Long γ-ray bursts are associated with energetic, broad-lined, stripped-envelope supernovae1,2 and as such mark the death of massive stars. The scarcity of such events nearby and the brightness of the γ-ray burst afterglow, which dominates the emission in the first few days after the burst, have so far prevented the study of the very early evolution of supernovae associated with γ-ray bursts3. In hydrogen-stripped supernovae that are not associated with γ-ray bursts, an excess of high-velocity (roughly 30,000 kilometres per second) material has been interpreted as a signature of a choked jet, which did not emerge from the progenitor star and instead deposited all of its energy in a thermal cocoon4. Here we report multi-epoch spectroscopic observations of the supernova SN 2017iuk, which is associated with the γ-ray burst GRB 171205A. Our spectra display features at extremely high expansion velocities (around 115,000 kilometres per second) within the first day after the burst5,6. Using spectral synthesis models developed for SN 2017iuk, we show that these features are characterized by chemical abundances that differ from those observed in the ejecta of SN 2017iuk at later times. We further show that the high-velocity features originate from the mildly relativistic hot cocoon that is generated by an ultra-relativistic jet within the γ-ray burst expanding and decelerating into the medium that surrounds the progenitor star7,8. This cocoon rapidly becomes transparent9 and is outshone by the supernova emission, which starts to dominate the emission three days after the burst.

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Data availability

The optical spectra obtained with GTC/OSIRIS and VLT/X-shooter are available in the GRBspec repository at http://grbspec.iaa.es. The optical spectra obtained with VLT/X-shooter are also available in the WISEREP repository at https://wiserep.weizmann.ac.il/object/7496. The optical data shown in the plots and tables and the Python codes used for the data analysis are available from the corresponding author on reasonable request. The entire photometric dataset is available at https://osf.io/apq3d/. Swift XRT and UVOT data are public (https://heasarc.gsfc.nasa.gov/docs/archive.html). The open-source code TARDIS used for the spectrum synthesis is available at https://tardis.readthedocs.io/en/latest/.

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Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.


  1. 1.

    Galama, T. J. et al. An unusual supernova in the error box of the γ-ray burst of 25 April 1998. Nature 395, 670–672 (1998).

  2. 2.

    Hjorth, J. et al. A very energetic supernova associated with the γ-ray burst of 29 March 2003. Nature 423, 847–850 (2003).

  3. 3.

    Cano, Z., Wang, S.-Q., Dai, Z.-G. & Wu, X.-F. The observer’s guide to the gamma-ray burst supernova connection. Adv. Astron. 2017, 8929054 (2017).

  4. 4.

    Piran, T., Nakar, E., Mazzali, P. & Pian, E. Relativistic jets in core collapse supernovae. Preprint at https://arxiv.org/abs/1704.08298 (2017).

  5. 5.

    Izzo, L. et al. GRB 171205A: VLT/X-shooter optical counterpart and spectroscopic observations. GCN Circ. 22180 (2017).

  6. 6.

    de Ugarte Postigo, A. et al. GRB 171205A: detection of the emerging SN. GCN Circ. 22204 (2017).

  7. 7.

    Bromberg, O., Nakar, E., Piran, T. & Sari, R. The propagation of relativistic jets in external media. Astrophys. J. 740, 100 (2011).

  8. 8.

    Harrison, R., Gottlieb, O. & Nakar, E. Numerically calibrated model for propagation of a relativistic unmagnetized jet in dense media. Mon. Not. R. Astron. Soc. 477, 2128–2140 (2018).

  9. 9.

    Ramirez-Ruiz, E., Celotti, A. & Rees, M. J. Events in the life of a cocoon surrounding a light, collapsar jet. Mon. Not. R. Astron. Soc. 337, 1349–1356 (2002).

  10. 10.

    Barthelmy, S. D. et al. GRB 171205A: Swift-BAT refined analysis. GCN Circ. 22184 (2017).

  11. 11.

    Perley, D. A. & Taggart, K. GRB 171205A: host galaxy photometric properties. GCN Circ. 22194 (2017).

  12. 12.

    Vergani, S. et al. Are long gamma-ray bursts biased tracers of star formation? Clues from the host galaxies of the Swift/BAT6 complete sample of LGRBs. I. Stellar mass at z ≤ 1. Astron. Astrophys. 581, A102 (2015).

