Spectral luminosity indicators in SNe Ia - Understanding the R(SiII) line strength ratio and beyond

TL;DR

This paper investigates the physical basis of the R(SiII) spectral line ratio in Type Ia supernovae, revealing its dependence on ionisation balance and proposing it as a potential luminosity indicator.

Contribution

It provides a detailed physical explanation for the R(SiII) ratio's correlation with luminosity using synthetic spectra and explores its effectiveness as a spectroscopic luminosity indicator.

Findings

R(SiII) depends on SiII f5972, not f6355, due to saturation.

The ratio correlates with luminosity through ionisation balance changes.

SiII f5972 may serve as the best spectroscopic luminosity indicator.

Abstract

SNe Ia are good distance indicators because the shape of their light curves, which can be measured independently of distance, varies smoothly with luminosity. This suggests that SNe Ia are a single family of events. Similar correlations are observed between luminosity and spectral properties. In particular, the ratio of the strengths of the SiII \lambda 5972 and \lambda 6355 lines, known as R(SiII), was suggested as a potential luminosity indicator. Here, the physical reasons for the observed correlation are investigated. A Monte-Carlo code is used to construct a sequence of synthetic spectra resembling those of SNe with different luminosities near B maximum. The influence of abundances and of ionisation and excitation conditions on the synthetic spectral features is investigated. The ratio R(SiII) depends ssentially on the strength of SiII \lambda 5972, because SiII \lambda 6355 is saturated. In less luminous objects, SiII \lambda 5972 is stronger because of a rapidly increasing SiII/SiIII ratio. Thus, the correlation between R(SiII) and luminosity is the effect of ionisation balance. The SiII \lambda 5972 line itself may be the best spectroscopic luminosity indicator for SNe Ia, but all indicators discussed show scatter which may be related to abundance distributions.