Infrared Emission from Interstellar Dust. III. The Small Magellanic Cloud

The infrared (IR) emission from interstellar dust in the Small MagellanicCloud (SMC) is modelled using a mixture of amorphous silicate and carbonaceousgrains, including a population of polycyclic aromatic hydrocarbon (PAH)molecules. (1) It is shown that this dust model is able to reproduce thespectral energy distribution from near-IR to far-IR for the entire SMC Barregion, provided the PAH abundance in the SMC Bar region is very low. (2) TheIR spectrum of the SMCB1#1 molecular cloud can also be reproduced by our dustmodel provided the PAH abundance is increased relative to the overall SMC Bar.The PAHs in SMCB1#1 incorporate ~3% of the SMC C abundance, compared to <0.4%in the SMC Bar. (3) The spectrum of SMCB1#1 is best reproduced if the PAHmixture has intrinsic IR band strengths which differ from the band strengthswhich best fit Milky Way PAH mixtures. The variation in the PAH IR bandstrengths would imply different PAH mixtures, presumably a consequence ofdiffering metallicity or environmental conditions. Other possibilities such assuper-hydrogenation of PAHs and softening of the starlight spectrum are alsodiscussed.Comment: 20 pages. Submitted to The Astrophysical Journa