Every supernova so far observed has been considered to be the terminal explosion of a star. Moreover, all supernovae with absorption lines in their spectra show those lines decreasing in velocity over time, as the ejecta expand and thin, revealing slower-moving material that was previously hidden. In addition, every supernova that exhibits the absorption lines of hydrogen has one main light-curve peak, or a plateau in luminosity, lasting approximately 100 days before declining1. Here we report observations of iPTF14hls, an event that has spectra identical to a hydrogen-rich core-collapse supernova, but characteristics that differ extensively from those of known supernovae. The light curve has at least five peaks and remains bright for more than 600 days; the absorption lines show little to no decrease in velocity; and the radius of the line-forming region is more than an order of magnitude bigger than the radius of the photosphere derived from the continuum emission. These characteristics are consistent with a shell of several tens of solar masses ejected by the progenitor star at supernova-level energies a few hundred days before a terminal explosion. Another possible eruption was recorded at the same position in 1954. Multiple energetic pre-supernova eruptions are expected to occur in stars of 95 to 130 solar masses, which experience the pulsational pair instability2,3,4,5. That model, however, does not account for the continued presence of hydrogen, or the energetics observed here. Another mechanism for the violent ejection of mass in massive stars may be required.
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I. Arcavi is an Einstein Fellow. B.S. is a Hubble Fellow and a Carnegie-Princeton Fellow. A.V.F. is a Miller Senior Fellow. See the Supplementary Information for a full list of Acknowledgements.
The authors declare no competing financial interests.
Reviewer Information Nature thanks P. Mazzali, S. Woosley 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
a, SDSS image centered at the position of iPTF14hls. b, Palomar 48-inch deep coadded pre-discovery reference image. c, Palomar 48-inch discovery image of iPTF14hls. d, The result of subtracting the reference image from the discovery image. The position of iPTF14hls is indicated by tick marks in each image.
The bolometric light curve of iPTF14hls (a) deduced from the blackbody fits shows a late-time decline rate that is slower than the radioactive decay of 56Co (black), but consistent with both delayed accretion power (blue; t0 is the onset of accretion at the last peak which could represent a final fallback event) and magnetar spindown power (red; t0 is the formation time of the magnetar, P0 is the initial spin period and B is the magnetic field in this simple analytic model). The magnetar model, however, is not consistent with the luminosity during the first 100 days, as implied by the P48, CSS and Gaia observations (b), unless the early-time magnetar emission is substantially adiabatically degraded. TNT photometry of iPTF14hls and publicly available CSS photometry (retrieved from the CSS website) and Gaia photometry (retrieved from the Gaia Alerts website) not presented in Fig. 1 are shown in b. Data from the P48 (dashed lines) and the LCO 1-m telescope (solid lines) presented in Fig. 1 are shown for comparison. Photometric points from the same day, instrument and filter are averaged for clarity. The B - V (c) and V - I/i (d) colour evolution of iPTF14hls from the LCO 1-m data (filled squares) differs from that of the normal type II-P SN 1999em (empty circles)22, even when contracting the iPTF14hls data by a factor of 10 in time (empty squares) to compensate for the slow evolution observed in its spectra compared to that of normal type II-P supernovae. All error bars, when available, denote 1σ uncertainties.
Data from P48 (R band, 3σ nondetections), CSS (unfiltered, obtained via the CSS website), and ASAS-SN (V band, 3σ nondetections—the dark-blue arrow is a deep coadd of the three images taken during the time range denoted by the horizontal line in the marker) are shown. The dashed line indicates the discovery magnitude and the shaded region shows the 1954 outburst magnitude and its uncertainty.
Weighted average best-fit phase of iPTF14hls spectra from Superfit47, compared to the true spectral phase are shown, when fitting the entire spectrum (black) or only certain line regions as noted. The dashed lines denote constant ratios between the observed and best-fit phases (assuming the explosion happened at discovery). The spectra of iPTF14hls evolve a factor of approximately 6–10 times slower than those of other type II supernovae.
The Hα region in our highest-resolution spectrum of iPTF14hls taken on 2016 June 4 using DEIMOS on Keck II (blue), expressed in terms of normalized flux density as a function of rest-frame wavelength (bottom axis), compared to the interaction-powered type IIn SN2005cl18 (red). The top axis is the corresponding velocity of Hα. iPTF14hls shows no signs of the narrow emission or narrow P Cygni features seen in interacting supernovae.
Spectra of iPTF14hls expressed in terms of normalized flux density as a function of rest-frame wavelength taken on rest-frame day 207 (right before the rise to the brightest peak in the light curve) and day 232 (at the brightest peak in the light curve) after discovery (solid lines) are shown. The similarity of the spectra indicates that the increase of about 50% in luminosity observed in the light curve between the two epochs is equal at all wavelengths. If the increase were due only to the continuum flux, then the line emission on day 232 would have been diluted by the continuum (as simulated by the dashed line).
Evolution of the measured velocity gradient in the normal type II-P SN 1999em22 (a) and in iPTF14hls (b) are shown. At a given time, the H-line-forming region is at material expanding with velocity v1, while the Fe-line-forming region is at material expanding with lower velocity v2 (top inset in a). For SN 1999em, the H-line-forming region soon reaches the material expanding at velocity v2 as it moves inward in mass (bottom inset in a) and v2 is measured in the H lines. For iPTF14hls, in contrast, the H-line-forming region does not reach the material expanding at v2 even after the time since discovery increases by a factor of 6. If the material were ejected at the time of discovery, this would indicate an increase in the radius of the line-forming regions by a factor of about 6, which is unlikely given the observed velocity gradient between the H and Fe lines. If the material were ejected before discovery, on the other hand, the relative expansion in radius would be much smaller, thus offering one possible explanation for the constant velocity gradient observed in iPTF14hls.
Blue-filter images of the position of iPTF14hls (marked by blue ticks) from 1954 February 23 (POSS; a) and 1993 January 2 (POSS-II; b) are shown. A source is visible at the position of iPTF14hls in the 1954 image, which is not there in the 1993 image. Using aperture photometry, we find that the 1954 source is 0.31 ± 0.14 mag brighter than the underlying host galaxy at that position, corresponding to a rough outburst magnitude of about −15.6 at the luminosity distance of iPTF14hls, after removing the host-galaxy contribution and calibrating the field to the SDSS u+g bands.
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Arcavi, I., Howell, D., Kasen, D. et al. Energetic eruptions leading to a peculiar hydrogen-rich explosion of a massive star. Nature 551, 210–213 (2017). https://doi.org/10.1038/nature24030
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