The extinction curve in the visible and the value of Rv

TL;DR

This paper examines the interstellar extinction curve in the visible spectrum, questioning previous estimates of Rv and suggesting a linear curve with a probable Rv value of 4, impacting how starlight extinction is understood.

Contribution

It provides a critical analysis of the visible extinction curve and reevaluates the typical Rv value, challenging previous indirect estimates and proposing a more natural value of 4.

Findings

Visible extinction curve is likely linear in the visible.

Indirect estimates of Rv from other observations are questionable.

R_v is most probably close to 4, not 3.

Abstract

This article discusses the interstellar extinction curve in the visible and the value of Rv. It is concluded that the visible extinction curve is likely to be linear in the visible, and that indirect estimates of Rv from tentative determinations of Av, infrared, or UV observations are questionable. There is currently no evidence of any variation of Rv with direction. If Rv is close to 3, as it has been inferred from mid-infrared data, starlight in the visible is extinguished by a factor F/F_0=(2.5exp{-2micron/lambda})^{E(B-V)} in the visible. But if the visible wavelength range alone is considered, 4 appears as its most natural and probable value, and F/F_0= exp{-2E(B-V)/lambda}.