Infrared spectropolarimetric detection of intrinsic polarization from a core-collapse supernova
Massive stars die an explosive death as a core-collapse supernova (CCSN). The exact physical processes that cause the collapsing star to rebound into an explosion are not well understood, and the key to resolving this issue may lie in the measurement of the shape of CCSNe ejecta. Spectropolarimetry is the only way to perform this measurement for CCSNe outside the Milky Way and Magellanic Clouds. We present the infrared spectropolarimetric detection of a CCSN enabled by the new highly sensitive WIRC+Pol instrument at Palomar Observatory, which can observe CCSNe (magnitude M = −17 mag) out to 20 Mpc at ~0.1% polarimetric precision. Infrared spectropolarimetry is less affected than optical spectropolarimetry by dust scattering in the circumstellar and interstellar media, thereby providing a less biased probe of the intrinsic geometry of the supernova ejecta. SN 2018hna, a SN 1987A-like explosion, shows 2.0 ± 0.3% continuum polarization in the J band oriented at ~160° on sky 182 days after the explosion. Assuming a prolate geometry as in SN 1987A, we infer an ejecta axis ratio of <0.48 with the axis of symmetry pointing at a 70° position angle. The axis ratio is similar to that of SN 1987A, suggesting that the two CCSNe may share intrinsic geometry and inclination angles. Our data do not rule out oblate ejecta. We also observe one other CCSN and two thermonuclear supernovae in the J band. Supernova 2020oi, a stripped-envelope type Ic SN in Messier 100 has broadband p = 0.37 ± 0.09% at peak light, indicative of either a 10% asymmetry or host interstellar polarization. The type Ia SNe 2019ein and 2020ue have <0.33% and <1.08% polarization near peak light, indicative of asymmetries of less than 10% and 20%, respectively.
© 2021 Nature Publishing Group. Received 14 May 2020; Accepted 27 January 2021; Published 11 March 2021. We thank A. Cikota and J. Sollerman for reading the manuscript and providing helpful comments and suggestions. We thank L. Dessart for helpful discussions, and for providing us with a model of optical depth in SN 1987A-like explosions. We thank the following for providing machine-readable data for the SNe listed: A. Cikota for SN 2005df; T. Nagao for SN 2017gmr; and M. Tanaka for SNe 2005bf, 2007gr and 2009mi. The data presented herein were obtained at Palomar Observatory, which is operated by a collaboration between the California Institute of Technology, the Jet Propulsion Laboratory, Yale University and the National Astronomical Observatories of China. 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 the National Aeronautics and Space Administration. This research made use of Astropy, a community-developed core Python package for Astronomy. 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 the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. We wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. Figure 4c is based on observations made with the NASA/ESA Hubble Space Telescope, and obtained from the Hubble Legacy Archive, which is a collaboration between the Space Telescope Science Institute (STScI/NASA), the Space Telescope European Coordinating Facility (ST-ECF/ESA) and the Canadian Astronomy Data Centre (CADC/NRC/CSA). Data availability: The data that support the findings of this study are available from the corresponding author upon reasonable request. Code availability: The WIRC+Pol data reduction pipeline can be publicly accessed at https://github.com/WIRC-Pol/wirc_drp. The specific scripts used to reproduce the results of this study are available from the corresponding author upon reasonable request. Author Contributions: S.T., M.M.-B., D.M., N.J., G.V. and E.S. were responsible for the design, construction and commissioning of the WIRC+Pol instrument, which enabled this study. S.T., M.M.K. and D.M. designed the experiment. S.T. and M.M.-B. obtained and analysed the data. S.T., D.C.L. and M.B. interpreted the results. M.M.K., K.D. and M.H. designed and built the Gattini-IR telescope and were responsible for providing photometric data for the analysis. S.T. and M.M.-B. prepared the manuscript with input and review from all authors. The authors declare no competing interests. Peer review information: Nature Astronomy thanks Grant Williams and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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