  13. 13.

    Campana, S. et al. Possible blackbody component in the X-ray spectrum of GRB171205A. GCN Circ. 22191 (2017).

  14. 14.

    Campana, S. et al. The association of GRB 060218 with a supernova and the evolution of the shock wave. Nature 442, 1008–1010 (2006).

  15. 15.

    Starling, R. L. C., Page, K., Pe’er, A., Beardmore, A. & Osborne, J. P. A search for thermal X-ray signatures in gamma-ray bursts – I. Swift bursts with optical supernovae. Mon. Not. R. Astron. Soc. 427, 2950–2964 (2012).

  16. 16.

    Bufano, F. et al. The highly energetic expansion of SN 2010bh associated with GRB 100316D. Astrophys. J. 753, 67 (2012).

  17. 17.

    Modjaz, M. et al. Early-time photometry and spectroscopy of the fast evolving SN 2006aj associated with GRB 060218. Astrophys. J. 645, L21–L24 (2006).

  18. 18.

    Xu, D. et al. Discovery of the broad-lined type Ic SN 2013cq associated with the very energetic GRB 130427A. Astrophys. J. 776, 98 (2013).

  19. 19.

    Modjaz, M., Liu, Y. Q., Bianco, F. B. & Graur, O. The spectral SN-GRB connection: systematic spectral comparisons between type Ic supernovae and broad-lined type Ic supernovae with and without gamma-ray bursts. Astrophys. J. 832, 108 (2016).

  20. 20.

    Iwamoto, K. et al. A hypernova model for the supernova associated with the γ-ray burst of 25 April 1998. Nature 395, 672–674 (1998).

  21. 21.

    Nakamura, T. et al. Explosive nucleosynthesis in hypernovae. Astrophys. J. 555, 880–899 (2001).

  22. 22.

    Matzner, C. D. & McKee, C. F. The expulsion of stellar envelopes in core-collapse supernovae. Astrophys. J. 510, 379–403 (1999).

  23. 23.

    Arnett, W. D. Type I supernovae. I – analytic solutions for the early part of the light curve. Astrophys. J. 253, 785–797 (1982).

  24. 24.

    Maeda, K. et al. Explosive nucleosynthesis in aspherical hypernova explosions and late-time spectra of SN1998bw. Astrophys. J. 565, 405–412 (2002).

  25. 25.

    Maeda, K. & Nomoto, K. Bipolar supernova explosions: nucleosynthesis and implications for abundances in extremely metal-poor stars. Astrophys. J. 598, 1163–1200 (2003).

  26. 26.

    Yoon, S.-C. & Langer, N. Evolution of rapidly rotating metal-poor massive stars towards gamma-ray bursts. Astron. Astrophys. 443, 643–648 (2005).

  27. 27.

    Moriya, T. J., Sanyal, D. & Langer, N. Extended supernova shock breakout signals from inflated stellar envelopes. Astron. Astrophys. 575, L10 (2015).

  28. 28.

    Suzuki, A. & Maeda, K. Broad-band emission properties of central engine-powered supernova ejecta interacting with a circumstellar medium. Mon. Not. R. Astron. Soc. 478, 110–125 (2018).

  29. 29.

    Nakar, E. & Piran, T. The observable signatures of GRB cocoons. Astrophys. J. 834, 28 (2017).

  30. 30.

    De Colle, F., Lu, W., Kumar, P., Ramirez-Ruiz, E. & Smoot, G. Thermal and non-thermal emission from the cocoon of a gamma-ray burst jet. Mon. Not. R. Astron. Soc. 478, 4553–4564 (2018).

  31. 31.

    Gehrels, N. et al. The Swift gamma-ray burst mission. Astrophys. J. 611, 1005–1020 (2004).

  32. 32.

    Greiner, J. et al. GROND – a 7-channel imager. Publ. Astron. Soc. Pacif. 120, 405–424 (2008).

  33. 33.

    Roming, P. W. A. et al. The Swift ultra-violet/optical telescope. Space Sci. Rev. 120, 95–142 (2005).

  34. 34.

