Improved Distances to Type Ia Supernovae with Two Spectroscopic Subclasses

TL;DR

This study classifies Type Ia supernovae into two spectroscopic subclasses based on Si II velocity, revealing differences in luminosity, color, and extinction, which improves distance measurement precision.

Contribution

It introduces a spectroscopic subclassification of SNe Ia that reduces luminosity dispersion, enhancing their use as standard candles.

Findings

HV SNe Ia have higher Si II velocities and redder colors.

Applying subclass-based corrections reduces luminosity dispersion from 0.178 to 0.125 mag.

HV SNe Ia prefer a lower extinction ratio Rv~1.6.

Abstract

We study the observables of 158 relatively normal Type Ia supernovae (SNe Ia) by dividing them into two groups in terms of the expansion velocity inferred from the absorption minimum of the Si II 6355 line in their spectra near B-band maximum brightness. One group ("Normal") consists of normal SNe Ia populating a narrow strip in the Si II velocity distribution, with an average expansion velocity v=10,600+/-400 km/s near B maximum; the other group ("HV") consists of objects with higher velocities, v > 11,800 km/s. Compared with the Normal group, the HV one shows a narrower distribution in both the peak luminosity and the luminosity decline rate dm_{15}. In particular, their B-V colors at maximum brightness are found to be on average redder by ~0.1, suggesting that they either are associated with dusty environments or have intrinsically red B-V colors. The HV SNe Ia are also found to prefer a lower extinction ratio Rv~1.6 (versus ~2.4 for the Normal ones). Applying such an absorption-correction dichotomy to SNe Ia of these two groups remarkably reduces the dispersion in their peak luminosity from 0.178 mag to only 0.125 mag.