Testing New Ideas Regarding the Nature of Interstellar Extinction

TL;DR

This paper challenges existing models of Galactic interstellar extinction by analyzing reddening parameters and diffuse interstellar bands, proposing a new extinction law that impacts our understanding of the universe's scale.

Contribution

It introduces a revised model of interstellar extinction based on new observational data, contradicting previous relations and suggesting a different wavelength dependence.

Findings

Identifies a minimum R_V=2.82 at X=0.83, indicating a new extinction law.

Shows inconsistencies with Cardelli et al. relations across multiple wavelengths.

Proposes that forward-scattered starlight affects observed extinction parameters.

Abstract

The nature of Galactic interstellar extinction is tested using reddening line parameters for several fields in conjunction with equivalent widths $W(\lambda4430)$ for the diffuse interstellar band at $4430$ \AA. The Cardelli et al.$\;$relations [29] at infrared, optical, and ultraviolet wavelengths are inconsistent with the newly-derived quadratic variation of $R_V({\rm observed})$ on reddening slope $X$. A minimum of $R_V=2.82\pm0.06$ exists for $X=0.83\pm0.10$, 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 \AA$\;$feature and a correlation of normalized $\lambda4430$ 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 $R_V({\rm observed})$ 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.