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

TL;DR

This study models interstellar dust properties causing steep extinction curves with low Rv ratios, revealing that smaller grain sizes and specific size distributions explain the unusual extinction laws observed.

Contribution

It demonstrates that simple power-law and lognormal dust models with smaller grain sizes can explain low Rv extinction curves, advancing understanding of interstellar dust characteristics.

Findings

Steep extinction curves are explained by smaller grain sizes (a_max < 0.24 um).

Both power-law and lognormal models fit the data with appropriate parameters.

Ultraviolet data are crucial for constraining dust composition and size distribution.

Abstract

It is well known that the extinction properties along lines of sight to Type Ia supernovae are described by steep extinction curves with unusually low total-to-selective extinction ratios of Rv = 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 Rv = 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} = 0.13 um, 0.094 um, and 0.057 um, respectively. These maximum cut-off radii are smaller than a_{max} ~ 0.24 um 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 Rv = 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.