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Red circles mark the locations where we have measured the metallicity in Paper II. North is up and east is to the left in the figure. Credits for the original image: ESA/NASA/Andre van der Hoeven. Abstract This thesis is based on research made by the intermediate Palomar Transient Factory [(i)PTF]. The focus is on stripped-envelope (SE) supernovae (SNe) discovered by (i)PTF, and it is closely tied to the research on the SE SN iPTF13bvn, that occurred in the nearby galaxy NGC 5806. This SN was initially thought to have been the explosion of a very massive Wolf-Rayet star, but we have shown that this is very likely not the case. We suggest instead that iPTF13bvn originated from a binary system where the envelope was stripped off from the SN progenitor by tidal forces from a companion (Paper I). PTF12os exploded in the same galaxy as iPTF13bvn, and our analysis shows that PTF12os and iPTF13bvn were very similar, and that both were also remarkably similar to the Type IIb SN 2011dh, in terms of their light-curves and spectra. In Paper II, hydrodynamical models were used to constrain the explosion parameters of iPTF13bvn, PTF12os and SN 2011dh; finding 56Ni masses in the range 0.063−0.075 M, ejecta masses in the range 1.85 − 1.91 M, and kinetic energies in the range 0.54 − 0.94 × 1051 erg. Furthermore, using nebular models and late-time spectroscopy we were able to constrain the Zero-Age Main Sequence (ZAMS) mass to ∼ 12 M for iPTF13bvn and 15 M for PTF12os. In current stellar evolution models, stars with these masses on the ZAMS cannot lose their envelopes and become SE SNe without binary interactions. In Paper III we investigate a peculiar SE SN, iPTF15dtg; this SN lacks both hydrogen and helium and shows a double- peaked LC with a broad main LC peak. Using hydrodynamical modeling we show that iPTF15dtg had a very large ejecta mass (∼ 10 M), resulting from an explosion of a very massive star (∼ 35 M). The initial peak in the LC can be explained by the presence of extended material around the star, likely due to an episode of strong mass-loss experienced by the progenitor prior to the explosion. In Paper IV we perform a statistical study of the spectra of all 176 SE SNe (Type IIb, Ib and Ic) discovered by (i)PTF. The spectra of Type Ic SNe show O absorption features that are both stronger and broader (indicating faster expansion velocities) compared to Type IIb and Type Ib SNe. These findings along with very weak He absorption support the traditional picture with Type Ic SNe being heavily stripped of their He envelopes prior to the explosions, and argue against alternative explanations, such as differences in explosive mixing of 56Ni among the SE SN subtypes. c Christoffer Fremling, Stockholm 2017 ISBN print 978-91-7649-929-0 ISBN PDF 978-91-7649-930-6 Printed by Universitetsservice US-AB, Stockholm 2017 Distributor: Department of Astronomy, Stockholm University To Paula List of Papers The four papers included in this PhD thesis will be referred to as Paper I, Paper II, Paper III and Paper IV. Short summaries of the papers are given in Chapt. 5. Paper I, Fremling et al. (2014); Fremling, C., Sollerman, J., Taddia, F., Er- gon, M., Valenti, S., Arcavi, I, Ben-Ami, S., Cao, Y., Cenko, S. B., Filippenko, A. V., Gal-Yam, A., Howell, D. A. (2014). The rise and fall of iPTF13bvn, not a Wolf-Rayet star. A&A, 565, A114. Paper II, Fremling et al. (2016); Fremling, C., Sollerman, J., Taddia, F., Ergon, M., Fraser, M., Karamehmetoglu, E., Valenti, S., Jerkstrand, A., Arcavi, I., Bufano, F., Elias Rosa, N., Filippenko, A. V., Fox, D., Gal-Yam, A., Howell, D. A., Kotak, R., Mazzali, P., Milisavljevic, D., Nugent, P. E., Nyholm, A., Pian, E., Smartt, S. (2016). PTF12os and iPTF13bvn. Two stripped-envelope supernovae from low-mass progenitors in NGC 5806. A&A, 593, A68. Paper III, Taddia et al. (2016a); Taddia, F., Fremling, C., Sollerman, J., Corsi, A., Gal-Yam, A., Karamehmetoglu, E., Lunnan, R., Bue, B., Ergon, M., Kasliwal, M., Vreeswijk, P. M., Wozniak, P. R. (2016). iPTF15dtg: a double-peaked Type Ic supernova from a massive pro- genitor. A&A 592, A89. Paper IV, Fremling et al. (2017); Fremling, C., Sollerman, J., Kasli- wal, M. M., Kulkarni, S. R., Barbarino, C., Ergon, M., Karamehme- toglu, E., Taddia, F., Arcavi, I., Cenko, S. B., Clubb, K., De