Physical Dust Models for the Extinction toward Supernova 2014J in M82
  Type Ia supernovae (SNe Ia) are powerful cosmological "standardizable
candles" and the most precise distance indicators. However, a limiting factor
in their use for precision cosmology rests on our ability to correct for the
dust extinction toward them. SN 2014J in the starburst galaxy M82, the closest
detected SN~Ia in three decades, provides unparalleled opportunities to study
the dust extinction toward an SN Ia. In order to derive the extinction as a
function of wavelength, we model the color excesses toward SN 2014J, which are
observationally derived over a wide wavelength range in terms of dust models
consisting of a mixture of silicate and graphite. The resulting extinction laws
steeply rise toward the far ultraviolet, even steeper than that of the Small
Magellanic Cloud (SMC). We infer a visual extinction of $A_V \approx 1.9~\rm
mag$, a reddening of $E(B-V)\approx1.1~ \rm mag$, and a total-to-selective
extinction ratio of $R_V \approx 1.7$, consistent with that previously derived
from photometric, spectroscopic, and polarimetric observations. The size
distributions of the dust in the interstellar medium toward SN 2014J are skewed
toward substantially smaller grains than that of the Milky Way and the SMC.
