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

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 $\alpha$ lines, the He lines started appearing in the near maximum spectrum. The near maximum spectrum of SN~2019gaf shows shallow H$\alpha$ 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 $\alpha$ 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.

14 pages, 16 figures, accepted for publication in Monthly Notices of the Royal Astronomical Society