PTF11mnb: the first analog of supernova 2005bf
  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 ($\sim85 M_{\odot}$) 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.
