Hubble Space TelescopeNICMOS Polarization Measurements of OMC‐1

Abstract

We present 2 m polarization measurements of positions in the IRc2 and BN regions of the Orion Molecular Cloud (OMC-1) made with NICMOS Camera 2 (0B2 resolution) on the Hubble Space Telescope. Our results are as follows: BN is $29% polarized by dichroic absorption and appears to be the illuminating source for most of the nebulosity to its north and for up to $5 00 to its south. Although the stars are probably all polarized by dichroic absorption, there are a number of compact but non-point-source objects that could be polarized by a combination of both dichroic absorption and local scattering of starlight. We identify several candidate YSOs, including an approximately edge-on bipolar YSO 8B7 east of BN, and a deeply embedded variable star. Additional strongly polarized sources are IRc2-B, IRc2-D, and IRc7, all of which are obviously self-luminous at mid-infrared wavelengths and may be YSOs. None of these is a reflection nebula illuminated by a star located near radio source I, as was previously suggested. Other IRc sources are clearly reflection nebulae: IRc3 appears to be illuminated by IRc2-B or a combination of the IRc2 sources, and IRc4 and IRc5 appear to be illuminated by an unseen star in the vicinity of radio source I, or by star n or IRc2-A. Trends in the magnetic field direction are inferred from the polarization of the 26 stars that are bright enough to be seen as NICMOS point sources. The most polarized star has a polarization position angle different from its neighbors by $40, but in agreement with the grain alignment inferred from millimeter polarization measurements of the cold dust cloud in the southern part of OMC-1.

3D model of bow shocks

“…This is in excellent agreement with the results in Kristensen et al (2008) and with polarization measurements that indicate a PA ∼ 140 • in this region (e.g. Hough et al 1986;Chrysostomou et al 1994;Simpson et al 2006;Tamura et al 2006). The b = 3 value of our best fit model is lower than the b = 4.8 estimated by Kristensen et al (2008) and the scaling factor b 7 inferred by Chrysostomou et al (1994), but we note that the latter was estimated in a slightly different region (southeast of Irc2) and could be uncertain by a factor 2.…”

“…The direction of the magnetic field is consistent with measurements of polarized light that indicate that the magnetic field has a position angle of ∼140 • (e.g. Hough et al 1986;Chrysostomou et al 1994;Simpson et al 2006;Tamura et al 2006). While the 2D model is successful in estimating the direction of the magnetic field it is not capable of simultaneously reproducing the width of the emission and the brightness.…”

Near-Infrared Polarization Images of the Orion Nebula

“…Imaging polarimetry has been conducted either only at optical wavelengths (Pallister et al 1977) or toward a small region near IRc2 or BN at near-infrared wavelengths (Minchin et al 1991;Jiang et al 2005;Simpson et al 2006). In this Letter, we present for the first time the nearinfrared polarization images of a ∼1 pc region of M42 and describe newly detected features seen as infrared nebulae on various scales.…”

Complex Organic Molecules at High Spatial Resolution Toward Orion-Kl. I. Spatial Scales

“…In Figure 9(a), the contours are centered very close to star n. NH 3 emission has also been detected toward this source at a similar rest velocity (∼−4 km s −1 ) by Migenes et al (1989). This indicates that the region has a high density, and it is possible that there is a dust shell or torus surrounding the star (Simpson et al 2006;Migenes et al 1989;Greenhill et al 2004). Figure 9(b) shows several strong concentrations of C 2 H 5 CN emission, the strongest of which is the peak to the east of the Hot Core and SMA1.…”