The Oxygen Features in Type Ia Supernovae and the Implications for the Nature of Thermonuclear Explosions

TL;DR

This study analyzes early spectral features of Type Ia supernovae, revealing that oxygen burning occurs at the outer layers and correlates with supernova luminosity, supporting the delayed-detonation explosion model.

Contribution

It provides new observational evidence linking oxygen features to explosion mechanisms and supernova diversity in Type Ia supernovae.

Findings

O I 7773 absorption feature correlates inversely with Si II and Ca II features.

Oxygen burning occurs at the outermost layers in a subgroup of SNe Ia.

Supports delayed-detonation model for Chandrasekhar mass white dwarf explosions.

Abstract

The absorption feature O I 7773 is an important spectral indicator for type Ia supernovae (SNe~Ia) that can be used to trace the unburned material at outer layers of the exploding white dwarf. In this work, we use a large sample of SNe~Ia to examine this absorption at early phases (i.e., -13 days <t <-7 days), and make comparisons with the absorption features of Si~II 6355 and Ca~II near-infrared (NIR) triplet. We show that for a subgroup of spectroscopically normal SNe with normal photospheric velocities (i.e., v_si < 12,500 km s^{-1} at optical maximum), the line strength of high velocity feature (HVF) of O~I is inversely correlated with that of Si~II (or Ca~II), and this feature also shows a negative correlation with the luminosity of SNe Ia. This finding, together with other features we find for the O~I HVF, reveal that for this subgroup of SNe~Ia explosive oxygen burning occurs at the outermost layer of supernova and difference in burning there could lead to the observed diversity, which are in remarkable agreement with the popular delayed-detonation model of Chandrasekhar mass WD.