Diversity in extinction laws of Type Ia supernovae measured between $0.2$ and $2\hat{I}¼\mathrm{m}$

Abstract

We present ultraviolet (UV) observations of six nearby Type~Ia supernovae (SNe~Ia) obtained with the Hubble Space Telescope, three of which were also observed in the near-IR (NIR) with Wide-Field Camera~3. UV observations with the Swift satellite, as well as ground-based optical and near-infrared data provide complementary information. The combined data-set covers the wavelength range $0.2$--$2\hat{I}¼$m. By also including archival data of SN 2014J, we analyse a sample spanning observed colour excesses up to $E(B-V)=1.4$mag. We study the wavelength dependent extinction of each individual SN and find a diversity of reddening laws when characterised by the total-to-selective extinction $R_V$. In particular, we note that for the two SNe with $E(B-V)\gtrsim 1$mag, for which the colour excess is dominated by dust extinction, we find $R_V=1.4\pm 0.1$ and $R_V=2.8\pm 0.1$. Adding UV photometry reduces the uncertainty of fitted $R_V$ by $\sim 50$% allowing us to also measure $R_V$ of individual low-extinction objects which point to a similar diversity, currently not accounted for in the analyses when SNe~Ia are used for studying the expansion history of the universe.

31 pages, 28 figures, accepted for publication in MNRAS