Infrared properties of molecular cirrus. II. Cloud-to-cloud variations in graphite and polycyclic aromatic hydrocarbon content

We have conducted a photometric survey of translucent molecular clouds in t he four IRAS wave bands. We find notable cloud-to-cloud variations in mid-I R emission, with the ratio I,,II,, varying by up to 1 order of magnitude an d often above unity. Because the clouds in our sample are nearby and transl ucent? the cloud heating is well constrained, so that the observed differen ces in infrared emission must represent differences in grain content. We show from first principles that a dust model containing only two compone nts, Mathis-Rumpl-Nordsieck (MRN) grains in thermal equilibrium and very sm all grains undergoing stochastic heating, can never produce I-12/I-25 great er than or equal to 1 under the conditions that prevail in the cloudy inter stellar medium. We fit the clouds in this sample with a three-component model which contain s continuum emission from both MRN grains and very small graphite grains, p lus emission features from a mixture of polycyclic aromatic hydrocarbon (PA H). The cloud-to-cloud variations in mid-IR emission require significant di fferences in their composition of small grains and PAH molecules. Models wh ich describe the formation and evolution of these particles in the interste llar medium must explain the presence of these variations in a cloud sample which is located in the solar neighborhood.