    Poole, T. S. et al. Photometric calibration of the Swift ultraviolet/optical telescope. Mon. Not. R. Astron. Soc. 383, 627–645 (2008).

  35. 35.

    Sari, R., Piran, T. & Narayan, R. Spectra and light curves of gamma-ray burst afterglows. Astrophys. J. 497, L17–L20 (1998).

  36. 36.

    Šimon, V., Hudec, R., Pizzichini, G. & Masetti, N. Colors and luminosities of the optical afterglows of the gamma-ray bursts. Astron. Astrophys. 377, 450–461 (2001).

  37. 37.

    Granot, J., Piran, T. & Sari, R. Images, light curves and spectra of GRB afterglow. Astron. Astrophys. Suppl. Ser. 138, 541–542 (1999).

  38. 38.

    Arnaud, K. A. XSPEC: the first ten years. ASP Conf. Ser. 101, 17–20 (1996).

  39. 39.

    Schady, P. et al. Dust and metal column densities in gamma-ray burst host galaxies. Mon. Not. R. Astron. Soc. 401, 2773–2792 (2010).

  40. 40.

    Schlegel, D. J., Finkbeiner, D. P. & Davis, M. Maps of dust infrared emission for use in estimation of reddening and cosmic microwave background radiation foregrounds. Astrophys. J. 500, 525–553 (1998).

  41. 41.

    Willingale, R., Starling, R. L. C., Beardmore, A. P., Tanvir, N. R. & O’Brien, P. T. Calibration of X-ray absorption in our Galaxy. Mon. Not. R. Astron. Soc. 431, 394–404 (2013).

  42. 42.

    Cardelli, J. A., Clayton, G. C. & Mathis, J. S. The relationship between infrared, optical, and ultraviolet extinction. Astrophys. J. 345, 245–256 (1989).

  43. 43.

    Pei, Y. C. Interstellar dust from the Milky Way to the Magellanic clouds. Astrophys. J. 395, 130–139 (1992).

  44. 44.

    Poznanski, D., Prochaska, J. X. & Bloom, J. S. An empirical relation between sodium absorption and dust extinction. Mon. Not. R. Astron. Soc. 426, 1465–1474 (2012).

  45. 45.

    Osterbrock, D. E. & Ferland, G. J. (eds) Astrophysics of Gaseous Nebulae and Active Galactic Nuclei (University Science Books, Sausalito, 2006).

  46. 46.

    Wilms, J., Allen, A. & McCray, R. On the absorption of X-rays in the interstellar medium. Astrophys. J. 542, 914–924 (2000).

  47. 47.

    de Ugarte Postigo, A. et al. The luminous host galaxy, faint supernova and rapid afterglow rebrightening of GRB 100418A. Preprint at https://arxiv.org/abs/1807.04281 (2018).

  48. 48.

    Clocchiatti, A., Suntzeff, N. B., Covarrubias, R. & Candia, P. The ultimate light curve of SN 1998bw/GRB 980425. Astron. J. 141, 163 (2011).

  49. 49.

    Ferrero, P. et al. The GRB 060218/SN 2006aj event in the context of other gamma-ray burst supernovae. Astron. Astrophys. 457, 857–864 (2006).

  50. 50.

    Hjorth, J. The supernova-gamma-ray burst-jet connection. Phil. Trans. R. Soc. Lond. A 371, 20120275 (2013).

  51. 51.

    Valenti, S. et al. The broad-lined type Ic supernova 2003jd. Mon. Not. R. Astron. Soc. 383, 1485–1500 (2008).

  52. 52.

    Cano, Z. et al. GRB 161219B/SN 2016jca: a low-redshift gamma-ray burst supernova powered by radioactive heating. Astron. Astrophys. 605, A107 (2017).

  53. 53.

    Dessart, J. A. et al. Radiative-transfer models for explosions from rotating and non-rotating single WC stars. Implications for SN 1998bw and LGRB/SNe. Astron. Astrophys. 603, A51 (2017).

  54. 54.

    Nousek, L. et al. Evidence for a canonical gamma-ray burst afterglow light curve in the Swift XRT data. Astrophys. J. 642, 389–400 (2006).

  55. 55.

    Perley, D. A., Schulze, S. & de Ugarte Postigo, A. GRB 171205A: ALMA observations. GCN Circ. 22252 (2017).

