Predicting peculiar interstellar extinction from gaseous abundances

Molecular and atomic abundances are examined for 19 lines of sight through dense clouds, each with a peculiar selective extinction curve. The interstellar clouds appear to fall into two distinct categories: CN-rich, with relatively small amounts of neutral iron, or CN-poor, with large amounts of neutral iron. Lines of sight having a CN/(Fe I) abundance ratio about two or greater, are found to have a shallow 2175 A feature relative to the underlying extinction, while the strength of bump is 3.60 + or - 0.36 for the other dense clouds in the present study. The difference is the strength of the extinction bump between these two ensembles is 1.03 + or - 0.23. Several atomic abundances are examined as potential indicators of peculiar extinction. The Mn I upper limits in the present study support that Mn is being preferentially depleted. Fe I is underabundant relative to K I by 0.7 (dex) in the large (CN)/(Fe I) compared to the small (CN)/(Fe I) lines of sight.