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A hybrid type Ia supernova with an early flash triggered by helium-shell detonation


Type Ia supernovae arise from the thermonuclear explosion of white-dwarf stars that have cores of carbon and oxygen1,2. The uniformity of their light curves makes these supernovae powerful cosmological distance indicators3,4, but there have long been debates about exactly how their explosion is triggered and what kind of companion stars are involved2,5,6. For example, the recent detection of the early ultraviolet pulse of a peculiar, subluminous type Ia supernova has been claimed as evidence for an interaction between a red-giant or a main-sequence companion and ejecta from a white-dwarf explosion7,8. Here we report observations of a prominent but red optical flash that appears about half a day after the explosion of a type Ia supernova. This supernova shows hybrid features of different supernova subclasses, namely a light curve that is typical of normal-brightness supernovae, but with strong titanium absorption, which is commonly seen in the spectra of subluminous ones. We argue that this early flash does not occur through previously suggested mechanisms such as the companion–ejecta interaction8,9,10. Instead, our simulations show that it could occur through detonation of a thin helium shell either on a near-Chandrasekhar-mass white dwarf, or on a sub-Chandrasekhar-mass white dwarf merging with a less-massive white dwarf. Our finding provides evidence that one branch of previously proposed explosion models—the helium-ignition branch—does exist in nature, and that such a model may account for the explosions of white dwarfs in a mass range wider than previously supposed11,12,13,14.

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Figure 1: The multi-band light curve of MUSSES1604D.
Figure 2: Comparative analysis of MUSSES1604D colour evolution.
Figure 3: Rest-frame B- and V-band light curves of MUSSES1604D and simulations.
Figure 4: An around-maximum spectral comparison of MUSSES1604D, other observed type Ia supernovae of different types, and models.


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Acknowledgements are contained within the Supplementary Information.

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Authors and Affiliations



J.J. initiated the study, carried out analysis and wrote the manuscript as the principal investigator of the MUSSES project. M.D. contributed to the initiation of the MUSSES project, and assisted with manuscript preparation and analysis together with K.M. and T.S. K.M. and T.S. organized the efforts for theoretical interpretation with J.J. and M.D. K.M. investigated the helium-detonation-triggered explosion models and conducted radiation-transfer calculations used to generate simulated light curves and spectra. T.S. developed and ran the radiation-transfer calculations used to generate simulated CEI-induced light curves. K.N. provided insights into the helium-detonation-triggered explosion models and assisted with analysis. N.Y., H.F. and S.M. are core software developers for HSC and are in charge of the HSC Subaru Strategic Program project. N.Y., N.T. and M.T. developed the HSC transient server for selecting real-time supernova candidates and contributed to Subaru/HSC observations and data reduction. T.M. contributed to the Subaru/HSC observations and to target-of-opportunity observations made with the 1.05-metre Kiso Schmidt telescope. S.W.J. contributed to SALT spectroscopy and data reduction. Ž.I., A.J.C., P.Y., P.R.-L., N.S., F.P., D.B., J.M., L.W., M.D.S., D.J., P.A.M. and C.A. are core collaborators of the MUSSES project who are in charge of follow-up observations (including proposal preparations) with the following telescopes: 3.5-metre ARC, 10.4-metre GTC, 8.1-metre VLT, 2.5-metre NOT, 2.5-metre INT and 2-metre LT. All of the authors contributed to discussions.

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Correspondence to Ji-an Jiang.

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Reviewer Information Nature thanks P. Nugent and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Figure 1 Comparison of MUSSES1604D early-phase observations and the outcomes of different model simulations.

Shown are the observed g-band (purple filled circles), rest-frame B-band (purple open squares), V-band (light green open squares), and BV (blue filled circles) curves for MUSSES1604D at flash phase. The red solid lines show the results from our best-fitting helium-detonation model (with a white dwarf (WD) of mass 1.38M and a helium shell of mass 0.03M). ac, Early B-band (a) and V-band (b) light curves and BV colour evolution (c) generated by different CEI simulations observed from the companion side. Dashed lines correspond to K10 models involving different binary-system compositions (MS, main-sequence star; RG, red-giant star)8. The magenta line denotes our best-fitting K–S CEI model19. Although an early flash as bright as that of MUSSES1604D can be produced with specific CEI models, the predicted colour is very blue in the flash phase. d, e, V-band light curves simulated by CSM–ejecta interaction models (P16 model)10 with deep (d) and shallow (e) 56Ni distribution for the inner ejecta. Dotted lines correspond to an external mass (Me) of 0.3M with different outer radii, Re. f, Colour evolution, under the same assumptions as in e. Like the CEI models, the CSM–ejecta simulations generate combinations of early light curves and colour evolution that differ from the observed features of MUSSES1604D.

Extended Data Figure 2 Spectral evolution of MUSSES1604D and analogues.

Spectra for MUSSES1604D (dark green) are compared with those of the analogous type Ia supernovae SN 2006bt, SN 2007cq and SN 2012df at similar epochs. Late-phase spectra of SN 2011fe are included for reference. SALT/Robert Stobie Spectrograph (RSS) follow-up observations were carried out −2 and 12 days after the B-band maximum, and the other two spectra were taken by the Gemini Multi-Object Spectrograph (GMOS) mounted on the Gemini-North telescope 3 and 26 days after the B-band maximum.

Extended Data Figure 3 Rest-frame B- and V-band light curves for MUSSES1604D and other type Ia supernovae.

K-corrections in the flash phase (open squares) and post-flash phase (filled squares with dashed lines) of MUSSES1604D were carried out with different methods (see Methods). The light curves of MUSSES1604D, SN 2006bt and SN 2007cq show an excellent match. Another peculiar early-flash type Ia supernova, iPTF14atg, also shows similar light curves, although its brightness is about one magnitude fainter than that of MUSSES1604D. The light curves of a normal-brightness type Ia supernova—SN 2011fe (black dotted lines)—are provided for reference. Magnitudes shown are in the Vega system; error bars denote 1σ uncertainties.

Extended Data Figure 4 Early Subaru/HSC g-band images for MUSSES1604D.

The left panel shows the earliest Subaru/HSC image of MUSSES1604D (α(J2000) = 12 h 18 min 19.85 s, δ(J2000) = +00° 15′ 17.38″) taken on 4.345 April 2016 ut, when the g-band magnitude of MUSSES1604D was 25.14 ± 0.15. The supernova then brightened rapidly to about 23.1 mag in one day (right panel).

Extended Data Figure 5 Composition structures of models used for radiation-transfer simulations.

We used the composition structures shown here in helium-detonation simulations with a sub-Chandrasekhar-mass white dwarf (a, c) or a Chandrasekhar-mass white dwarf (b, d). The mass fractions of selected elements are shown as a function of velocity (a, b) or mass coordinate (c, d). The colours are the same for all panels.

Extended Data Figure 6 Density structures of the models used for radiation-transfer simulations.

The density structures (as a function of velocity) shown here are the input models used for the radiation-transfer simulations, representing helium-detonation models for a sub-Chandrasekhar-mass white dwarf (black dashed line) and a Chandrasekhar-mass white dwarf (red line).

Extended Data Table 1 Imaging observations of MUSSES1604D
Extended Data Table 2 Properties of MUSSES1604D- and iPTF14atg-like type Ia supernovae

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Jiang, Ja., Doi, M., Maeda, K. et al. A hybrid type Ia supernova with an early flash triggered by helium-shell detonation. Nature 550, 80–83 (2017).

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