  56. 56.

    Patat, F. et al. The metamorphosis of SN 1998bw. Astrophys. J. 555, 900–917 (2001).

  57. 57.

    Clocchiatti, A. et al. The type IC SN 1990B in NGC 4568. Astrophys. J. 553, 886–896 (2001).

  58. 58.

    Mazzali, P. A. et al. The type Ic hypernova SN 2002ap. Astrophys. J. 572, L61–L65 (2002).

  59. 59.

    Kerzendorf, W. E. & Sim, S. A. A spectral synthesis code for rapid modelling of supernovae. Mon. Not. R. Astron. Soc. 440, 387–404 (2014).

  60. 60.

    Lucy, L. B. Nonthermal excitation of helium in type Ib supernovae. Astrophys. J. 383, 308–313 (1991).

  61. 61.

    Mazzali, P. A., Iwamoto, K. & Nomoto, K. A spectroscopic analysis of the energetic type Ic hypernova SN 1997EF. Astrophys. J. 545, 407–419 (2000).

  62. 62.

    Ashall, C. et al. GRB 161219B-SN 2016jca: a powerful stellar collapse. Preprint at https://arxiv.org/abs/1702.04339 (2017).

  63. 63.

    MacFadyen, A. I. & Woosley, S. E. Collapsars: gamma-ray bursts and explosions in “failed supernovae”. Astrophys. J. 524, 262–289 (1999).

  64. 64.

    Metzger, B. D. et al. The diversity of transients from magnetar birth in core collapse supernovae. Mon. Not. R. Astron. Soc. 454, 3311–3316 (2015).

  65. 65.

    Hatano, K. et al. Ion signatures in supernova spectra. Astrophys. J. Suppl. Ser. 121, 233–246 (1999).

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We acknowledge A. S. Esposito for the rendering of the figures presented in this work. L.I. acknowledges support from funding associated with Juan de la Cierva Incorporacion fellowship IJCI-2016-30940. L.I., A.d.U.P., C.C.T. and D.A.K. acknowledge support from the Spanish research project AYA2017-89384-P. A.d.U.P. acknowledges support from funding associated with Ramón y Cajal fellowship RyC-2012-09975. C.C.T. acknowledges support from funding associated with Ramón y Cajal fellowship RyC-2012-09984. D.A.K. acknowledges support from funding associated with Juan de la Cierva Incorporacion fellowship IJCI-2015-26153. K.M. acknowledges support from JSPS Kakenhi grants (18H05223, 18H04585 and 17H02864). S. Schmidl acknowledges support from grant DFG Klose 766/16-3 and discussions with S. Klose. R.L.C.S. acknowledges funding from STFC. M.J.M. acknowledges the support of the National Science Centre, Poland, through POLONEZ grant 2015/19/P/ST9/04010; this project has received funding from the European Union’s Horizon 2020 research and innovation programme under Marie Skłodowska-Curie grant agreement number 665778. R.S.-R. acknowledges support from ASI (Italian Space Agency) through contract number 2015-046-R.0 and from the European Union’s Horizon 2020 programme under the AHEAD project (grant agreement number 654215). The Cosmic Dawn Center is funded by the DNRF. J.H. was supported by a VILLUM FONDEN Investigator grant (project number 16599). G.L. was supported by a research grant from VILLUM FONDEN (project number 19054). K.E.H. acknowledges support by a Project Grant (162948–051) from The Icelandic Research fund. J.J. and L.K. acknowledge support from NOVA and NWO-FAPESP grant for advanced instrumentation in astronomy.

