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G.A. Topasna, B. Dennison, J.H. Simonetti (Virginia Tech)
Polarimetric CCD images of the Rosette Nebula (NGC 2237-9) were obtained using a rotating polarizer device designed for use in conjunction with the Spectral Line Imaging Camera at Virginia Tech's Martin Observatory. Using a narrow bandpass interference filter, coupled with a 58 mm camera lens and cryogenically-cooled CCD, diffuse H\alpha emission over a 10-degree angular extent was imaged. Stoke's I, Q, and U images were calculated and a polarization map of the Rosette Nebula was made.
The observed polarization reaches 10 percent in the northwest quadrant at a distance greater than 40 arcminutes from the center of the nebula. Using the empirical relationship between maximum polarization and color excess predicts that observed polarization by selective extinction should be no more than 3 to 4 percent. A radial comparison of [SII] and H\alpha intensities in the scattering region suggests that the H\alpha is dominated by scattered H\alpha light from the Rosette Nebula. A correlation was found to exist between the H\alpha intensity and infrared emission by dust grains in all four IRAS waveband images in the scattering region. Scattered continuum light from the central star cluster (NGC 2244) in the H\alpha bandpass was ruled out.
The evidence suggests that the H\alpha intensity observed in the northwest quadrant of the Rosette Nebula is dominated by H\alpha light from the Rosette Nebula which has been scattered by dust grains in that region. Thus, in H\alpha, the Rosette Nebula appears larger than it actually is. Polarimetric images of the North America Nebula (NGC 7000), which were also obtained and are currently being analyzed, also support the hypothesis that scattering may enhance the apparent sizes of HII regions in H\alpha.