.

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