A hypernova is a type of supernova explosion with an energy substantially higher than that
of standard supernovae. An alternative term for most hypernovae is "superluminous supernovae".
Such explosions are believed to be the origin of long-duration gamma-ray bursts.
Just like supernovae in general, hypernovae are produced by several different types of
stellar explosion: some well modelled and observed in recent years, some still tentatively
suggested for observed hypernovae, and some entirely theoretical. Numerous hypernovae
have been observed corresponding to supernovae type Ic and type IIn, and possibly also at
least one of type IIb. 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, as discussed below.
History of the term Before the 1990s, the term "hypernova" was
used sporadically to describe the theoretical extremely energetic explosions of extremely
massive population III stars. It has also been used to describe other extreme energy
events, such as mergers of supermassive black holes.
In 1998, a paper suggesting a link between gamma-ray bursts and young massive stars formally
proposed to use the term "hypernova" for the visible after-glow from those gamma-ray bursts.
The energy of such events was speculated to be up to several hundred times that of known
supernovae. Almost simultaneously, various over-luminous
supernovae were being discovered and investigated. These events were described as hypernovae
and varied from less than five to around 50 times as energetic as other supernovae and
up to 20 times as luminous as a standard type Ia supernova at its peak. This definition
has become standard for the term "hypernova", although not all of them are associated with
gamma-ray bursts. Investigation of these types of luminous supernovae
suggests that some of them are due to explosions of extremely massive low metallicity stars
by the pair instability mechanism, although not with the energies that were speculated
for them decades earlier. Gamma-ray bursts
Gamma-ray bursts are some of the most energetic events observed in the universe, but their
origin was entirely speculative until around the year 2000. Now, hypernova explosions are
known to cause at least some gamma-ray bursts, although some gamma-ray bursts are likely
from completely different events and not all hypernovae are necessarily associated with
gamma-ray bursts. A gamma-ray burst from a nearby hypernova
could destroy life on Earth; however, no likely candidate progenitors are close enough to
be a danger. Some have suggested that a hypernova may have caused the Ordovician–Silurian
mass extinction on Earth 440 million years ago, but no categorical evidence for this
hypothesis exists. Causes of hypernovae
A wide variety of models have been proposed to explain events an order of magnitude or
more greater than standard supernovae. The collapsar and CSM models are widely accepted
and a number of events are well-observed. Other models are still only tentatively observed
or entirely theoretical. Collapsar model
The collapsar model is a type of hypernova that produces a gravitationally collapsed
object, or black hole. When core collapse occurs in a star with a core at least around
fifteen times as massive as the sun—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. A star with a core mass slightly below this
level—in the range of 5–15 times the sun—will undergo a supernova explosion, but so much
of the ejected mass falls back onto the core remnant that it still collapses into a black
hole. If such a star is rotating slowly, then it will produce a faint supernova, but if
the star is rotating quickly enough, then the fallback to the black hole will produce
relativistic jets. The energy that these jets transfer into the ejected shell renders the
visible outburst substantially more luminous than a standard supernova. The jets also beam
high energy particles and gamma rays directly outward and thereby produce x-ray or gamma-ray
bursts; the jets can last for several seconds or longer and correspond to long-duration
gamma-ray bursts, but they do not appear to explain short-duration gamma-ray bursts.
A star with a 5–15 solar mass core has an approximate total mass of 25–90 suns if
the star has not undergone mass loss. Such a star will still have a hydrogen envelope
and will explode as a type II supernova. Faint type II supernovae have been observed, but
no definite candidates for a type II hypernova. Only the very lowest metallicity population
III stars will reach this stage of their life with little mass loss. Other stars, including
most of those visible to us, will have had most of their outer layers blown away by their
high luminosity to become a Wolf-Rayet star and will explode as type Ib or type Ic supernovae.
Many observed hypernovae are type Ic and those associated with gamma-ray bursts are almost
all type Ic, and these are very good candidates for having relativistic jets produced by fallback
to a black hole. Not all type Ic hypernovae correspond to observed gamma-ray bursts but
the burst would only be visible if one of the jets were aimed towards us.
In recent years a great deal of observational data on long-duration gamma-ray bursts has
significantly increased our understanding of these events and made clear that the collapsar
model produces explosions that differ only in detail from more or less ordinary supernovae
and have energy ranges from approximately normal to around 100 times larger. Nevertheless,
they continue sometimes to be referred to in the literature as hypernovae. The word
hypernova itself was coined by S.E. Woosley. A good example of a collapsar hypernova is
Sn1998bw, which was associated with the gamma-ray burst GRB 980425. It is classified as a type Ic
supernova due to its distinctive spectral properties in the radio spectrum, indicating
the presence of relativistic matter. CSM model
Almost all observed hypernovae have had spectra similar to either a type Ic or type IIn supernova.
The type Ic hypernovae are thought to be produced by jets from fallback to a black hole, but
type IIn hypernovae have significantly different light curves and are not associated with gamma-ray
bursts. Type IIn supernovae are all embedded in a dense nebula probably expelled from the
progenitor star itself, and this circumstellar material is thought to be the cause of the
extra luminosity. When material expelled in an initial normal supernova explosion meets
dense nebular or material or dust close to the star, the shockwave converts kinetic energy
efficiently into visible radiation. Thus we see an extremely luminous supernova of extended
duration even though the initial explosion energy was the same as that of a normal supernova.
Although any supernova type could potentially produce a type IIn hypernova, given suitable
surrounding CSM, the constraints on the size and density of the CSM mean that it will almost
always be produced from the star itself immediately prior to the supernova explosion. Such stars
are hypergiants and LBVs undergoing substantial mass loss due to Eddington instability, for
example SN2005gl. Pair-instability supernova
Another type of hypernova is a pair-instability supernova, of which SN 2006gy may possibly
be the first observed example. This supernova event was observed in a galaxy about 238 million
light years from Earth. The theoretical basis for pair-instability
collapse has been known for many decades and was suggested as a dominant source of higher
mass elements in the early universe as super-massive population III stars exploded. In a pair-instability
supernova, the pair production effect causes a sudden pressure drop in the star's core,
leading to a rapid partial collapse. Gravitational potential energy from the collapse causes
runaway fusion of the core which entirely destroys the star, leaving no remnant.
Current models show that this phenomenon only happens in stars with extreme low metallicity
and masses between about 140 and 260 times the sun, making observing them in the local
universe extremely unlikely. Although originally expected to produce hypernova explosions hundreds
of times greater than a supernova, they actually produce luminosities ranging from about the
same as a normal core collapse supernova to perhaps 50 times brighter, although remaining
bright for much longer. Magnetar energy release
Models of the creation and subsequent spin down of a magnetar yield much higher luminosities
than regular supernova events and match the observed properties of at least some hypernova.
In cases where pair-instability supernova may not be a good fit for explaining a hypernova,
a magnetar explanation is more plausible. Other models
There are still models for hypernova explosions produced from binary systems, white dwarf
or neutron stars in unusual arrangements or undergoing mergers, and some of these are
proposed to account for gamma-ray bursts. See also
Gamma-ray burst progenitors Quark star
References
Further reading