Stripped-envelope supernovae were thought to be the explosions of very massive stars (  20 M⊙) that lost their outer layers of hydrogen and/or helium in strong stellar winds. However, recent studies have highlighted that a majority of stripped-envelope supernovae seem to be arising from relatively lower-mass progenitor stars in the 12 − 20 M⊙ range, creating a mystery about the fate of the higher-mass stars. In this licentiate thesis, we review our knowledge of stripped-envelope supernovae, and present the astrophysical problem of their missing high-mass progenitors. The thesis focuses on observations of unique and rare stripped-envelope supernovae classified with modern optical surveys such as the intermediate Palomar Transient Factory (iPTF) and the Public European Spectroscopic Survey of Transient Objects (PESSTO). In these surveys, we have discovered stripped-envelope supernovae with long-lasting broad lightcurves, which are thought to be a marker for highly massive (  20 M⊙) progenitor stars. Despite this exciting association, there are only a handful of existing examples of stripped-envelope supernovae with broad lightcurves published in the literature, not numerous enough to account for the missing high-mass stars.

In our efforts, the first of the objects we focused on was OGLE-2014-SN-131, a long-lasting supernova in the southern sky initially classified by PESSTO. We re-classified it as a supernova Type Ibn interacting with a helium-rich circumstellar environment. Unlike all other Type Ibn’s in the literature, OGLE-2014-SN-131 was found to have a long rise-time and large lightcurve broadness. By modeling its bolometric lightcurve, we concluded that OGLE-2014-SN-131 must have had an unusually massive progenitor star. Furthermore, since an ordinary radioactive-decay model could not reproduce the lightcurve, we investigated both a magnetar and circumstellar interaction as potential powering scenarios, and favored the latter due to the signatures of interaction present in the spectra. Next, we looked for similar objects in the supernova dataset of the iPTF, which has over 200 stripped-envelope supernovae. Searching in a sub-sample of ∼100 well-observed supernovae, we identified 11 to have unusually broad lightcurves. We also constrained the distribution of lightcurve broadness for iPTF stripped-envelope supernovae. The 11 with broad lightcurves will be studied carefully in an upcoming paper. The first part of this upcoming paper, which describes the careful statistical identification of these supernovae, is included in this thesis. In it we identify that ∼10% of the iPTF stripped-envelope supernova sample have broad lightcurves, a surprisingly high fraction given their rarity in the published literature. Finally, we evaluate whether our estimate of the fraction of broad stripped-envelope supernovae could help explain the missing high-mass progenitors. We conclude that they can only be a small fraction of the missing high-mass progenitors.