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 wav elengths. Light curv e 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 hy drogen-dominat ed spectra t o helium-dominat ed spectra within a month after maximum brightness. The velocity evolution of SN 2008aq matches well with those of other well-studied Type IIb SN e, while SN 2019g af shows higher v elocities. Close t o maximum light, the H α and He i line velocities of SN 2019g af ar e ∼2000 and ∼4000 km s −1 higher than other well-studied Type IIb SNe. Semi-analytical modelling 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 progenit or estimat ed using the nebular spectra of SN 2008aq ranges between 13 and 20 M , while for SN 2019gaf, the inferred ZAMS mass is between 13 and 25 M . The [O i ] to [Ca ii ] lines flux ratio favours a less massiv e progenit or star in a binary system for both the SNe.