PTF11mnb: the first analog of supernova 2005bf

Abstract

We study PTF11mnb, a He-poor supernova (SN) whose pre-peak light curves (LCs) resemble those of SN 2005bf, a peculiar double-peaked stripped-envelope (SE) SN. LCs, colors and spectral properties are compared to those of SN 2005bf and normal SE SNe. A bolometric LC is built and modeled with the SNEC hydrodynamical code explosion of a MESA progenitor star, as well as with semi-analytic models. The LC of PTF11mnb turns out to be similar to that of SN 2005bf until $\sim$50 d, when the main (secondary) peaks occur at $-18.5$ mag. The early peak occurs at $\sim$20 d, and is about 1.0 mag fainter. After the main peak, the decline rate of PTF11mnb is remarkably slower than that of SN 2005bf, and it traces the ${}^{56}$Co decay rate. The spectra of PTF11mnb reveal no traces of He unlike in the case of SN Ib 2005bf. The bolometric LC is well reproduced by the explosion of a massive ($M_{ej}=$ 7.8 $M_{\odot }$), He-poor star with a double-peaked ${}^{56}$Ni distribution, a total ${}^{56}$Ni mass of 0.59 $M_{\odot }$ and an explosion energy of 2.2$\times$10${}^{51}$ erg. Alternatively, a normal SN Ib/c explosion [M(${}^{56}$Ni)$=$0.11 $M_{\odot }$, $E_K$ = 0.2$\times$10${}^{51}$ erg, $M_{ej}=$ 1 $M_{\odot }$] can power the first peak while a magnetar ($B$=5.0$\times$10${}^{14}$ G, $P=18.1$ ms) provides energy for the main peak. The early $g$-band LC implies a radius of at least 30 $R_{\odot }$. If PTF11mnb arose from a massive He-poor star characterized by a double-peaked ${}^{56}$Ni distribution, the ejecta mass and the absence of He imply a large ZAMS mass ($\sim 85M_{\odot }$) for the progenitor, which most likely was a Wolf-Rayet star, surrounded by an extended envelope formed either by a pre-SN eruption or due to a binary configuration. Alternatively, PTF11mnb could be powered by a normal SE SN during the first peak and by a magnetar afterwards.

Abstract abridged to fit allowed limit; revised version after addressing referee's comments; comments are welcome, 14 pages, 12 figures