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The electron-capture origin of supernova 2018zd

Abstract

In the transitional mass range (~8–10 solar masses) between white dwarf formation and iron core-collapse supernovae, stars are expected to produce an electron-capture supernova. Theoretically, these progenitors are thought to be super-asymptotic giant branch stars with a degenerate O + Ne + Mg core, and electron capture onto Ne and Mg nuclei should initiate core collapse1,2,3,4. However, no supernovae have unequivocally been identified from an electron-capture origin, partly because of uncertainty in theoretical predictions. Here we present six indicators of electron-capture supernovae and show that supernova 2018zd is the only known supernova with strong evidence for or consistent with all six: progenitor identification, circumstellar material, chemical composition5,6,7, explosion energy, light curve and nucleosynthesis8,9,10,11,12. For supernova 2018zd, we infer a super-asymptotic giant branch progenitor based on the faint candidate in the pre-explosion images and the chemically enriched circumstellar material revealed by the early ultraviolet colours and flash spectroscopy. The light-curve morphology and nebular emission lines can be explained by the low explosion energy and neutron-rich nucleosynthesis produced in an electron-capture supernova. This identification provides insights into the complex stellar evolution, supernova physics, cosmic nucleosynthesis and remnant populations in the transitional mass range.

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Fig. 1: Normalized pseudobolometric light curves.
Fig. 2: UV-colour light curves.
Fig. 3: Flash spectral time series.
Fig. 4: Nebular spectral time series.

Data availability

The data that support the plots within this paper and other findings of this study are available from the Open Supernova Catalog (https://sne.space/) and the Weizmann Interactive Supernova Data Repository (https://wiserep.weizmann.ac.il/), or from the corresponding author upon reasonable request.

Code availability

MESA is publicly available at http://mesa.sourceforge.net/.

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Acknowledgements

We are grateful to A. Suzuki, T. Takiwaki, T. Nozawa, M. Tanaka, C. Kobayashi, R. Ouchi, T. Matsuoka, T. Hayakawa, S. I. Blinnikov, K. Chen, L. Bildsten and B. Paxton for comments and discussions, to C. P. Gutiérrez and A. Pastorello for sharing the velocity data of the type II SN sample and SN 2005cs (respectively), and to P. Iláš for creating the colour-composite image. D.H., D.A.H., G.H., C.M. and J.B. were supported by the US National Science Foundation (NSF) grants AST-1313484 and AST-1911225, as well as by the National Aeronautics and Space Administration (NASA) grant 80NSSC19kf1639. D.H. is thankful for support and hospitality by the Kavli Institute for the Physics and Mathematics of the Universe (IPMU) where many discussions of this work took place. J.A.G. is supported by the NSF GRFP under grant 1650114. K.M. acknowledges support by JSPS KAKENHI grants 20H00174, 20H04737, 18H04585, 18H05223 and 17H02864. K.N.’s work and D.H.’s visit to Kavli IPMU have been supported by the World Premier International Research Center Initiative (WPI Initiative), MEXT, and JSPS KAKENHI grants JP17K05382 and JP20K04024, Japan. I.A. is a CIFAR Azrieli Global Scholar in the Gravity and the Extreme Universe Program and acknowledges support from that programme, from the Israel Science Foundation (grants 2108/18 and 2752/19), from the United States – Israel Binational Science Foundation (BSF), and from the Israeli Council for Higher Education Alon Fellowship. Research by K.A.B., S.V. and Y.D. is supported by NSF grant AST-1813176. J.E.A. and N.S. receive support from NSF grant AST-1515559. Research by D.J.S. is supported by NSF grants AST-1821967, 1821987, 1813708, 1813466 and 1908972. G.S.A. acknowledges support from the Infrared Processing and Analysis Center (IPAC) Visiting Graduate Student Fellowship and from NASA/HST grant SNAP-15922 from the Space Telescope Science Institute (STScI), which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under NASA contract NAS5-26555. A.V.F. is grateful for financial assistance from the Christopher R. Redlich Fund, the TABASGO Foundation, and the UC Berkeley Miller Institute for Basic Research in Science (where he is a Senior Miller Fellow); additional funding was provided by NASA/HST grant AR-14295 from STScI. G.F. acknowledges support from CONICET through grant PIP-2015-2017-11220150100746CO and from ANPCyT through grant PICT-2017-3133. This paper made use of data from the Las Cumbres Observatory global network of telescopes through the Global Supernova Project. Some of the observations reported herein were obtained at the Bok 2.3 m telescope, a facility of the University of Arizona, at the MMT Observatory, a joint facility of the University of Arizona and the Smithsonian Institution, and 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 Keck Observatory was made possible by the generous financial support of the W. M. Keck Foundation. This work is partly based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at STScI. These observations are associated with programmes GO-9788, GO-13007 and GO-15151. Financial support for programme GO-15151 was provided by NASA through a grant from STScI. This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. We thank the support of the staffs at the Neil Gehrels Swift Observatory. This research has made use of the NASA/IPAC Extragalactic Database (NED), which is funded by NASA and operated by the California Institute of Technology, as well as IRAF, which is distributed by NOAO (operated by AURA, Inc.), under cooperative agreement with NSF. Numerical computations were in part carried out on the PC cluster at the Center for Computational Astrophysics, the National Astronomical Observatory of Japan. We recognize and acknowledge the very significant cultural role and reverence that the summits of Maunakea and Haleakal\(\bar{\,\text{a}\,}\) have always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from these mountains.

