D.-S.M., M.R.D., and C.D.M. are supported by Discovery Grants from the Natural Sciences and Engineering Research Council of Canada. D.-S.M. was supported in part by a Leading Edge Fund from the Canadian Foundation for Innovation (project No. 30951). M.R.D. was supported in part by the Canada Research Chairs Program, the Canadian Institute for Advanced Research (CIFAR), and the Dunlap Institute at the University of Toronto. D.J.S. acknowledges support by NSF grants AST-1821987, 1821967, 1908972 and from the Heising-Simons Foundation under grant No. 2020-1864. S.G.-G. acknowledges support by FCT under Project CRISP PTDC/FIS-AST-31546 and Project UIDB/00099/2020. S.C.K., Y.L., and H.S.P. acknowledge support by KASI under the R&D program (Project No. 2022-1-868-04) supervised by the Ministry of Science and ICT. H.S.P. was supported in part by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT, Ministry of Science and ICT; No. NRF-2019R1F1A1058228). P.J.B. acknowledges support from the Swift GI program 80NSSC19K0316. S.V., Y.D., and K.A.B. acknowledge support by NSF grants AST-1813176 and AST-2008108. C. M. acknowledges support by NSF grant AST-1313484. R.L.B. acknowledges support by NASA through Hubble Fellowship grant No. 51386.01 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS 5-26555. A.G.-Y’s research is supported by the EU via ERC grant No. 725161, the ISF GW excellence center, an IMOS space infrastructure grant and BSF/Transformative and GIF grants, as well as the André Deloro Institute for Advanced Research in Space and Optics, the Schwartz/Reisman Collaborative Science Program and the Norman E. Alexander Family M Foundation ULTRASAT Data Center Fund, Minerva and Yeda-Sela; A.G.-Y. is the incumbent of The Arlyn Imberman Professorial Chair. L.G. acknowledges financial support from the Spanish Ministerio de Ciencia e Innovación (MCIN), the Agencia Estatal de Investigación (AEI) 10.13039/501100011033, and the European Social Fund (ESF) “Investing in your future” under the 2019 Ramón y Cajal program RYC2019-027683-I and the PID2020-115253GA-I00 HOSTFLOWS project, and from Centro Superior de Investigaciones Científicas (CSIC) under the PIE project 20215AT016. G.P. acknowledges support by ANID—Millennium Science Initiative—ICN12_009 and by FONDECYT Regular 1201793. J.A. is supported by the Stavros Niarchos Foundation (SNF) and the Hellenic Foundation for Research and Innovation (H.F.R.I.) under the 2nd Call of “Science and Society” Action Always strive for excellence—“Theodoros Papazoglou” (project No. 01431).

SN 2018aoz is a Type Ia SN with a B-band plateau and excess emission in infant-phase light curves ≲1 day after the first light, evidencing an over-density of surface iron-peak elements as shown in our previous study. Here, we advance the constraints on the nature and origin of SN 2018aoz based on its evolution until the nebular phase. Near-peak spectroscopic features show that the SN is intermediate between two subtypes of normal Type Ia: core normal and broad line. The excess emission may be attributable to the radioactive decay of surface iron-peak elements as well as the interaction of ejecta with either the binary companion or a small torus of circumstellar material. Nebular-phase limits on Hα and He i favor a white dwarf companion, consistent with the small companion size constrained by the low early SN luminosity, while the absence of [O i] and He i disfavors a violent merger of the progenitor. Of the two main explosion mechanisms proposed to explain the distribution of surface iron-peak elements in SN 2018aoz, the asymmetric Chandrasekhar-mass explosion is less consistent with the progenitor constraints and the observed blueshifts of nebular-phase [Fe ii] and [Ni ii]. The helium-shell double-detonation explosion is compatible with the observed lack of C spectral features, but current 1D models are incompatible with the infant-phase excess emission, Bmax - Vmax color, and weak strength of nebular-phase [Ca II]. Although the explosion processes of SN 2018aoz still need to be more precisely understood, the same processes could produce a significant fraction of Type Ia SNe that appear to be normal after ∼1 day.

Peer reviewed

Ni et al.

and the