The Outermost Ejecta of Type Ia Supernovae

TL;DR

This study analyzes the outermost ejecta of Type Ia supernovae using early optical spectra, revealing differences in velocity, composition, and potential asymmetries that influence spectral diversity and light curve properties.

Contribution

It provides new insights into the high-velocity ejecta properties and their impact on spectral diversity, highlighting the roles of asphericity and composition variations.

Findings

HVG SNe have higher photospheric velocities than LVG SNe at early times.

LVG SNe show less intense burning and lower unburned carbon mass.

Outer layer Fe-group abundances vary, affecting spectral diversity.

Abstract

The properties of the highest velocity ejecta of normal Type Ia supernovae (SNe Ia) are studied via models of very early optical spectra of 6 SNe. At epochs earlier than 1 week before maximum, SNe with a rapidly evolving Si II 6355 line velocity (HVG) have a larger photospheric velocity than SNe with a slowly evolving Si II 6355 line velocity (LVG). Since the two groups have comparable luminosities, the temperature at the photosphere is higher in LVG SNe. This explains the different overall spectral appearance of HVG and LVG SNe. However, the variation of the Ca II and Si II absorptions at the highest velocities (v >~ 20,000 km/s) suggests that additional factors, such as asphericity or different abundances in the progenitor white dwarf, affect the outermost layers. The C II 6578 line is marginally detected in 3 LVG SNe, suggesting that LVG undergo less intense burning. The carbon mass fraction is small, only less than 0.01 near the photosphere, so that he mass of unburned C is only <~ 0.01 Msun. Radioactive 56Ni and stable Fe are detected in both LVG and HVG SNe. Different Fe-group abundances in the outer layers may be one of the reasons for spectral diversity among SNe Ia at the earliest times. The diversity among SNe Ia at the earliest phases could also indicate an intrinsic dispersion in the width-luminosity relation of the light curve.