Abstract
The final fate of massive stars, and the nature of the compact remnants they leave behind (black holes and neutron stars), are open questions in astrophysics. Many massive stars are stripped of their outer hydrogen envelopes as they evolve. Such Wolf–Rayet stars1 emit strong and rapidly expanding winds with speeds greater than 1,000 kilometres per second. A fraction of this population is also helium-depleted, with spectra dominated by highly ionized emission lines of carbon and oxygen (types WC/WO). Evidence indicates that the most commonly observed supernova explosions that lack hydrogen and helium (types Ib/Ic) cannot result from massive WC/WO stars2,3, leading some to suggest that most such stars collapse directly into black holes without a visible supernova explosion4. Here we report observations of SN 2019hgp, beginning about a day after the explosion. Its short rise time and rapid decline place it among an emerging population of rapidly evolving transients5,6,7,8. Spectroscopy reveals a rich set of emission lines indicating that the explosion occurred within a nebula composed of carbon, oxygen and neon. Narrow absorption features show that this material is expanding at high velocities (greater than 1,500 kilometres per second), requiring a compact progenitor. Our observations are consistent with an explosion of a massive WC/WO star, and suggest that massive Wolf–Rayet stars may be the progenitors of some rapidly evolving transients.
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Data availability
The photometry of SN 2019hgp is available in Supplementary Table 1, and all the observations (photometry and spectra) are available from WISeREP52 (http://wiserep.weizmann.ac.il/). Matlab scripts that generate most of the plots within this paper are available from the corresponding author upon request. Opticon observations were obtained under programme ID OPT/2019A/024, PI A.G.-Y.
Code availability
Relevant software sources have been provided in the text, web locations provided as references, and are publicly available.
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Acknowledgements
This work is based on observations obtained with the Samuel Oschin 48-inch Telescope and the 60-inch Telescope at Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the US National Science Foundation (NSF) under grant AST-1440341 and a collaboration including Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein Center at Stockholm University, the University of Maryland, the University of Washington, Deutsches Elektronen-Synchrotron and Humboldt University, Los Alamos National Laboratories, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, and Lawrence Berkeley National Laboratories. Operations are conducted by COO, IPAC, and UW. This work includes observations made with the Nordic Optical Telescope (NOT), owned in collaboration by the University of Turku and Aarhus University, and operated jointly by Aarhus University, the University of Turku and the University of Oslo (representing Denmark, Finland and Norway, respectively), the University of Iceland and Stockholm University, at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofísica de Canarias. These data were obtained with ALFOSC, which is provided by the Instituto de Astrofísica de Andalucía (IAA) under a joint agreement with the University of Copenhagen and NOT. This work includes observations made with the GTC telescope, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, under Director’s Discretionary Time. 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 (NASA); the Observatory was made possible by the generous financial support of the W. M. Keck Foundation. This work includes observations obtained at the international Gemini Observatory, a programme of NSF’s NOIRLab, which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the NSF on behalf of the Gemini Observatory partnership: the NSF (USA), National Research Council (Canada), Agencia Nacional de Investigación y Desarrollo (Chile), Ministerio de Ciencia, Tecnología e Innovacíon (Argentina), Ministério da Ciência, Tecnologia, Inovações e Comunicações (Brazil), and Korea Astronomy and Space Science Institute (Republic of Korea). 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. This work includes observations obtained at the Liverpool Telescope, which is 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. Research at Lick Observatory is partially supported by a generous gift from Google. This work includes observations obtained with the Hobby–Eberly Telescope, which is a joint project of the University of Texas at Austin, the Pennsylvania State University, Ludwig-Maximilians-Universität München, and Georg-August-Universität Göttingen. These results made use of the Lowell Discovery Telescope (LDT) at Lowell Observatory. Lowell is a private, non-profit institution dedicated to astrophysical research and public appreciation of astronomy and operates the LDT in partnership with Boston University, the University of Maryland, the University of Toledo, Northern Arizona University and Yale University. This work benefited from the OPTICON telescope access programme (https://www.astro-opticon.org/index.html), funded from the European Union’s Horizon 2020 research and innovation programme under grant agreement 730890. We made use of IRAF, which is distributed by the NSF NOIRLab. A.G.-Y. is supported by the EU via ERC grant no. 725161, the ISF GW excellence centre, an IMOS space infrastructure grant, BSF/Transformative and GIF grants, as well as by the Benoziyo Endowment Fund for the Advancement of Science, the Deloro Institute for Advanced Research in Space and Optics, The Veronika A. Rabl Physics Discretionary Fund, Minerva, Yeda-Sela and the Schwartz/Reisman Collaborative Science Program; A.G.-Y. is the incumbent of the The Arlyn Imberman Professorial Chair. M.M.K. acknowledges generous support from the David and Lucile Packard Foundation; the GROWTH project was funded by the NSF under grant AST-1545949. E.C.K., J.S. and S.S. acknowledge support from the G.R.E.A.T. research environment funded by Vetenskapsrå det, the Swedish Research Council, under project number 2016-06012; E.C.K. also received support from The Wenner-Gren Foundations. The Oskar Klein Center’s participation in ZTF was made available by the K.A.W. Foundation. G.L. is supported by a research grant (19054) from VILLUM FONDEN. J.C.W. and B.P.T. are supported by NSF grant AST-1813825. A.V.F.’s supernova group at UC Berkeley is supported by the TABASGo Foundation, the Christopher R. Redlich Fund, and the Miller Institute for Basic Research in Science (A.V.F. is a Senior Miller Fellow).
