The Behavior of the Aromatic Features in M101 HII Regions: Evidence for Dust Processing

The aromatic features in M101 were studied spectroscopically and photometrically using observations from all three instruments on the Spitzer Space Telescope. The global SED of M101 shows strong aromatic feature (commonly called PAH feature) emission. The spatially resolved spectral and photometric measurements of the aromatic feature emission show strong variations with significantly weaker emission at larger radii. We compare these variations with changes in the ionization index (as measured by [NeIII]/[NeII] and [SIV/SIII], which we probe over the ranges 0.03-20 and 0.044-15 respectively) and metallicity (expressed as log(O/H)+12, which ranges from 8.1 to 8.8). Over these ranges, the spectroscopic equivalent widths of the aromatic features from 7 HII regions and the nucleus were found to correlate better with ionization index than with metallicity. This implies that the weakening of the aromatic emission in massive star forming regions is due primarily to processing of the dust grains in these environments, not to differences in how they form (although formation could still be important on a secondary basis). The behavior of the correlation between the aromatic feature equivalent widths and ionization index can be described as a constant equivalent width until a threshold in ionization index is reached ([NeIII]/[NeII] ~ 1), above which the equivalent widths decrease with a power law dependence. This behavior for M101 HII regions is also seen for the sample of starburst galaxies presented in the companion study of Engelbracht et al. (2008) which expands the range of [NeIII]/[NeII] ratios to 0.03-25 and log(O/H)+12 values to 7.1-8.8. The form of the correlation explains seemingly contradictory results present in the literature. [abridged]Comment: 20 pages, 18 figures, ApJ, in press, (version with full resolution figures at http://dirty.as.arizona.edu/~kgordon/papers/PS_files/m101_aromatics.pdf

Effects of structural and chemical disorders on the vis/UV spectra of carbonaceous interstellar grains

Contains fulltext : 111493.pdf (preprint version ) (Open Access

Steric and Electronic Effects of Ligand Substitution on Redox-Active Fe4S4-Based Coordination Polymers

One of the notable advantages of molecular materials is the ability to precisely tune structure, properties, and function via molecular substitutions. While many studies have demonstrated this principle with classic carboxylate‐based coordination polymers, there are comparatively fewer examples where systematic changes to sulfur‐based coordination polymers have been investigated. Here we present such a study on 1D coordination chains of redox‐activeiron-sulfur clusters linked by methylated 1,4‐benzene‐dithiolates. A series of new iron-sulfur based coordination polymers were synthesized with either 2,5‐dimethyl‐1,4‐benzenedithiol (DMBDT) or 2,3,5,6‐tetramethyl‐1,4‐benzenedithiol. The structures of these compounds have been characterized based on synchrotron Xraypowder diffraction while their chemical and physical properties have been characterized by techniques including X‐ray photoelectron spectroscopy, cyclic voltammetry and UV–visible spectroscopy. Methylation results in the general trend of increasing electron‐richness in the series, but the tetramethyl version exhibits unexpected properties arising from steric constraints. All these results highlight how substitutions on organic linkers can modulate electronic factors to fine‐tune the electronic structures of metal‐organic materials.</div