Friday, May 14, 2021

What is a Hypernova?

Hypernova is a very energetic supernova thought to result from an extreme core-collapse scenario. In this case, a massive star (>30 M☉) collapses to form a rotating black hole emitting twin energetic jets and surrounded by an accretion disk.
It is a type of stellar explosion that ejects material with an unusually high kinetic energy, an order of magnitude higher than most supernovae, with a luminosity at least 10 times greater. They usually appear similar to a type Ic supernova, but with unusually broad spectral lines indicating an extremely high expansion velocity.

Hypernovae are one of the mechanisms for producing long gamma ray bursts, which range from 2 seconds to over a minute in duration. They have also been referred to as superluminous supernovae, though that classification also includes other types of extremely luminous stellar explosions that have different origins.

Hypernovae are now widely accepted to be supernovae with ejecta having a kinetic energy larger than about 1052 erg (1055 J), an order of magnitude higher than a typical core collapse supernova. The ejected nickel masses are large and the ejection velocity up to 99% of the speed of light.

These are typically of type Ic, and some are associated with long-duration gamma-ray bursts. The electromagnetic energy released by these events varies from comparable to other type Ic supernova, to some of the most luminous supernovae known such as SN 1999as.

The archetypal hypernova, SN 1998bw, was associated with GRB 980425. Its spectrum showed no hydrogen and no clear helium features, but strong silicon lines identified it as a type Ic supernova. The main absorption lines were extremely broadened and the light curve showed a very rapid brightening phase, reaching the brightness of a type Ia supernova at day 16. The total ejected mass was about 10 M☉ and the mass of nickel ejected about 0.4 M☉. All supernovae associated with GRBs have shown the high-energy ejecta that characterises them as hypernovae.

Unusually bright radio supernovae have been observed as counterparts to hypernovae, and have been termed “radio hypernovae”.

Models for hypernova focus on the efficient transfer of energy into the ejecta.In normal core collapse supernovae, 99% of neutrinos generated in the collapsing core escape without driving the ejection of material. It is thought that rotation of the supernova progenitor drives a jet that accelerates material away from the explosion at close to the speed of light.

Binary systems are increasingly being studied as the best method for both stripping stellar envelopes to leave a bare carbon-oxygen core, and for inducing the necessary spin conditions to drive a hypernova.

The collapsar model describes a type of supernova that produces a gravitationally collapsed object, or black hole. The word “collapsar”, short for “collapsed star”, was formerly used to refer to the end product of stellar gravitational collapse, a stellar-mass black hole. The word is now sometimes used to refer to a specific model for the collapse of a fast-rotating star. When core collapse occurs in a star with a core at least around fifteen times the sun’s mass (M☉)—though chemical composition and rotational rate are also significant—the explosion energy is insufficient to expel the outer layers of the star, and it will collapse into a black hole without producing a visible supernova outburst.

The mechanism for producing the stripped progenitor, a carbon-oxygen star lacking any significant hydrogen or helium, of type Ic supernovae was once thought to be an extremely evolved massive star, for example a type WO Wolf-Rayet star whose dense stellar wind expelled all its outer layers. Observations have failed to detect any such progenitors.

It is still not conclusively shown that the progenitors are actually a different type of object, but several cases suggest that lower-mass “helium giants” are the progenitors. These stars are not sufficiently massive to expel their envelopes simply by stellar winds, and they would be stripped by mass transfer to a binary companion. Helium giants are increasingly favoured as the progenitors of type Ib supernovae, but the progenitors of type Ic supernovae is still uncertain.

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