Herein we analyse late-time (post-plateau; 103 < t < 1229 d) optical spectra of low-redshift (z < 0.016), hydrogen-rich Type IIP supernovae (SNe IIP). Our newly constructed sample contains 91 nebular spectra of 38 SNe IIP, which is the largest data set of its kind ever analysed in one study, and many of the objects have complementary photometric data. The strongest and most robust result we find is that the luminosities of all spectral features (except those of helium) tend to be higher in objects with steeper late-time V-band decline rates. A steep late-time V-band slope likely arises from less efficient trapping of γ-rays and positrons, which could be caused by multidimensional effects such as clumping of the ejecta or asphericity of the explosion itself. Furthermore, if γ-rays and positrons can escape more easily, then so can photons via the observed emission lines, leading to more luminous spectral features. It is also shown that SNe IIP with larger progenitor stars have ejecta with a more physically extended oxygen layer that is well-mixed with the hydrogen layer. In addition, we find a subset of objects with evidence for asymmetric 56Ni ejection, likely bipolar in shape. We also compare our observations to theoretical late-time spectral models of SNe IIP from two separate groups and find moderate-to-good agreement with both sets of models. Our SNe IIP spectra are consistent with models of 12-15 M☉ progenitor stars having relatively low metallicity (Z ≤ 0.01).
Bibliographical noteFunding Information:
We would like to thank the referee, in addition to J. Anderson, A. Clocchiatti, L. Dessart, A. Jerkstrand, K. Maguire, A. Piro, I. Shivvers, J. Spyromilio and S. Valenti, for helpful discussions that helped improve this paper. We are also indebted to many observers and data reducers, especially M. Childress, B. Cobb, O. Fox, M. Ganeshalingam, L. Ho, I. Kleiser, F. Serduke, I. Shivvers, T. Steele, B. Tucker, D. Wong and W. Zhang, as well as the staffs at the Lick, Keck, McDonald and Siding Spring Observatories, who made this work possible. Research at Lick Observatory is partially supported by a generous gift from Google.
JMS is supported by an NSF Astronomy and Astrophysics Postdoctoral Fellowship under award AST-1302771. JCW’s supernova group at UT Austin is supported by NSF Grant AST 11-09801. JV is supported by Hungarian OTKA Grant NN 107637. AVF’s group at UC Berkeley has been supported by Gary & Cynthia Bengier, the Richard & Rhoda Goldman Fund, the Christopher R. Redlich Fund, the TABASGO Foundation and NSF grant AST-1211916. RJF is supported in part by NSF grant AST-1518052 and from fellowships from the Alfred P. Sloan Foundation and the David and Lucile Packard Foundation.
- Methods: data analysis
- Supernovae: general
- Techniques: spectroscopic