On the Lira law and the nature of extinction towards Type Ia Supernovae

TL;DR

This study investigates the color evolution of Type Ia Supernovae and its relation to circumstellar material, revealing that late-time color changes correlate with spectral features and host galaxy types, suggesting CSM influence.

Contribution

It provides new evidence linking late-time color evolution and spectral absorption features to circumstellar material in SNe Ia, enhancing understanding of their environments.

Findings

Faster Lira law B-V decliners have higher Na I D absorption.

Color and reddening law differences are present at maximum and late times.

Host galaxy type influences late-time color variations.

Abstract

We have studied the relation between the color evolution of Type Ia Supernovae (SNe Ia) from maximum light to the Lira law regime and the presence of narrow absorption features. Based on a nearby sample of 89 SNe Ia we have found that the rate of change of B-V colors at late phases (between 35 and 80 days after maximum) varies significantly among different SNe Ia. At maximum light, faster Lira law B-V decliners have significantly higher equivalent widths of blended Na I D1 & D2 narrow absorption lines, redder colors and lower R_V reddening laws. We do not find faster Lira law B-V decliners to have a strong preference for younger galaxy environments, where higher interstellar material (ISM) column densities would be expected. We interpret these results as evidence for the presence of circumstellar material (CSM). The differences in colors and reddening laws found at maximum light are also present 55 days afterward, but unlike the colors at maximum, they show a significant variation among different host galaxy morphological types. This suggests that the effect of ISM on the colors is more apparent at late times. Finally, we discuss how the transversal expansion of the ejecta in an inhomogeneous ISM could mimic some of these findings.