Author information


  1. Instituto de Astrofísica de Andalucía (IAA-CSIC), Granada, Spain

    • L. Izzo
    • , A. de Ugarte Postigo
    • , C. C. Thöne
    • , D. A. Kann
    • , M. Della Valle
    • , K. Bensch
    •  & Z. Cano
  2. DARK, Niels Bohr Institute, University of Copenaghen, Copenhagen, Denmark

    • A. de Ugarte Postigo
    • , J. Selsing
    • , J. Hjorth
    • , G. Leloudas
    •  & D. B. Malesani
  3. Department of Astronomy, Kyoto University, Kyoto, Japan

    • K. Maeda
  4. INAF—Osservatorio Astronomico di Capodimonte, Napoli, Italy

    • M. Della Valle
  5. International Center for Relativistic Astrophysics Network, Pescara, Italy

    • M. Della Valle
  6. LAPTh, Université de Savoie, CNRS, Annecy-le-Vieux, France

    • M. Della Valle
  7. The Oskar Klein Centre, Physics Department, Stockholm University, Stockholm, Sweden

    • A. Sagues Carracedo
  8. Astronomical Observatory Institute, Faculty of Physics, Adam Mickiewicz University, Poznań, Poland

    • M. J. Michałowski
    • , K. Kamiński
    • , M. Krużyński
    • , T. Kwiatkowski
    •  & T. Michałowski
  9. Max-Planck-Institut für Extraterrestrische Physik, Garching, Germany

    • P. Schady
    •  & J. Bolmer
  10. Department of Physics, University of Bath, Bath, UK

    • P. Schady
  11. Thüringer Landessternwarte Tautenburg, Tautenburg, Germany

    • S. Schmidl
  12. The Cosmic Dawn Center (DAWN), Niels Bohr Institute, University of Copenhagen, Copenhagen Ø, Denmark

    • J. Selsing
    • , J. P. U. Fynbo
    • , K. E. Heintz
    •  & D. B. Malesani
  13. The Cosmic Dawn Center (DAWN), DTU-Space, Technical University of Denmark, Kongens Lyngby, Denmark

    • J. Selsing
    • , J. P. U. Fynbo
    • , K. E. Heintz
    •  & D. B. Malesani
  14. Department of Physics and Astronomy, University of Leicester, Leicester, UK

    • R. L. C. Starling
    • , N. R. Tanvir
    •  & K. Wiersema
  15. Division of Theoretical Astronomy, National Astronomical Observatory of Japan, National Institutes of Natural Sciences, Tokyo, Japan

    • A. Suzuki
  16. European Southern Observatory, Vitacura, Chile

    • J. Bolmer
  17. INAF—Osservatorio Astronomico di Brera, Merate, Italy

    • S. Campana
    •  & S. Covino
  18. Department of Physics and Astronomy, Clemson University, Clemson, SC, USA

    • D. H. Hartmann
  19. Centre for Astrophysics and Cosmology, Science Institute, University of Iceland, Reykjavik, Iceland

    • K. E. Heintz
  20. Astronomical Institute Anton Pannekoek, University of Amsterdam, Amsterdam, The Netherlands

    • J. Japelj
    • , L. Kaper
    •  & G. Pugliese
  21. Department of Physics, The George Washington University, Washington, DC, USA

    • C. Kouveliotou
  22. Astronomy, Physics and Statistics Institute of Sciences (APSIS), The George Washington University, Washington, DC, USA

    • C. Kouveliotou
  23. DTU Space, National Space Institute, Technical University of Denmark, Kongens Lyngby, Denmark

    • G. Leloudas
  24. Department of Physics, University of Warwick, Coventry, UK

    • A. J. Levan
    • , D. Steeghs
    • , K. Ulaczyk
    •  & K. Wiersema
  25. INAF—Osservatorio Astronomico di Roma, Monte Porzio Catone, Italy

    • S. Piranomonte
  26. INAF—Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Bologna, Italy

    • A. Rossi
  27. INAF—Istituto di Astrofisica e Planetologia Spaziali, Roma, Italy

    • R. Sánchez-Ramírez
  28. Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot, Israel

    • S. Schulze
  29. GEPI, Observatoire de Paris, PSL University, CNRS, Meudon, France

