Extremely stripped supernova reveals a silicon and sulfur formation site

M.W.C. acknowledges support from the U.S. National Science Foundation (NSF) with grants PHY-2308862 and PHY-2117997. A.V.F.’s group at UC Berkeley is grateful for financial assistance from the Christopher R. Redlich Fund, Gary and Cynthia Bengier, Clark and Sharon Winslow, Alan Eustace (W.Z. is a Bengier–Winslow–Eustace Specialist in Astronomy), William Draper, Timothy and Melissa Draper, Briggs and Kathleen Wood, Sanford Robertson (T.G.B. is a Draper–Wood–Robertson Specialist in Astronomy) and many other donors. A.G.-Y.’s research is supported by the ISF GW excellence centre, an IMOS space infrastructure grant and BSF/Transformative and GIF grants, as well as the André Deloro Institute for Space and Optics Research, the Center for Experimental Physics, a WIS-MIT Sagol grant, the Norman E. Alexander Family M Foundation ULTRASAT Data Center Fund and Yeda-Sela; A.G.-Y. is the incumbent of the Arlyn Imberman Professorial Chair. N.K. was supported by the Ministry of Science, Technological Development and Innovation of the Republic of Serbia (MST-DIRS) through contract no. 451-03-66/2024-03/200002 made with the Astronomical Observatory Belgrade and contract no. 451-03-66/2024-03/200104 made with the Faculty of Mathematics at the University of Belgrade. R.L. acknowledges support from the European Research Council (ERC) under the European Union’s Horizon Europe research and innovation programme (grant agreement 1010422). Views and opinions expressed are, however, those of the author(s) only and do not necessarily reflect those of the European Union or the European Research Council Executive Agency. Neither the European Union nor the granting authority can be held responsible for them. K. Maeda acknowledges support from the JSPS KAKENHI grants JP20H00174 and JP24H01810. A.A.M. and S.S. are partially supported by LBNL subcontract no. 7707915. N.S. acknowledges support from the Knut and Alice Wallenberg Foundation through the ‘Gravity Meets Light’ project. Y.T. acknowledges support from the JSPS KAKENHI grant 23H04900. D.T. is supported by the Sherman Fairchild Postdoctoral Fellowship at the California Institute of Technology. Y. Yang’s research is partially supported by the Tsinghua University Dushi programme and he was a Bengier–Winslow–Robertson Postdoctoral Fellow in Astronomy at UC Berkeley. We appreciate the excellent assistance provided by the staff at the various observatories in which the data were obtained. UC Berkeley undergraduate student E. Liu is thanked for her efforts in obtaining the Lick/Nickel data. Based in part on observations obtained with the 48-inch Samuel Oschin Telescope and the 60-inch Telescope (P60) at the Palomar Observatory as part of the Zwicky Transient Facility (ZTF) project. ZTF is supported by the U.S. NSF under grants AST-1440341 and AST-2034437 and a collaboration including present partners Caltech, IPAC, the Weizmann Institute of Science, the Oskar Klein Centre at Stockholm University, the University of Maryland, Deutsches Elektronen-Synchrotron and Humboldt University, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, Trinity College Dublin, Lawrence Livermore National Laboratories, IN2P3, University of Warwick, Ruhr University Bochum, Northwestern University and former partners the University of Washington, Los Alamos National Laboratories and Lawrence Berkeley National Laboratories. Operations are conducted by COO, IPAC and UW. ZTF access was supported by Northwestern University and the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). The SED Machine on P60 is based on work supported by NSF grant 1106171. Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c)3 nonprofit organization 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. Based in part on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programme(s) 105.20KC and 105.20PN. Data presented here were obtained in part with ALFOSC, which is provided by the Instituto de Astrofísica de Andaluciá (IAA) under a joint agreement with the University of Copenhagen and NOT. KAIT and its continuing operation at Lick Observatory were made possible by donations from Sun Microsystems, Inc., the Hewlett-Packard Company, AutoScope Corporation, Lick Observatory, the U.S. NSF, the University of California, the Sylvia & Jim Katzman Foundation and the TABASGO Foundation. A notable upgrade of the Kast spectrograph on the Shane 3-m telescope at Lick Observatory was made possible through generous gifts from William and Marina Kast as well as the Heising-Simons Foundation. Research at Lick Observatory is partially supported by a generous gift from Google. Based on observations made with the Liverpool Telescope operated on the island of La Palma by Liverpool John Moores University in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias with financial support from the UK Science and Technology Facilities Council. We acknowledge the use of public data from the Swift data archive.

The reduced spectra and photometry of SN 2021yfj are available on the WISeREP archive (https://www.wiserep.org/object/19115). The raw data of the observations acquired with the European Southern Observatory (X-shooter; programme IDs: 105.20PN, 105.20KC), the W. M. Keck Observatory (LRIS), the Lick Observatory (Nickel and Kast), the Liverpool Telescope (IO:O; programme IDs: JL21A14, JZ21B01), the Neil Gehrels Swift Observatory (UVOT, XRT; object ID: 00014807), the Nordic Optical Telescope (ALFOSC; programme IDs: 61-501, 64-501) and the Zwicky Transient Facility (P48) can be retrieved from their designated public data repositories.

Much analysis for this paper has been performed with publicly available software packages. The details required to reproduce the analysis are contained in the manuscript.