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D.H. initiated the study, triggered follow-up observations, reduced the Las Cumbres data, produced the light-curve models, performed the analysis and wrote the manuscript. D.A.H. is the principal investigator of the Las Cumbres Observatory Global Supernova Project through which all of the Las Cumbres data were obtained; he also assisted with data interpretation and the manuscript. S.D.V.D. is the principal investigator of the HST programme ‘The Stellar Origins of Supernovae’ (GO-15151) through which the post-explosion HST data were obtained; he also found the progenitor candidate in the pre-explosion HST F814W image, calculated the upper limits in the pre-explosion HST and Spitzer images, and assisted with data interpretation and the manuscript. J.A.G. produced the progenitor and light-curve models and assisted with their interpretation and the manuscript. K.M. assisted with theoretical nebular spectral model interpretation and the manuscript. T.J.M. and N.T. assisted with theoretical SAGB progenitor and ECSN light-curve model interpretations and the manuscript. K.N. assisted with theoretical SAGB progenitor and ECSN explosion model interpretation and the manuscript. G.H. assisted in obtaining the Las Cumbres data, reduced the FLOYDS spectra and contributed comments to the manuscript. I.A., C.M. and J.B. assisted in obtaining the Las Cumbres data; I.A. and C.M. also contributed comments to the manuscript. K.A.B. obtained the Keck LRIS and DEIMOS spectra, reduced the LRIS spectra and contributed comments to the manuscript. S.V. is the principal investigator of the Keck proposals (2018B, project code U009; 2019A, project code U019; 2019B, project code U034) under which the nebular spectra were obtained; he also built the Las Cumbres photometric and spectroscopic reduction pipelines, reduced the Keck DEIMOS spectrum and contributed comments to the manuscript. Y.D. assisted in obtaining the Keck LRIS and DEIMOS spectra. P.J.B. obtained and reduced the Swift UVOT data. J.E.A. obtained and reduced the MMT and Bok spectra. C.B. reduced and analysed the MMT SPOL spectropolarimetry. G.G.W. is the principal investigator of the Supernova Spectropolarimetry (SNSPOL) project. P.S.S. is the principal investigator of the SPOL instrument. G.G.W. and P.S.S. collected the spectropolarimetric data with the SPOL instrument at the MMT Observatory. N.S. is the principal investigator of the MMT and Bok programmes; he also contributed comments to the manuscript. D.J.S. co-leads the University of Arizona team that obtained the MMT and Bok spectra; he also contributed comments to the manuscript. G.S.A. reduced and analysed the archival HST WFC3/IR data (GO-12206) of the host galaxy NGC 2146. C.X. and C.M. analysed and rejected the cosmic rays in the pre-explosion HST F814W image. A.V.F., M.C.B., G.F. and P.L.K. are co-investigators of the HST programme (GO-15151); they also contributed comments to the manuscript (which A.V.F. edited in detail). T.N. monitored the supernova and provided his photometry. K.I. is the discoverer of the supernova; he also monitored the supernova and provided his photometry.

Corresponding author

Correspondence to Daichi Hiramatsu.

