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 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.
