Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992aas...181.4803c&link_type=abstract
American Astronomical Society, 181st AAS Meeting, #48.03; Bulletin of the American Astronomical Society, Vol. 24, p.1198
Astronomy and Astrophysics
Astronomy
2
Scientific paper
Detailed comparisons between new wide-field CCD images of high galactic latitude cirrus fields and cleaned and coadded IRAS Skyflux maps of the same areas reveal variations in the optical and infrared colors within selected clouds that are difficult to understand in the context of current dust models. These color discrepancies include: 1) The brightest blue emission from the clouds often does not correspond to the peak of the 100microns emission, as might be expected if the warm (T ~ 25K) grains that produce the bulk of the 60 and 100microns emission are the primary scatterers of interstellar light. Paley et al. (1991 ApJ, 376, 335) first noted variations in the ratio of 100microns to blue light within the cirrus, and they suggested that the variations were produced by the presence of an inhomogeneously mixed population of very cold grains (T<15K) that scatter very efficiently. 2) When it is detected, the 12 and 25mu emission in the clouds is sometimes displaced from the bulk of the 60 and 100microns light. The mid-infrared emission from the cirrus exhibits a much warmer color temperature (T ~ 175K) than that at longer wavelengths, and has been suggested to arise from transiently heated very small dust grains or large molecules. 3) The peaks of the red and blue optical light are displaced in certain clouds. In the regions of peak red emission, the cirrus has colors far too red to be produced by simple scattering processes alone. Guhathakurta and Tyson (1989 ApJ, 356, 773) suggested that the excess red light arises from the luminescence of small hydrogenated amorphous carbon grains, as is observed in bright reflection nebulae. 4) The brightest red emission does not appear to correspond to the regions of brightest 12 and 25microns emission, as might be expected if both arose from the small grains or molecules. If the cirrus clouds are optically thin, these color variations imply that the relative line-of-sight abundances of the "classical" silicate and graphite grains, the cold grains, and the very small transiently heated grains and molecules must be non-uniform within individual clouds and possibly from cloud-to-cloud.
Cutri Roc M.
Guhathakurta Puraga
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