Unburned Material in the Ejecta of Type Ia Supernovae

TL;DR

This study investigates the presence of unburned carbon in Type Ia supernovae ejecta using early spectra, revealing a higher incidence of carbon than previously thought, which impacts models of the explosion process.

Contribution

It provides new observational evidence of carbon in SNeIa ejecta, accounting for previous detection biases, and discusses implications for explosion models and asymmetries.

Findings

At least 30% of SNeIa show C II absorption features.

Detected carbon corresponds to a mass of 10^-3 to 10^-2 solar masses.

Objects with carbon tend to be bluer and less luminous.

Abstract

(Abridged) The presence of unburned material in the ejecta of normal Type Ia supernovae (SNeIa) is investigated using early-time spectroscopy obtained by the CSP. The tell-tale signature of pristine material from a C+O white dwarf progenitor star is the presence of carbon. The most prominent carbon lines in optical spectra of SNeIa are due to C II. We find that at least 30% of the objects in the sample show an absorption at ~6300 AA which is attributed to C II 6580. An association to H_alpha is considered to be unlikely. These findings imply a larger incidence of carbon in SNeIa ejecta than previously noted. We show how observational biases and physical conditions may hide these C II lines, and account for the scarcity of previous carbon detections. This relatively large frequency of carbon detections has crucial implications on our understanding of the explosive process. The observations indicate that carbon is present merely ~1000 km s^-1 above the bulk of Si II. Spectral modeling shows the detections are consistent with a mass of carbon of 10^-3--10^-2 M_sun. This would imply substantial mixing, which may be related to asymmetries of the flame propagation or clumps of unburned material along the line of sight. However, the uniformity of the relation between C II and Si II velocities is not consistent with the latter scenario. The observational properties of SNeIa with and without carbon signatures are compared. A trend toward bluer color and lower luminosity is found for objects with carbon.