    • S. D. Vergani


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L.I., K.M., A.d.U.P., D.A.K., M.D.V., P.S. and C.C.T. wrote the manuscript. L.I., D.A.K. and A.d.U.P. coordinated the follow-up efforts. L.I., main coordination, X-ray and optical data reduction, spectral analysis and SED interpretation. A.d.U.P., GTC spectroscopic data reduction and analysis, discovery of the emerging supernova and the high-velocity components. K.M. and A.S., spectral synthesis modelling and interpretation. A.S.C., supernova data analysis and interpretation. N.R.T., C.C.T. and D.A.K., principal investigators of the VLT and GTC afterglow/GRB-associated supernova proposals with which all spectra were obtained. M.J.M., T.M., K.K., T.K. and M.K., planning and analysis of the RBT/PST2 observations. J.S. and J.J., VLT data reduction and analysis. K.E.H. and D.B.M. led and planned the NOT observations. P.S. and S. Schmidl contributed to UVOT and GROND data analysis and interpretation. R.L.C.S. contributed to X-ray data analysis and interpretation. D.S., K.U. and R.L.C.S. planned and analysed the GOTO observations. L.I., A.d.U.P., D.A.K., C.C.T., M.D.V., K.B., J.B., S. Campana, Z.C., S. Covino, J.P.U.F., D.H.H., K.E.H., J.H., L.K., C.K., G.L., A.J.L., D.B.M., G.P., S.P., A.R., R.S.-R., S. Schulze, D.S., N.R.T., S.D.V. and K.W. contributed to observation strategy and planning for X-shooter observations. All authors contributed to the discussion and presentation of the results and reviewed the manuscript.

Competing interests

The authors declare no competing interests.

Corresponding author

Correspondence to L. Izzo.

Extended data figures and tables

  1. Extended Data Fig. 1 Early evolution of the colour index.

    The evolution of the ub, bv, uvw1 − u and uvw2 − uvw1 colour indices, computed from UVOT data in the first 18 days after the GRB trigger. Error bars represent 1 s.d.

  2. Extended Data Fig. 2 Modelling the SEDs.

    SEDs for the epochs at TSED1 = 0.003 days and TSED2 = 0.06 days (top), TSED3 = 0.17 days and TSED4 = 0.55 days (middle), and TSED5 = 0.97 days and TSED6 = 1.97 days (bottom). All datasets use photometric data points obtained with Swift UVOT for the low-energy part of the spectrum (red). Error bars represent 1 s.d. Faint dotted lines represent the entire spectral model used, in flux density (Fv) units. The SED is complemented with VLT/X-shooter at day 0.06 (green) and GTC/OSIRIS spectra at day 0.97 and day 1.97 (red), whereas for the X-ray energy range we built specific Swift XRT spectra (black). An additional spectrum is shown in the top-left panel (black data) together with the best-fit results obtained for the Swift windowed-timing (WT) mode spectrum computed at 0.003 days using a black body plus power-law spectral model (solid line).

  3. Extended Data Fig. 3 Evolution of the light curve of SN 2017iuk.

    Evolution of the BVRCIC magnitude of SN 2017iuk as observed with the RBT/PST2 telescope. Coloured curves represent the interpolation functions used to estimate the peak brightness. Error bars represent 1 s.d.

  4. Extended Data Fig. 4 SN 2017iuk versus SN 1998bw and SN 2006aj.

    Evolution of the V (green) and RC (red) absolute magnitudes for SN 2017iuk (symbols), as observed from the NOT, OSN, RBT/PST2, GOTO and smaller telescopes (iTelescope, OASDG). The evolution in the first 30 days of SN 1998bw (dashed curves) and SN 2006aj (dot-dashed curves) are also shown, considering a common rest-frame time interval. Error bars represent 1 s.d.

  5. Extended Data Fig. 5 Spectrum of GRB 171205A/ SN 2017iuk obtained 1.5 h after the GRB detection.

    This spectrum was obtained with VLT/X-shooter in the range 3,200–19,000 Å. The inset shows the UVB arm (3,200–5,500 Å), where the emission excess at wavelengths up to 4,000 Å and a possible absorption feature at about 3,700 Å are shown.

  6. Extended Data Fig. 6 Spectroscopic evolution of SN 2017iuk in the NIR.

    Grey regions indicate telluric features in the spectra. The possible He i λ10830/Mg ii λ10914 feature is visible in the day-21 spectrum, while the Ca ii triplet shows a P-Cygni profile at bluer wavelengths.

  7. Extended Data Table 1 Log of the spectroscopic observations
  8. Extended Data Table 2 Fit results of the SEDs built from GROND, Swift UVOT and XRT data
  9. Extended Data Table 3 Model parameters
  10. Extended Data Table 4 Elemental abundances obtained from the synthesis model as a function of the expansion velocity

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