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Extended data

Extended Data Fig. 1 The host galaxy and post- and pre-explosion images of SN 2018zd.

a, Las Cumbres 2 m BVgr-composite image of SN 2018zd and the host starburst galaxy NGC 2146 (Supplementary Information), courtesy of Peter Iláš. At the assumed luminosity distance of 9.6 Mpc, 1’ corresponds to 2.8 kpc. SN 2018zd is on a tidal stream which was likely ejected during a galaxy merger event. b, Portion of an HST WFC3/UVIS F814W mosaic obtained on 2019 May 19, 443.7 d after the explosion of SN 2018zd (indicated by the tick marks). c, Portion of an HST ACS/WFC F814W mosaic from 2004 April 10; the SN site is similarly indicated by tick marks. This mosaic consists of a single exposure, so to remove a number of cosmic-ray hits in the image, we use a masked mean filter to smooth any pixels that have a score of 0.001 or higher from our deep-learning model (Methods). The pixels associated with the progenitor candidate had scores < 4 × 10−5, so are not affected. d, Same as panel (c), but with F658N on the same epoch. e, Portion of a Spitzer IRAC 3.6 μm mosaic obtained on 2011 November 15, with the SN site again indicated by tick marks. All panels (b)–(e) are shown to the same scale and orientation, with north up and east to the left. The progenitor candidate is identified only in the single HST ACS/WFC F814W image (c).

Extended Data Fig. 2 SN progenitor and SAGB candidate SEDs.

The SED for the SN 2018zd progenitor candidate resulting from pre-explosion HST and Spitzer archival data (Methods; black solid circles). For comparison we show model SEDs from BPASS v2.251 for SAGB stars (in the initial mass range Minit = 6 − 8 M with bolometric luminosities L ≈ 105L in the last model timestep; navy curves) and RSG stars at Minit = 8 M (purple curves) and Minit = 15 M (orange curves), at metallicities Z = 0.02 (solar; short-dashed line) and Z = 0.01 (subsolar; long-dashed line). The SEDs of the BPASS models are extrapolated into the mid-infrared via MARCS79 model stellar atmospheres of similar temperatures as the last BPASS model timesteps, deriving synthetic photometry from those atmosphere models using the bandpass throughputs provided in the Spitzer IRAC and MIPS Instrument Handbooks. Also shown for comparison are the SEDs for the SAGB candidate MSX SMC 055 (assuming Galactic foreground extinction and adjusted to a Small Magellanic Cloud distance modulus of μ = 18.90 mag from the Extragalactic Distance Database80; red open pentagons52) and for the progenitor of the low-luminosity Type II-P SN 2005cs (assuming the total reddening from the two studies53,54 and adjusted to a recent accurate distance for M5181; blue open squares53, green open diamonds54). The luminosity of the HST ACS/WFC F814W detection of the SN 2018zd progenitor candidate lies between MSX SMC 055 and the SN 2005cs progenitor.

Extended Data Fig. 3 Multiband light curve of SN 2018zd.

a, Multiband light curve of SN 2018zd focusing on the early rise. A quadratic function \({F}_{1}{(t-{t}_{0})}^{2}\) is fitted to the unfiltered optical Itagaki and the first three Noguchi points to estimate an explosion epoch t0 = MJD 58178.4 ± 0.1 (Supplementary Information). The observed flash-spectroscopy epochs (Extended Data Fig. 4) are marked by the vertical dashed lines. Note the sharper rise in the Swift UVW2 than in the V and unfiltered photometry during the flash-spectroscopy epochs. b, Multiband light curve of SN 2018zd up to the 56Co decay tail. The data gap is due to the Sun constraint. Error bars denote 1σ uncertainties and are sometimes smaller than the marker size. The light-curve shape resembles that of a typical Type II-P SN. Comparing the luminosity on the tail to that of SN 1987A82, we estimate a 56Ni mass of (8.6 ± 0.5) × 10−3 M at the assumed luminosity distance of 9.6 Mpc.

Extended Data Fig. 4 Optical spectral time series of SN 2018zd.

The flash features (for example, He ii, C iii, and C iv) persist up to > 8.8 d and disappear before 16.8 d. Then the broad Balmer-series P Cygni lines appear, typical of the photospheric phase of a Type II-P SN. After ~ 200 d, the nebular emission lines (for example, Hα, [Ca ii], and [Ni ii]) dominate over the relatively flat continuum.

Extended Data Fig. 5 Expansion velocities as a function of time.

Comparison of the unnormalised (a, b, c) and normalized (to day 50; d, e, f) Hα, Hβ, and Fe iiλ5169 expansion velocities of SN 2018zd (Supplementary Information) with a Type II SN sample83 (transparent lines), including archetypal SN 1999em, along with low-luminosity SN 2005cs84, early-flash SN 2013fs21, and low-luminosity and early-flash SN 2016bkv77,85. Error bars denote 1σ uncertainties and are sometimes smaller than the marker size. Note the pronounced early Hα and Hβ rises and the relatively flat velocity evolution (up to ~ 30 d) of SN 2018zd, indicating shock propagation inside the dense, optically-thick CSM.

