R[SUB]nu[/SUB]-dependent optical and near-ultraviolet extinction

Abstract

We have derived extinctions A(lambda)/A(V) at the wavelengths of the uvby filters for 22 stars, with a range of values of Rnu, from the sample of Cardelli, Clayton, & Mathis (1989, hereafter CCM). We have fit these extinctions, and also UBVRIJHKL, IUE and ANS extinction measurements, with linear relations A(lambda)/A/(V) = a+b/Rnu and fit a and b as a function of x(=1/lambda) with polynomials to obtain an Rnu-dependent mean extinction law (A(x)/A(V) = a(x) + b(x)/Rnu)in the optical and near-ultraviolet (1.1/micrometer less than or equal to 3.3/micrometer). This law is virtually identical to the CCM extinction law for large values of Rnu(Rnu approximately 5) but is slightly lower in the near-ultraviolet for smaller Rnu (Rnu approximately 3). The extinction law presented here agrees much better with a high-resolution extinction curve for the diffuse interstellar medium (Rnu approximately 3.1), presented by Bastiaansen (1992), than CCM. The deviations of individual extinction curves from the mean are dominated by observational errors. The wavelength resolution of this work is not high enough to show evidence for or against the existence of very broad structure in optical extinction curves.