Intrinsic and extinction colour components in SNe Ia and the determination of R_V

¹ Université de Lyon, Université Lyon 1, CNRS/IN2P3, IP2I, 4 rue Enrico Fermi, 69622 Villeurbanne, France
e-mail: g.smadja@ipnl.in2p3.fr
² Centre de Physique des Particules de Marseille, Aix-Marseille Université, CNRS/IN2P3, 163 avenue de Luminy-Case 902, 13288 Marseille Cedex 09, France
³ Max-Planck-Institut fûr Astrophysik, Karl-Schwartzschild-Str. 1, 85748 Garching, Germany
⁴ Princeton University, Department of Astrophysics, 4 Ivy Lane, Princeton, NJ 08544, USA
⁵ Tsinghua Center for Astrophysics, MongManWai building, Tsinghua University, Beijing 100084, PR China

Received: 18 November 2022
Accepted: 25 July 2023

Abstract

Context. The colour fluctuations of type Ia supernovae (SNe Ia) include intrinsic and extrinsic components, which both contribute to the observed variability. Previous works proposed a statistical separation of these two contributions, but the individual intrinsic colour contributions of each SN Ia were not extracted. In addition, a large uncertainty remains on the value of the parameter R_V, which characterises the dust extinction formula.

Aims. Leveraging the known parameterisation of the extinction formula for dust in our Galaxy, and applying it to the host galaxy of SNe Ia, we propose a new method of separation –valid for each SN– using the correlations between colour fluctuations. This also allows us to derive a well-constrained value of the extinction parameter R_V with different, possibly smaller systematic errors. We also define a three-dimensional space of intrinsic colour fluctuations.

Methods. The key ingredients in this attempt at separating the intrinsic and extinction colour components for each SN –and subsequently measuring R_V– are the assumption of a linearized dependence of magnitude on the extinction component of colour, a one-dimensional extra-intrinsic colour space (in addition to Ca II H&Kλ3945 and Si IIλ4131 contributions) over four independent colours, and the absence of correlation between the intrinsic and extrinsic variabilities.

Results. We show that a consistent solution is found under the previous assumptions, but the observed systematic trends point to a (small) inadequacy of the extinction formula. Once corrected, all systematic extinction effects can be cancelled by choosing a single scaling of the extinction colour component as well as an appropriate value of R_V = 2.181 ± 0.117. The observed colours are described within an accuracy of 0.025 mag. The resulting magnitude variability is 0.13 over all UBVRI bandpasses, and this fluctuation is shown to be independent of the bandpass to within 0.02 mag.