Extended Data Fig. 6 MESA+STELLA progenitor and degenerate light-curve models.

a, b, Ejecta mass Mej and explosion energy \({E}_{\exp }\) inferred from Eq. (1) (Methods) as a function of progenitor radius R consistent with the bolometric light curve of SN 2018zd at the assumed luminosity distance of 9.6 ± 1.0 Mpc, along with the properties of the three degenerate explosion models. The blue and red shaded regions show explosion parameters expected for ECSNe6,7,10 and typical of Fe CCSNe86, respectively. c, d, Three degenerate MESA+STELLA explosion models providing good fits to the light curve and velocities inferred from the Fe iiλ5169 line during the plateau phase. Models are labelled by \({\rm{M}}[{M}_{{\rm{ej}},\odot }]\_{\rm{R}}[{R}_{\odot }]\_{\rm{E}}[{E}_{\exp ,51}]\). Error bars denote 1σ uncertainties. Note the observed early-time excess luminosity and suppressed velocity of SN 2018zd. This light-curve degeneracy highlights the inability to distinguish ECSNe from Fe CCSNe solely based on their light curves, suggesting that many ECSNe might have been overlooked owing to the lack of additional observations. e, f, Same as panels (c, d), but adding a dense wind profile (\({\dot{M}}_{{\rm{wind}}}=0.01\ {{\rm{M}}}_{\odot }\) yr−1, vwind = 20 km s−1, and twind = 10 yr) to the three degenerate MESA models before handoff to STELLA. g, Comparison of the UV-colour models with the same wind CSM parameters as in panels (e, f). Error bars denote 1σ uncertainties. All three models with the same wind CSM parameters are able to reproduce the early-time luminosity excess and blueward UV-colour evolution almost identically, suggesting the insensitivity of a particular model choice. Despite a possible artificial velocity kink when the Fe line-forming region transitions from the CSM to the stellar ejecta, the velocity evolution with the early suppression is also reproduced.

Extended Data Fig. 7 ECSN candidate checklist.

Check marks, check+question marks, and cross marks (respectively) indicate observations consistent, perhaps consistent, and inconsistent with theoretical expectations. Dashed lines indicate the lack of observational constraints, and lone question marks indicate unknowns (Supplementary Information). For SN 2018zd, we identify a faint progenitor candidate that may be consistent with an SAGB star (Extended Data Figs. 1 & 2), and the explosion energy is consistent within the light-curve degeneracy (Extended Data Fig. 6).

Extended Data Fig. 8 ECSN rate estimators.

Comparison of the ECSN rate estimates: ‘SAGB’ is the SAGB mass window from stellar evolutionary calculations at solar metallicity5; ‘IIn’ is the observed Type IIn SN rate from a volume-limited (≤60 Mpc) sample75; ‘IIn-P+2018zd’ is a rough lower limit of the Type IIn-P SN rate within 60 Mpc combined with SN 2018zd (Methods); ‘ILRT’ is a rough estimate from ILRTs within 30 Mpc87; ‘NS’ is an estimated rate from the bimodality in the neutron star mass distribution31 assuming that the low-mass and high-mass peaks originate from ECSNe and Fe CCSNe, respectively; and ‘86Kr’ is an upper limit from the ECSN nucleosynthesis calculation76 assuming that ECSNe are the dominant production source of 86Kr. The conversion between the fraction of all CCSNe and the SAGB mass window is performed assuming the Salpeter initial mass function with lower and upper CCSN mass limits of 7.5 M and 120 M (respectively) and maximum and minimum SAGB masses of 9.25 M and 9.25 M − ΔMSAGB (respectively) at solar metallicity5. The grey vertical dotted line is where the minimum SAGB mass equals the assumed lower CCSN mass limit of 7.5 M. The grey shaded region shows a rough ECSN rate constraint by the IIn-P+2018zd lower limit and the nucleosynthesis upper limit.

Supplementary information

Supplementary Information

Supplementary Methods, Discussion, References and Figs. 1 and 2.

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Hiramatsu, D., Howell, D.A., Van Dyk, S.D. et al. The electron-capture origin of supernova 2018zd. Nat Astron 5, 903–910 (2021). https://doi.org/10.1038/s41550-021-01384-2

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