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A.G.-Y. initiated the project, planned the observations, conducted spectroscopic and physical analysis, and wrote the manuscript. R.B. identified the transient, initiated follow-up observations, conducted photometric analysis, and contributed to the WIS infant SN programme. S.S. contributed to follow-up design and execution, conducted multiwavelength and host-galaxy analysis, and contributed to the manuscript. Y. Yang. contributed to follow-up design and execution, reduced the Gemini spectra, and contributed to the manuscript. D.A.P. conducted follow-up observations with the LT and contributed to the manuscript. I.I. conducted photometric and spectroscopic analysis and contributed to physical interpretation and manuscript writing. J.S. helped to plan and develop the manuscript and provided NOT data. E.C.K. analysed the bolometric light curve and provided physical modelling. M.T.S. contributed to photometric analysis. O.Y. conducted spectroscopic modelling. N.L.S. conducted a prediscovery variability search and contributed to the manuscript. E.Z. contributed to spectroscopic analysis. C.B. provided the sample of SNe Ic from PTF and reduced the NOT spectroscopy. S.R.K. is the ZTF PI. S.R.K., M.M.K., C. Fremling. and L.Y. provided Palomar and Keck data. K.D. and Y. Yao. reduced the Palomar and Keck data. E.O.O. and C. Fransson. contributed to physical interpretation. A.V.F., W.Z., T.G.B., R.J.F., J.B. and M.S. contributed Lick and Keck data; A.V.F. also contributed to the manuscript. C.M.C. contributed LT data. A.L.C.-L., D.G.-A. and A.M.-B. provided GTC observations. S.F. and T.H. provided LDT observations. J.C.W., B.P.T. and J.V. planned, obtained and reduced the HET observations. G.L. reduced the GTC spectra and contributed to the manuscript. M.J.G., D.A.D., A.J.D., R.D., E.C.B., B.R., D.L.S., I.A., Y.S., R.R. and J.v.R. are ZTF builders. N.K. contributed to the spectroscopic analysis. Many authors provided comments on the manuscript, and all authors have approved it.
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Extended data figures and tables
Extended Data Fig. 1 Early spectroscopy shows strong emission lines of carbon, oxygen and neon.
During the initial hot phase (1–5.6 d after explosion) these highly ionized lines (see Fig. 1 for detailed line identification) weaken with time, with pure emission lines evolving to P Cygni profiles, and then to absorption-dominated profiles. Major emission features are marked; the spectral area around 5,250 Å in restframe is impacted by imperfect subtraction of the strong atmospheric 5,577 Å skyline (grey shade). Five additional P60 and LT spectra with lower signal-to-noise and spectral resolution obtained during this period are omitted for clarity.
Extended Data Fig. 2 Evolution toward lower ionization.
The spectroscopic series obtained during the intermediate phase (5–19 d after explosion) follows the weakening and disappearance of the CIII and OIII absorption features seen earlier, and the emergence of a set of low-ionization emission lines, initially of CII (red) and later OI (blue; 50% intensity lines extracted as in ref. 29). Higher resolution spectra resolve the broad features in the blue into multiple narrow components better described by CIII and OIII at zero velocity than by OII blends sometimes seen in hot early phases of stripped SNe, including Type I SLSNe23,72. By day 19 (bottom) broad features appear and the spectrum shows a marked blue excess. Seven additional spectra omitted for clarity.
Extended Data Fig. 3 Development of the photospheric spectrum.
The spectroscopic series obtained during the late phase (19–27 d after explosion) evolves as features of heavier elements (for example, Mg) begin to emerge, while broad absorption features develop. Initially, strong features (such as OI 7774Å and CII 6580Å) present both a narrow (~2,000 km s−1 blue edge) absorption feature as well as a broader (~6,000 km s−1 minimum) component. At 27 d after explosion, relatively broad absorption features have developed that are reminiscent of spectra of type Ic SNe, with features from Mg, Ca and Fe appearing in addition to C and O. Excess continuum in the blue is evident, probably arising from the Fe ii pseudo-continuum often seen in spectra of interacting SNe (types IIn and Ibn23).
Extended Data Fig. 4 Blackbody SED fits calculated using PhotoFit142.
