Testing New Ideas Regarding the Nature of Interstellar Extinction

Abstract

The nature of Galactic interstellar extinction is tested using reddening line parameters for several fields in conjunction with equivalent widths $W(\lambda 4430)$ for the diffuse interstellar band at $4430$ Å. The Cardelli et al.relations [29] at infrared, optical, and ultraviolet wavelengths are inconsistent with the newly-derived quadratic variation of $R_V(\mathrm{observed})$ on reddening slope $X$. A minimum of $R_V=2.82\pm 0.06$ exists for $X=0.83\pm 0.10$, and is argued to represent true Galactic extinction described by $A(\lambda )\propto {\lambda }^{-1.375}$. It matches expectations for a new description of extinction in the infrared, optical, and ultraviolet by Zagury [32]. Additional consequences, reddened stars with no 2175 Åfeature and a correlation of normalized $\lambda 4430$ absorption with $X$, are not predicted by the Cardelli et al.relation [29]. Known variations in $X$ from 0.62 to 0.83, and corresponding variations in $R_V(\mathrm{observed})$ from 4.0 to 2.8, presumably result from forward-scattered starlight in the ultraviolet contaminating optical light of stars affected by dust extinction. A new understanding of the true nature of interstellar extinction is important for establishing an accurate picture of the extragalactic distance scale, which in turn is related to our understanding of the nature of the Universe.

arXiv commentTo appear in the CJP

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• Cepheid Abundances: Multiphase Results and Spatial Gradients
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• The complex stellar populations in the lines of sight to open clusters in the third Galactic quadrant
• Solving the distance discrepancy for the open cluster NGC 2453. The planetary nebula NGC 2452 is not a cluster member
• On the relationship between the continuum of interstellar extinction curves, the 2200 Å bump, and the diffuse interstellar bands