Diversity in extinction laws of Type Ia supernovae measured between $0.2$ and $2\,\mu\mathrm{m}$

TL;DR

This study analyzes the extinction laws of six nearby Type Ia supernovae across UV to NIR wavelengths, revealing diverse reddening laws characterized by varying R_V values, which impacts their use in cosmology.

Contribution

It provides the first detailed UV to NIR extinction law measurements for multiple SNe Ia, highlighting the diversity in dust properties affecting cosmological measurements.

Findings

Diverse R_V values observed among SNe Ia.

UV data reduces uncertainty in R_V measurements.

Dust extinction properties vary significantly among supernovae.

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 $0.2$--$2~\mu$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 $E(B-V)\gtrsim1~$mag, for which the colour excess is dominated by dust extinction, we find $R_V=1.4\pm0.1$ and $R_V=2.8\pm0.1$. Adding UV photometry reduces the uncertainty of fitted $R_V$ by $\sim50\,$% 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.