A study of the color diversity around maximum light in Type Ia supernovae

TL;DR

This study analyzes the color diversity in Type Ia supernovae around maximum light, revealing correlations with nebular line velocities that suggest viewing angle effects influence observed colors and have implications for cosmological measurements.

Contribution

It provides new evidence linking early color variations to nebular line velocities, supporting off-center explosion models and emphasizing the importance of viewing angles in supernova observations.

Findings

Correlations between early phase colors and nebular line velocities.

Nebular velocity shifts relate to Ca II line properties.

Viewing angle effects impact supernova color and extinction estimates.

Abstract

From a sample of 12 well-observed Type Ia supernovae, we find clear evidence of correlations between early phase (U-B), (V-R), and (V-I) colors and the velocity shifts of the [Fe II] \lambda 7155 and [Ni II] \lambda 7378 nebular lines measured from late-phase spectra. As these lines are thought to trace the ashes of the initial deflagration process, our findings provide additional support to the new paradigm of off-center explosions in Type Ia supernovae, and we interpret these correlations as viewing angle effects in the observed colors. We also show that the nebular velocity shifts are related to the strength and width of the Ca II H&K and IR-triplet lines near-maximum light. The evidence presented here implies that the viewing angle must be taken into account when deriving extinction values and distances in future cosmological studies based on Type Ia supernovae.