Cia, A., Duggan, G., Filippenko, A. V., Gal-Yam, A., Graham, M. L., Horesh, A., Hosseinzadeh, G., Howell, D. A., Kuesters, D., Lunnan, R., Matheson, T., Nugent, P. E., Perley, D. A., Quimby, R. M., Saunders, C. Oxygen and helium in stripped-envelope supernovae. Submitted to A&A. Reprints made with permission from Astronomy & Astrophysics, c ESO. Declarations Contribution to Paper I. The author of this thesis (CF) used photometric and spectroscopic data collected by the iPTF collaboration, along with archival HST images, to perform the analysis presented in the paper. Spectra were reduced by collaborators within the iPTF. The host-subtracted photometry presented in the paper was reduced using the pipeline developed by CF (FPIPE; see Chapt. 4). CF designed all figures, and wrote the entirety of the text in the paper. The hydrodynamical model fitting results presented in the paper were computed by M. Ergon. Changes were incorporated after discussing the first draft with the coauthors. Contribution to Paper II. The author of this thesis (CF) analyzed and pre- sented results based on photometric and spectroscopic data collected by the iPTF collaboration and a large European collaboration. HST archival im- ages were analyzed by CF and M. Fraser. CF performed the astrometric progenitor identification presented in the paper, and M. Fraser performed the HST photometry and image subtractions. F. Taddia performed the metallicity measurements based on long-slit spectroscopy presented in the paper. Some of the metallicity measurements were based on spectra obtained via a service- mode proposal at the Nordic Optical Telescope by CF (Proposal ID 48- 408, PI C. Fremling). All previously unpublished spectra of PTF12os and iPTF13bvn were reduced by people within the collaborations. The host- subtracted photometry presented in the paper was reduced using the pipeline developed by CF (FPIPE; which is also presented and described in the paper). CF produced all figures from the relevant data, except Fig. 4 (provided by M. Fraser) and Fig. 1 (which is composed by CF from the original color image created by ESA/NASA/Andre van der Hoeven). CF wrote the entirety of the text in the paper, except for Sect. 5.1, which contains significant contributions from M. Fraser, and Sect. 3 which contains contributions from F. Taddia. The hydrodynamical model fitting results presented in the paper were computed by M. Ergon. Changes and additions were incorporated after discussing the first draft with the coauthors. Contribution to Paper III. The main author of this paper is F. Taddia. The author of this thesis (CF) reduced the P60 and NOT imaging of the supernova to produce the lightcurves for the paper. CF assisted in the calculation of stellar evolution models using MESA, assisted in modeling the early-time shock-breakout cooling emission peak in the observed lightcurves, and helped with interpretations of the results presented in the paper. Contribution to Paper IV. The author of this thesis performed all of the analysis and wrote the entirety of the text presented in the paper. The majority of the spectra included in the paper were reduced by members of the PTF and iPTF collaboration, but the author of this thesis reduced several spectra from the Gemini N telescope that are included in the paper. Minor changes to the paper were made based on feedback from the co-authors, among which the paper was circulated several times prior to submission. Figures in this thesis. The figures in this thesis were created by C. Fremling if there is no reference or credit in the figure caption. Information about the sources for the figures can also be found within brackets for each entry in the List of Figures section. Re-used text from the licentiate thesis of C. Fremling. The licentiate thesis of C. Fremling is the basis of this doctoral thesis. Parts of all sections, including the abstract, have been directly re-used with minor modifications and updates. A presentation of iPTF15dtg (Paper III) has been added to Chapter 1. Sections of text presenting research on iPTF15dtg (Paper III) and the spectral analysis of SE SN spectra discovered by (i)PTF (Paper IV) have been added to Chapter 2.