Evolution of very small particles in the southern part of Orion B observed by ISOCAM
A. Abergel, J. P. Bernard, F. Boulanger, D. Cesarsky, E. Falgarone, A. Jones, M. A. Miville-Deschênes, M. Pérault, J. L. Puget, M. Huldtgren, A. A. Kaas, L. Nordh, G. Olofsson, P. André, S. Bontemps, M. M. Casali, C. J. Cesarsky, M. E. Copet, J. Davies, T. Montmerle, P. Persi, F. Sibille;
AaA, 2002, 389, 239
ABSTRACT:We present ISOCAM observations (5-18 mu m) of the southern part of Orion B, including the reflection nebula NGC 2023 and the Horsehead nebula
illuminated by the B star HD 37903 and the O star sigma Orionis, respectively.
Due to
the limited radiation field, the emission in these wavelengths is due to
very small particles which are heated each time they absorb a UV photon.
A
filamentary structure is detected at small angular scales (down to the angular
resolution of 6'') on top of a smoother background.
The particular
case of the Horsehead nebula suggests that the filaments in general result
from the illuminated surfaces of dense structures, while the smoother
background comes from lower density matters probably ionised.
Striking spatial
variations of the infrared colour (5-8.5 mu m/12-18 mu m) are also
detected.
Spectroscopic observations show that they are due to variations of the intensity of
the aromatic features (especially at 7.7 mu m) relative to a continuum
emission increasing in intensity towards longer wavelengths.
The
contribution of the continuum relative to the aromatic features appears
significantly higher at the illuminated surfaces of dense structures than in lower
density matter.
This effect could be the signature of the evolution of the very
small particles from shielded molecular material to photo-dissociated and
photo-ionised matter.
We also show that size segregation due to grain dynamics in
uni-directional radiation fields may play a major role.
KEYWORDS: methods: data analysis, ism: individual objects: orion, ism: dust, extinction, ism: clouds, ism: structure, infrared: ism
CODE: abergel2002