A test for the universality of extinction curve shape

Abstract

We present an analysis of 436 lines of sight with extinction data covering wavelength range from near-infrared (NIR) to ultraviolet (UV). We use J, H, K photometry from 2MASS database, the IR intrinsic colors from Wegner (1994), and UV extinction data from Wegner (2002). We exclude 19 lines of sight (4%) from the original sample because of suspected photometric problems. We derive total to selective extinction ratios (R_V) based on the Cardelli, Clayton, and Mathis (1989; CCM) law, which is typically used to fit the extinction data for both diffuse and dense interstellar medium. We conclude that CCM law is able to fit well most of the extinction curves in our sample (71%), and we present a catalog of R_V and visual extinction (A_V) values for those cases. We divide the remaining lines of sight with peculiar extinction into two groups according to two main behaviors: a) the NIR or/and UV wavelength regions cannot be reproduced by CCM formula (14% of the entire sample), b) the NIR and UV extinction data taken separately are best fit by CCM laws with significantly different values of R_V (10% of the entire sample). We present examples of such curves. We caution that some peculiarities of the extinction curves may not be intrinsic but simply caused by faulty data. The study of the intrinsically peculiar cases could help us to learn about the physical processes that affect dust in the interstellar medium, e.g., formation of mantles on the surface of grains, evaporation, growing or shattering.