The Extinction Curve in the Visible and the Value of Rv. Part II: Addendum to AN 333, 160

TL;DR

This study refines the understanding of the interstellar extinction curve in the visible and infrared, suggesting a near-linear power law with p close to 1 and exploring its implications for Rv and dust properties.

Contribution

It provides a corrected, continuous power law model for the extinction curve and analyzes how slight variations in the power index p affect Rv and dust extinction interpretations.

Findings

Extinction law proportional to 1/λ^p with p ≈ 1 fits observations.

Values of p > 1 explain increased infrared extinction beyond 1 μm.

Rv varies from 4.04 to 2.76 as p increases from 1 to 1.4.

Abstract

This paper corrects and completes a previous study of the shape of the extinction curve in the visible and the value of Rv. A continuous visible/infrared extinction law proportional to 1/{\lambda}^p with p close to 1 ({\pm}0.4) is indistinguishable from a perfectly linear law (p = 1) in the visible within observational precision, but the shape of the curve in the infrared can be substantially modified. Values of p slightly larger than 1 would account for the increase of extinction (compared to the p = 1 law) reported for {\lambda} > 1{\mu}m and deeply affect the value of Rv. In the absence of gray extinction Rv must be 4.04 if p = 1. It becomes 3.14 for p = 1.25, 3.00 for p = 1.30, and 2.76 for p = 1.40. Values of p near 1.3 are also attributed to extinction by atmospheric aerosols, which indicates that both phenomena may be governed by similar particle size distributions. A power extinction law may harmonize visible and infrared data into a single, continuous, and universal, interstellar extinction law.