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Diversity of Type Ia supernova optical light curves among different spectroscopic subclasses

R. B. Koshi, M. Doi, S. Sako, K. Maeda, M. Tanaka, N. Takanashi

Abstract

Attempts to reveal the spectroscopic diversity of Type Ia supernovae (SNe Ia) have led to subclassification schemes such as the Branch system, which classifies SNe Ia into four categories: core normal (CN), broad line (BL), cool (CL), and shallow silicon (SS). The physical origin of these spectroscopic differences, including progenitor channels, explosion mechanisms, or other parameters, however, remains unclear. Moreover, previous work has concentrated primarily on properties near peak luminosity, yielding limited insight into their behavior at later epochs. In this study, we compile $UBVRI$ photometry for 109 SNe Ia and construct the first set of average light curves for each Branch subgroup, spanning from pre-maximum through the late tail. We find pronounced diversity in the $I$-band, especially in the timing of the secondary maximum across subgroups and in the late-time decline of CL events. After correcting for light curve stretch, which reflects the combined influence of ${}^{56}$Ni and ejecta masses, we show that the secondary maximum is powered by Fe II recombination, and its timing is particularly sensitive to the amount of stable iron-group elements (IGEs) synthesized in the explosion. This implies an anti-correlation between the mass of stable IGEs and ${}^{56}$Ni: CL (and possibly BL) events have a larger mass ratio of IGEs/$^{56}$Ni resulting in earlier secondary maxima, while SS events have a smaller ratio and thus later secondary maxima. This trend is naturally explained within the near-$M_{\rm Ch}$ delayed-detonation scenario, whereas it is inconsistent with the positive correlation predicted by the sub-$M_{\rm Ch}$ double-detonation scenario. Finally, we show that stretch-corrected late-time slopes provide a practical diagnostic for CL events, likely linked to an emission feature around $7,200$ Angstroms.

Diversity of Type Ia supernova optical light curves among different spectroscopic subclasses

Abstract

Attempts to reveal the spectroscopic diversity of Type Ia supernovae (SNe Ia) have led to subclassification schemes such as the Branch system, which classifies SNe Ia into four categories: core normal (CN), broad line (BL), cool (CL), and shallow silicon (SS). The physical origin of these spectroscopic differences, including progenitor channels, explosion mechanisms, or other parameters, however, remains unclear. Moreover, previous work has concentrated primarily on properties near peak luminosity, yielding limited insight into their behavior at later epochs. In this study, we compile photometry for 109 SNe Ia and construct the first set of average light curves for each Branch subgroup, spanning from pre-maximum through the late tail. We find pronounced diversity in the -band, especially in the timing of the secondary maximum across subgroups and in the late-time decline of CL events. After correcting for light curve stretch, which reflects the combined influence of Ni and ejecta masses, we show that the secondary maximum is powered by Fe II recombination, and its timing is particularly sensitive to the amount of stable iron-group elements (IGEs) synthesized in the explosion. This implies an anti-correlation between the mass of stable IGEs and Ni: CL (and possibly BL) events have a larger mass ratio of IGEs/Ni resulting in earlier secondary maxima, while SS events have a smaller ratio and thus later secondary maxima. This trend is naturally explained within the near- delayed-detonation scenario, whereas it is inconsistent with the positive correlation predicted by the sub- double-detonation scenario. Finally, we show that stretch-corrected late-time slopes provide a practical diagnostic for CL events, likely linked to an emission feature around Angstroms.
Paper Structure (3 sections, 2 figures)

This paper contains 3 sections, 2 figures.

Table of Contents

  1. Introduction
  2. Data
  3. Archival data

Figures (2)

  • Figure 1: Histogram of the redshift distribution of the SN Ia sample used in this work. Bins have a width of $\Delta z=0.01$. Alt text: The x axis shows the redshift from 0.00 to 0.10. The y axis shows the number of supernovae, from 0 to 35.
  • Figure 2: Branch diagram showing pseudo-equivalent widths of Si II $\lambda 6355$ versus Si II $\lambda 5972$ for the SN Ia sample used in this work. Different subgroups are denoted by symbols: CN (orange open circles), BL (blue filled circles), CL (red crosses), and SS (open triangles). Values are from branch09 and folatelli13. Objects that lack measurements of either Si II line are not plotted here, but are retained for analyses that do not require pEWs. Alt text: A scatter plot diagram. The x axis shows the pseudo-equivalent width of the Si II absorption line at 6355 Angstroms, and the y axis shows those of the Si II line at 5972 Angstroms.