Infrared Properties of Molecular Cirrus. II. Cloud-to-Cloud Variations in Graphite and Polycyclic Aromatic Hydrocarbon Content
F. Verter, L. Magnani, E. Dwek, L. J. Rickard;
ApJ, 2000, 536, 831
ABSTRACT:We have conducted a photometric survey of translucent molecular clouds in the four IRAS wave bands.
We find notable cloud-to-cloud
variations in mid-IR emission, with the ratio I12/I25
varying by up to 1 order of magnitude and often above unity.
Because the clouds
in our sample are nearby and translucent, the cloud heating is well
constrained, so that the observed differences in infrared emission must represent
differences in grain content.
We show from first principles that a dust model
containing only two components, Mathis-Rumpl-Nordsieck (MRN) grains in thermal
equilibrium and very small grains undergoing stochastic heating, can never
produce I12/I25>=1 under the conditions that
prevail in the cloudy interstellar medium.
We fit the clouds in this sample
with a three-component model which contains continuum emission from both
MRN grains and very small graphite grains, plus emission features from a
mixture of polycyclic aromatic hydrocarbon (PAH).
The cloud-to-cloud
variations in mid-IR emission require significant differences in their
composition of small grains and PAH molecules.
Models which describe the
formation and evolution of these particles in the interstellar medium must
explain the presence of these variations in a cloud sample which is located in
the solar neighborhood.
KEYWORDS: ism: dust, extinction, ism: clouds, ism: molecules
PERSOKEY:cirrus, fir, mir, iras, ,
CODE: verter2000