Letters to the Editor

The reddening law of type Ia supernovae: separating intrinsic variability from dust using equivalent widths^⋆

¹ Université de Lyon, Université Lyon 1, CNRS/IN2P3, Institut de Physique Nucléaire de Lyon, 69622 Villeurbanne, France
e-mail: nchotard@ipnl.in2p3.fr
² Physics Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA
³ LPNHE, Université Pierre et Marie Curie Paris 6, Université Paris Diderot Paris 7, CNRS-IN2P3, 75252 Paris Cedex 05, France
⁴ Department of Physics, Yale University, New Haven, CT 06250-8121, USA
⁵ Physikalisches Institut Universitat Bonn, Nussallee 12 53115 Bonn, Germany
⁶ Department of Physics, University of California Berkeley, 366 LeConte Hall MC 7300, Berkeley, CA, 94720-7300, USA
⁷ Computational Cosmology Center, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA
⁸ Department of Astronomy, University of California, Berkeley, CA 94720-3411, USA
⁹ Observatoire de Lyon, 69230 Saint-Genis Laval, Université de Lyon, Université Lyon 1, 69003 Lyon, France
¹⁰ Australian National University, Mt. Stromlo Observatory, The RSAA, Weston Creek, ACT 2611, Australia
¹¹ Centre de Physique des Particules de Marseille, 163 avenue de Luminy, Case 902, 13288 Marseille Cedex 09, France
¹² Tsinghua Center for Astrophysics, Tsinghua University, Beijing 100084, PR China
¹³ New York University, Center for Cosmology and Particle Physics, 4 Washington Place, New York, NY 10003, USA
¹⁴ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, PR China

Received: 15 February 2011
Accepted: 28 March 2011

Abstract

We employ 76 type Ia supernovae (SNe Ia) with optical spectrophotometry within 2.5 days of B-band maximum light obtained by the Nearby Supernova Factory to derive the impact of Si and Ca features on the supernovae intrinsic luminosity and determine a dust reddening law. We use the equivalent width of Si ii λ4131 in place of the light curve stretch to account for first-order intrinsic luminosity variability. The resulting empirical spectral reddening law exhibits strong features that are associated with Ca ii and Si ii λ6355. After applying a correction based on the Ca ii H&K equivalent width we find a reddening law consistent with a Cardelli extinction law. Using the same input data, we compare this result to synthetic rest-frame UBVRI-like photometry to mimic literature observations. After corrections for signatures correlated with Si ii λ4131 and Ca ii H&K equivalent widths and introducing an empirical correlation between colors, we determine the dust component in each band. We find a value of the total-to-selective extinction ratio, R_V = 2.8 ± 0.3. This agrees with the Milky Way value, in contrast to the low R_V values found in most previous analyses. This result suggests that the long-standing controversy in interpreting SN Ia colors and their compatibility with a classical extinction law, which is critical to their use as cosmological probes, can be explained by the treatment of the dispersion in colors, and by the variability of features apparent in SN Ia spectra.