The Carnegie Supernova Project: Intrinsic Colors of Type Ia Supernovae

TL;DR

This study refines the understanding of intrinsic colors of Type Ia Supernovae using extensive data, introduces a new light-curve parameter, and investigates the reddening law and metallicity effects across various wavelengths.

Contribution

It introduces a new light-curve parameter suited for fast-declining SNe Ia and provides an updated analysis of intrinsic colors and reddening laws with a larger dataset.

Findings

Peculiar SNe Ia are extensions of normal populations.

Low Rv values are favored but vary significantly.

Large scatter in u-band colors linked to metallicity.

Abstract

We present an updated analysis of the intrinsic colors of SNe Ia using the latest data release of the Carnegie Supernova Project. We introduce a new light-curve parameter very similar to stretch that is better suited for fast-declining events, and find that these peculiar types can be seen as extensions to the population of "normal" SNe Ia. With a larger number of objects, an updated fit to the Lira relation is presented along with evidence for a dependence on the late-time slope of the B-V color-curves with stretch and color. Using the full wavelength range from u to H band, we place constraints on the reddening law for the sample as a whole and also for individual events/hosts based solely on the observed colors. The photometric data continue to favor low values of Rv, though with large variations from event to event, indicating an intrinsic distribution. We confirm the findings of other groups that there appears to be a correlation between the derived reddening law, Rv, and the color excess, E(B-V), such that larger E(B-V) tends to favor lower Rv. The intrinsic u-band colors show a relatively large scatter that cannot be explained by variations in Rv or by the Goobar (2008) power-law for circumstellar dust, but rather is correlated with spectroscopic features of the supernova and is therefore likely due to metallicity effects.