@article{b230966673014573adde3aa86b4c66e1,
title = "After the fall: Late-time spectroscopy of type IIP supernovae",
abstract = "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).",
keywords = "Methods: data analysis, Supernovae: general, Techniques: spectroscopic",
author = "Silverman, {Jeffrey M.} and Stephanie Pickett and {Craig Wheeler}, J. and Filippenko, {Alexei V.} and J{\'o}zsef Vink{\'o} and Marion, {G. H.} and {Bradley Cenko}, S. and Ryan Chornock and Clubb, {Kelsey I.} and Foley, {Ryan J.} and Graham, {Melissa L.} and Kelly, {Patrick L.} and Thomas Matheson and Shields, {Joseph C.}",
note = "Funding Information: Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and NASA; the observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community; we are most fortunate to have the opportunity to conduct observations from this mountain. Funding 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. The HET is a joint project of the University of Texas at Austin, the Pennsylvania State University, Stanford University, Ludwig-Maximilians-Universit{\"a}t M{\"u}nchen, and Georg-August-Universit{\"a}t G{\"o}ttingen. The HET is named in honour of its principal benefactors, William P. Hobby and Robert E. Eberly. The Marcario Low-Resolution Spectrograph is named for Mike Marcario of High Lonesome Optics who fabricated several optics for the instrument but died before its completion. The LRS is a joint project of the HET partnership and the Instituto de Astronom{\'i}a de la Universidad Nacional Aut{\'o}noma de M{\'e}xico. Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and NASA; the observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community; we are most fortunate to have the opportunity to conduct observations from this mountain. This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. 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. Funding 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. Publisher Copyright: {\textcopyright} 2017 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society",
year = "2017",
month = may,
day = "1",
doi = "10.1093/mnras/stx058",
language = "English (US)",
volume = "467",
pages = "369--411",
journal = "Monthly Notices of the Royal Astronomical Society",
issn = "0035-8711",
publisher = "Oxford University Press",
number = "1",
}