A Wolf-Rayet-like progenitor of SN 2013cu from spectral observations of a stellar wind.

The explosive fate of massive Wolf-Rayet stars (WRSs) is a key open question in stellar physics. An appealing option is that hydrogen-deficient WRSs are the progenitors of some hydrogen-poor supernova explosions of types IIb, Ib and Ic (ref. 2). A blue object, having luminosity and colours consistent with those of some WRSs, has recently been identified in pre-explosion images at the location of a supernova of type Ib (ref. 3), but has not yet been conclusively determined to have been the progenitor. Similar work has so far only resulted in non-detections. Comparison of early photometric observations of type Ic supernovae with theoretical models suggests that the progenitor stars had radii of less than 10(12) centimetres, as expected for some WRSs. The signature of WRSs, their emission line spectra, cannot be probed by such studies. Here we report the detection of strong emission lines in a spectrum of type IIb supernova 2013cu (iPTF13ast) obtained approximately 15.5 hours after explosion (by 'flash spectroscopy', which captures the effects of the supernova explosion shock breakout flash on material surrounding the progenitor star). We identify Wolf-Rayet-like wind signatures, suggesting a progenitor of the WN(h) subclass (those WRSs with winds dominated by helium and nitrogen, with traces of hydrogen). The extent of this dense wind may indicate increased mass loss from the progenitor shortly before its explosion, consistent with recent theoretical predictions.

An outburst from a massive star 40 days before a supernova explosion.

Some observations suggest that very massive stars experience extreme mass-loss episodes shortly before they explode as supernovae, as do several models. Establishing a causal connection between these mass-loss episodes and the final explosion would provide a novel way to study pre-supernova massive-star evolution. Here we report observations of a mass-loss event detected 40 days before the explosion of the type IIn supernova SN 2010mc (also known as PTF 10tel). Our photometric and spectroscopic data suggest that this event is a result of an energetic outburst, radiating at least 6 × 10(47) erg of energy and releasing about 10(-2) solar masses of material at typical velocities of 2,000 km s(-1). The temporal proximity of the mass-loss outburst and the supernova explosion implies a causal connection between them. Moreover, we find that the outburst luminosity and velocity are consistent with the predictions of the wave-driven pulsation model, and disfavour alternative suggestions.

Hydrogen-poor superluminous stellar explosions.

Supernovae are stellar explosions driven by gravitational or thermonuclear energy that is observed as electromagnetic radiation emitted over weeks or more. In all known supernovae, this radiation comes from internal energy deposited in the outflowing ejecta by one or more of the following processes: radioactive decay of freshly synthesized elements (typically (56)Ni), the explosion shock in the envelope of a supergiant star, and interaction between the debris and slowly moving, hydrogen-rich circumstellar material. Here we report observations of a class of luminous supernovae whose properties cannot be explained by any of these processes. The class includes four new supernovae that we have discovered and two previously unexplained events (SN 2005ap and SCP 06F6) that we can now identify as members of the same class. These supernovae are all about ten times brighter than most type Ia supernova, do not show any trace of hydrogen, emit significant ultraviolet flux for extended periods of time and have late-time decay rates that are inconsistent with radioactivity. Our data require that the observed radiation be emitted by hydrogen-free material distributed over a large radius (∼10(15) centimetres) and expanding at high speeds (>10(4) kilometres per second). These long-lived, ultraviolet-luminous events can be observed out to redshifts z > 4.

A faint type of supernova from a white dwarf with a helium-rich companion.

Supernovae are thought to arise from two different physical processes. The cores of massive, short-lived stars undergo gravitational core collapse and typically eject a few solar masses during their explosion. These are thought to appear as type Ib/c and type II supernovae, and are associated with young stellar populations. In contrast, the thermonuclear detonation of a carbon-oxygen white dwarf, whose mass approaches the Chandrasekhar limit, is thought to produce type Ia supernovae. Such supernovae are observed in both young and old stellar environments. Here we report a faint type Ib supernova, SN 2005E, in the halo of the nearby isolated galaxy, NGC 1032. The 'old' environment near the supernova location, and the very low derived ejected mass ( approximately 0.3 solar masses), argue strongly against a core-collapse origin. Spectroscopic observations and analysis reveal high ejecta velocities, dominated by helium-burning products, probably excluding this as a subluminous or a regular type Ia supernova. We conclude that it arises from a low-mass, old progenitor, likely to have been a helium-accreting white dwarf in a binary. The ejecta contain more calcium than observed in other types of supernovae and probably large amounts of radioactive (44)Ti.

Supernova 2007bi as a pair-instability explosion.

Stars with initial masses such that 10M[symbol: see text] or= 140M[symbol: see text] (if such exist) develop oxygen cores with masses, M(core), that exceed 50M[symbol: see text], where high temperatures are reached at relatively low densities. Conversion of energetic, pressure-supporting photons into electron-positron pairs occurs before oxygen ignition and leads to a violent contraction which triggers a nuclear explosion that unbinds the star in a pair-instability supernova. Transitional objects with 100M[symbol: see text] < M(initial) < 140M[symbol: see text] may end up as iron-core-collapse supernovae following violent mass ejections, perhaps as a result of brief episodes of pair instability, and may already have been identified. Here we report observations of supernova SN 2007bi, a luminous, slowly evolving object located within a dwarf galaxy. We estimate the exploding core mass to be M(core) approximately 100M[symbol: see text], in which case theory unambiguously predicts a pair-instability supernova. We show that >3M[symbol: see text] of radioactive (56)Ni was synthesized during the explosion and that our observations are well fitted by models of pair-instability supernovae. This indicates that nearby dwarf galaxies probably host extremely massive stars, above the apparent Galactic stellar mass limit, which perhaps result from processes similar to those that created the first stars in the Universe.