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Molecular and atomic abundances are examined for 19 lines of sight through dense clouds, each with a peculiar selective extinction curve. The interstellar clouds in the present study 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 (~ 0.3 dex) or greater, are found to have a shallow (2.57 ± 0.55 mag) 2175 Å feature relative to the underlying extinction, while the strength of the bump is 3.60 ± 0.36 for the other dense clouds in the present study. The difference in the strength of the extinction bump between these two ensembles is 1.03 ± 0.23. Several atomic abundances are examined as potential indicators of peculiar extinction. Mn i abundances in particular are sought at 10 times greater sensitivity than previous studies because of a possible empirical connection between a small (Mn ii)/(Fe ii) abundance ratio and a weak 2175 Å bump reported in the literature. Unfortunately, the abundances of the neutral atoms do not appear to scale with the abundance of CN, reducing the effectiveness of Mn i as a diagnostic tool. Nevertheless, the Mn i upper limits in the present study support Mn 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. In addition, the data suggest that potassium is substantially depleted in both types of dense clouds

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Astrophys. J.


Copyright 1989 American Astronomical Society

Erratum: Predicting peculiar interstellar extinction from gaseous abundances [Astrophys. J., 340, 314 (1989)]
Charles L. Joseph et al.
Astrophys. J. 347, 561 (1989)