Our well sampled photometry extending from the Swift UV bands to the near-infrared (NIR) z′ band is well fit by a blackbody curve during the first 12 days after explosion. From day 15 onwards, a clear blue excess develops initially in the UV and extending into the blue part of the optical band from day 21 onward. The derived blackbody parameters (radius and temperature) are therefore less reliable from that date. Standard 1σ error bars marked.
Extended Data Fig. 5 Light curves of SN 2019hgp extending from the UV to the NIR.
Post-peak Swift B- and V-band photometry is inconsistent with data from other sources and probably unreliable. Five outlying P60 points (1 u, 2 g and 2 r) are inconsistent with the rest of the data to well above their formal errors and have been removed. Standard 1σ error bars marked.
Extended Data Fig. 6 Low-order polynomial fits to the early r-band photometry indicate that the explosion occurred on 2019 June 7.1 ± 0.2 d.
Although a linear fit does not provide a good description of the data, low-order (degrees 2–4) polynomials fit the data well and converge on an estimated explosion time occurring ~1 d prior to discovery (stars denote extrapolated times of zero flux). Stacked pre-discovery data recover a detection during the prior night. All times in the paper are reported relative to this fiducial explosion time. The last 5σ non-detection is also marked. Standard 1σ error bars marked.
Extended Data Fig. 7 A comparison of spectra of interacting SNe.
Our spectrum of SN 2019hgp is overall quite similar to those of SNe Ibn (SN 2016jc20 and SN 2018bcc75), sharing in particular the unusual non-thermal continuum that is flat on the red side, and has a pronounced elevation bluewards of ~5,500 Å (dotted line); this emission probably arises from a quasi-continuum of multiple Fe ii emission lines (resolved in some cases, for example, the Type IIn SN 2005cl110, bottom). The hallmark strong He I emission lines common to SNe Ibn (λλ5876, 6678, 7065, 7281) are absent from the spectrum of SN 2019hgp. Remarkably, this object does, however, show broad absorption features that are missing from spectra of Type Ibn and Type IIn, suggesting that strong shocks are not obscuring our line of sight at 27.4 d after explosion.
Extended Data Fig. 8 SN 2019hgp (marked by the crosshair) exploded in the outskirts of its host galaxy at a projected distance of 4.4 kpc (3.54″).
The host shows elongated arms of diffuse emission which could suggest a spiral arm or a recent episode of galaxy interaction. In this image east is to the left and north up. The image size is 40″ on the side.
Extended Data Fig. 9 Extinction fits to our first-epoch SED (+1.5 d) using various extinction laws.
a–c, Extinction fits using MW (a), LMC (b) and SMC (c) extinction laws. A fit with negligible host extinction (red) fits the data well. Values of extinction, extending up to EB−V = 0.15 mag (requiring blackbody temperatures of ~100 kK) are allowed; higher extinction is ruled out regardless of extinction law parameters (MW (d) law shown, SMC and LMC are similar). χ2 minimization is done using epochs well fit by blackbody curves (<15 d). Standard 1σ error bars marked.
Extended Data Fig. 10 Modelling of the emission complex around 4,660Å during the first two Gemini epochs.
1 day (a, b) and 3 days (c, d) after explosion. We fit a combination of a Lorentzian emission component of CIII λ4650Å along with a blueshifted Gaussian absorption component. Including an additional Lorentzian emission from He ii λ4686Å (b, d) is preferred by the data (in the χ2 sense) even though this feature does not appear as a distinct emission peak. We conclude that the presence of He ii in these spectra cannot be ruled out.
Extended Data Fig. 11 A comparison of our +27.4 d Keck spectrum of SN 2019hgp to SYNOW models.
The spectrum can be well represented by a combination of common elements seen in supernovae (oxygen, sodium, magnesium, calcium and iron); the addition of neon, which is unique to this object, seems to improve the fit substantially around 6,200–7,000 Å (yellow). We compare models without (green) and with (red) He I; we find that the contribution of helium compromises the fit around 6,000−7,000 Å, owing to the expected but unobserved contribution of the P Cygni profile of He I λ6678Å. Perhaps this could be reconciled by more sophisticated modelling, though we note that recent analysis75 suggests that the emission component from this particular transition grows stronger with time in spectra of He-rich SNe Ibn.
Supplementary information
Supplementary Information
This file contains Supplementary Figs 1–5 and Supplementary Tables 1–3 (a full machine-readable file for Supplementary Table 1 is also provided separately).
Supplementary Table 1
Full photometry of SN 2019hgp.
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Gal-Yam, A., Bruch, R., Schulze, S. et al. A WC/WO star exploding within an expanding carbon–oxygen–neon nebula. Nature 601, 201–204 (2022). https://doi.org/10.1038/s41586-021-04155-1
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DOI: https://doi.org/10.1038/s41586-021-04155-1
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