Properties of interstellar dust responsible for extinction laws with unusually low total-to-selective extinction ratios of RV=1-2

It is well known that the extinction properties along lines of sight to Type Ia supernovae (SNe Ia) are described by steep extinction curves with unusually low total-to-selective extinction ratios of R-v = 1.0-2.0. In order to reveal the properties of interstellar dust that causes such peculiar extinction laws, we perform the fitting calculations to the measured extinction curves by applying a two-component dust model composed of graphite and silicate. As for the size distribution of grains, we consider two function forms of the power-law and lognormal distributions. We find that the steep extinction curves derived from the one-parameter formula by Cardelli et al. (1989) with R-v = 2.0, 1.5, and 1.0 can be reasonably explained even by the simple power-law dust model that has a fixed power index of -3.5 with the maximum cut-off radii of a(max) similar or equal to 0.13 mu m, 0.094 mu m, and 0.057 mu m, respectively. These maximum cutoff radii are smaller than a(max) similar or equal to 0.24 mu m considered to be valid in the Milky Way, clearly demonstrating that the interstellar dust responsible for steep extinction curves is highly biased to smaller sizes. We show that the lognomal size distribution can also lead to good fits to the extinction curves with R-V = 1.0-3.1 by taking the appropriate combinations of the relevant parameters. We discuss that the extinction data at ultraviolet wavelengths are essential for constraining the composition and size distribution of interstellar dust. (C) 2016 Elsevier Ltd. All rights reserved.