SN 2020acat: A purr-fect example of a fast rising Type IIb Supernova

Abstract

The Ultra-Violet (UV) and Near Infrared (NIR) photometric and optical spectroscopic observations of SN 2020acat covering $\sim 250$ days after explosion are presented here. Using the fast rising photometric observations, spanning from the UV to NIR wavelengths, a pseudo-bolometric light curve was constructed and compared to several other well-observed Type IIb supernovae (SNe IIb). SN 2020acat displayed a very short rise time reaching a peak luminosity of $\mathrm{Lo}{\mathrm{g}}_{10}(L)=42.49\pm 0.15\mathrm{erg}{\mathrm{s}}^{-1}$ in only $\sim 14.6\pm 0.3$ days. From modelling of the pseudo-bolometric light curve, we estimated a total mass of ${}^{56}\mathrm{Ni}$ synthesised by SN 2020acat of $0.13\pm 0.02{\mathrm{M}}_{\odot }$, with an ejecta mass of $2.3\pm 0.3{\mathrm{M}}_{\odot }$ and a kinetic energy of $1.2\pm 0.2\times 10^{51}$ erg. The optical spectra of SN 2020acat display hydrogen signatures well into the transitional period ($\gtrsim 100$ days), between the photospheric and the nebular phases. The spectra also display a strong feature around $4900\tilde{A}$ that cannot be solely accounted for by the presence of the $\mathrm{F}{\mathrm{e}}_{\mathrm{II}}$ $5018$ line. We suggest that the $\mathrm{F}{\mathrm{e}}_{\mathrm{II}}$ feature was augmented by $\mathrm{H}{\mathrm{e}}_{\mathrm{I}}$ $5016$ and possibly by the presence of ${\mathrm{N}}_{\mathrm{II}}$ $5005$. From both photometric and spectroscopic analysis, we inferred that the progenitor of SN\,2020acat was an intermediate mass compact star with a $M_{\mathrm{ZAMS}}$ of $18-22{\mathrm{M}}_{\odot }$.

18 pages, 13 figures