A Study of Two Type IIb Supernovae: SNe 2008aq and 2019gaf

TL;DR

This paper presents a detailed optical photometric and spectroscopic study of two Type IIb SNe, 2008aq and 2019gaf, to characterize their light curves, spectral evolution, and explosion parameters. Using Arnett's approximation, scaling relations, and a two-component semi-analytic model, the authors derive nickel masses around 0.05–0.08 M⊙, ejecta masses of a few solar masses, and kinetic energies spanning roughly 5–12 × 10^51 erg, with SN 2019gaf being the more energetic event. Late-time nebular spectroscopy and comparisons to nucleosynthesis models yield ZAMS progenitor masses in the range ∼13–25 M⊙, with [O I]/[Ca II] flux ratios favoring binary progenitors for both SNe. The results reinforce the view that Type IIb SNe arise from binary-stripped progenitors and illustrate the diversity of explosion energies and envelope stripping within this subclass, providing constraints for progenitor evolution and explosion physics.

Abstract

We present photometric and spectroscopic studies of two core-collapse supernovae (SNe) 2008aq and 2019gaf in the optical wavelengths. Light curve and spectral sequence of both the SNe are similar to those of other Type IIb SNe. The pre-maximum spectrum of SN~2008aq showed prominent H $α$ lines, the He lines started appearing in the near maximum spectrum. The near maximum spectrum of SN~2019gaf shows shallow H$α$ absorption and He lines with almost similar strength. Both the SNe show transition from hydrogen-dominated spectra to helium-dominated spectra within a month after maximum brightness. The velocity evolution of SN~2008aq matches well with those of other well-studied Type IIb SNe, while SN~2019gaf shows higher velocities. Close to maximum light, the H $α$ and He I line velocities of SN~2019gaf are $\sim$ 2000 km sec$^{-1}$ and $\sim$ 4000 km sec$^{-1}$ higher than other well-studied Type IIb SNe. Semi-analytical modeling indicates SN~2019gaf to be a more energetic explosion with a smaller ejecta mass than SN~2008aq. The zero-age main-sequence (ZAMS) mass of the progenitor estimated using the nebular spectra of SN~2008aq ranges between 13 to 20 M$_\odot$, while for SN~2019gaf, the inferred ZAMS mass is between 13 to 25 M$_\odot$. The [O I] to [Ca II] lines flux ratio favors a less massive progenitor star in a binary system for both the SNe.

Figures (16)

• Figure 1: Light curve evolution of SN 2008aq in BVRI bands.
• Figure 2: Light curve evolution of SN 2019gaf as observed in the UBgVri bands.
• Figure 3: BVRI light curves for SNe 2008aq and 2019gaf, compared with those of other Type IIb SNe.
• Figure 4: Color evolution of SNe 2008aq and 2019gaf compared with color evolution of other well-studied Type IIb SNe.
• Figure 5: Evolution of the pseudo-bolometric luminosity for SNe 2008aq and 2019gaf, along with a comparison to other Type IIb SNe.