PTF12os and iPTF13bvn: Two stripped-envelope supernovae from low-mass progenitors in NGC 5806

This is the author accepted manuscript. The final version is available from EDP Sciences via http://dx.doi.org/10.1051/0004-6361/201628275$\textit{Context.}$ We investigate two stripped-envelope supernovae (SNe) discovered in the nearby galaxy NGC 5806 by the (intermediate) Palomar Transient Factory [(i)PTF].  These SNe, designated PTF12os/SN 2012P and iPTF13bvn, exploded within ~ 520 days of  one another at a similar distance from the host-galaxy center. We classify PTF12os  as a Type IIb SN based on our spectral sequence; iPTF13bvn has previously been  classified as Type Ib having a likely progenitor with zero age main sequence  (ZAMS) mass below ~ 17 M$_\odot$ . Because of the shared and nearby host, we are  presented with a unique opportunity to compare these two SNe.

$\textit{Aims.}$ Our main objective is to constrain the explosion parameters of  iPTF12os and iPTF13bvn, and to put constraints on the SN progenitors. We also  aim to spatially map the metallicity in the host galaxy, and to investigate  the presence of hydrogen in early-time spectra of both SNe.

$\textit{Methods.}$ We present comprehensive datasets collected on PTF12os and  iPTF13bvn, and introduce a new automatic reference-subtraction photometry pipeline  (FPipe) currently in use by the iPTF. We perform a detailed study of the light  curves (LCs) and spectral evolution of the SNe. The bolometric LCs are modeled  using the hydrodynamical code $\Tiny{HYDE}$. We analyze early spectra of both SNe to  investigate the presence of hydrogen; for iPTF13bvn we also investigate the  regions of the Paschen lines in infrared spectra. We perform spectral line  analysis of helium and iron lines to map the ejecta structure of both SNe.  We use nebular models and late-time spectroscopy to constrain the ZAMS mass  of the progenitors. We also perform image registration of ground-based images  of PTF12os to archival $\small{HST}$ images of NGC 5806 to identify a potential progenitor candidate.

$\textit{Results.}$ We find that our nebular spectroscopy of iPTF13bvn remains consistent with a low-mass progenitor, likely having a ZAMS mass  of ~ 12 M$_\odot$ . Our late-time spectroscopy of PTF12os is consistent with a  ZAMS mass of ~ 15 M$_\odot$. We successfully identify a source in pre-explosion  $\small{HST}$ images coincident with PTF12os. The colors and absolute magnitude of  this object are consistent between pre-explosion and late-time $\small{HST}$ images,  implying it is a cluster of massive stars. Our hydrodynamical modeling  suggests that the progenitor of PTF12os had a compact He core with a  mass of 3.25$^{+0.77}_{−0.56}$ M$_\odot$ at the time of the explosion, which had a  total kinetic energy of 0.54$^{+0.41}_{−0.25}$ × 10$^{51}$ erg and synthesized  0.063$^{+0.020}_{−0.011}$ M$_\odot$ of strongly mixed $^{56}$Ni. Spectral comparisons  to the Type IIb SN 2011dh indicate that the progenitor of PTF12os was  surrounded by a thin hydrogen envelope with a mass lower than 0.02 M$_\odot$.  We also find tentative evidence that the progenitor of iPTF13bvn could  have been surrounded by a small amount of hydrogen prior to the explosion.  This result is supported by possible weak signals of hydrogen in both optical  and infrared spectra.Knut and Alice Wallenberg Foundation; Swedish Research Council; Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231; W. M. Keck Foundation; Google; EU/FP7 via ERC grant No. 307260; Quantum Universe I-Core program by the Israeli Committee for Planning and Budgeting; Minerva and ISF grants; Weizmann-UK making connections program; Kimmel and ARCHES awards; NASA/HST grant AR-14295 from the Space Telescope Science Institute; Christopher R. Redlich Fund; TABASGO Foundation; NSF grant AST-1211916; PRININAF 2014 with the project: Transient Universe: unveiling new types of stellar explosions with PESSTO; European Union FP7 program though ERC grant number 320360; European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013)/ERC Grant agreement no [291222]