Supernovae, Hypernovae and Gamma-Ray Bursts

There are two fundamentally different types of supernovae: thermonuclear explosions which occur when a white dwarf is pushed above its maximum mass of 1.4 solar masses (the Chandrasekhar limit) and which leads to the complete disruption of the star (referred to as a 'Type Ia supernova'), and core-collapse supernovae which take place when the core of a massive star has exhausted all of its nuclear fuel. In the latter case, the core of the star collapses to leave a compact remnant: a neutron star or in some cases a black hole. A small fraction of the energy released in the collapse is deposited in the envelope, leading to an explosion and the bright, spectacular display we refer to as a supernova. The appearance of the supernova, however, depends sensitively on the pre-supernova structure of the envelope and hence the star's evolutionary history in a binary. The various binary interactions (mass loss, mass accretion, tidal spin-up and merging) can fundamentally change the structure of a massive star and may thereby account for many of the observational supernova sub-classes.

As with most of stellar physics, supernovae are not just interesting in themselves, but their study has implications for cosmology, galaxy formation and probably our understanding of fundamental physics.

Type Ia supernovae (SN Ia) have recently been used as cosmological distance candles to measure the spacetime curvature of the Universe. At face value, the results suggest an accelerating Universe dominated by a cosmological constant (or "dark energy") a dramatic break from the previous picture. However, these results do not take into account possible evolutionary changes in the population of type Ia supernova progenitors. Considering that there is no agreement on what the progenitor systems of these supernovae actually are, this seriously weakens the cosmological argument. We are presently studying various types of binary systems proposed as progenitors for type Ia supernovae and model the evolution of the population of these progenitors since the early Universe, using binary population synthesis methods.

One of the most interesting speculations about type Ia supernovae arises from the observation that they can be separated into three different classes by their intrinsic luminosity: the standard "Branch-normal" SN Ia, overluminous "91T-like" supernovae and underluminous "91bg-like" supernovae. Do these three types arise via the same explosion mechanism and progenitors, and if so what is the origin of these differences in luminosity?

In trying to understand the progenitors of type Ia supernovae, we must model the mass transfer in binary systems (as it is believed that only a narrow range of mass transfer rates can lead to a successful thermonuclear explosion). The figure on the right shows the results of one of those studies. Currently the "single degenerate" scenario seems physically favoured as the explanation for the bulk of observed SN Ia but it has not been proven that we can produce enough such explosions to account for the observed rate; we are currently investigating how to increase the theoretically predicted SN Ia rate.

It is essential to study the detailed physics in the core of pre-explosion white dwarfs in order to determine the conditions at the time of ignition. We have made great progress towards modelling this phase: a large step towards producing realistic models of SN Ia explosions (see Lesaffre, Podsiadlowski and Tout, 2005 and this more accessible explanation [pdf]).

Important information about the progenitors of supernovae can gleaned by examining their host galaxies. It is rare that we have direct knowledge about the star that exploded, but different galaxy types contain different stellar populations, so statistical conclusions can be drawn. It seems likely that only double-degenerate progenitors could reasonably be expected to produce supernovae several Gyr after the end of star formation in a galaxy.

We also examine the spectra of supernovae. As the spectra change with time we can see different layers of material from the supernova and hence can attempt to reconstruct the internal structure of the pre-supernova object. In cosmological studies it is also important to understand supernova classification when only poor-quality spectra are available, to make sure that only true type Ia supernovae are included in any sample (see, e.g. astro-ph/0509195).

Gamma-Ray Bursts and Hypernovae

Gamma-ray bursts are extraordinarily bright bursts of high-energy radiation. If they emit isotropically, then some 100 to 1000 times the visible energy production of a typical supernova must be concentrated into X-ray and gamma photons production. They are currently the subject of intensive study. If the energy is isotropically emitted then we have no idea what engine could produce such vast power. Current theories require less energetic events, as we strongly suspect that the energy is emitted in a narrow beam: however, nor do we have a final idea how those jets are produced.

It was once noted that there were more theories on how GRBs were produced than there were observed GRBs. At least the theories are now converging: "short" Gamma-ray bursts are believed to result from the mergers of two neutron-stars, whilst "long" ones are thought to be produced in a particular class of core-collapse supernova. (Recently it has been acknowledged that some GRB events may also be from more local and less energetic events -- flares from soft gamma-ray repeaters, which themselves form an intriguing class of systems.)

Here we concentrate on "long" GRBs and the hypernova connection. Improved observations of GRBs and their afterglows should give us more information with which to test our models, but in the currrently favoured picture - called the "collapsar" model - a rapidly rotating massive star undergoes core collapse, leaving a torus of extremely dense material orbiting a proto-neutron star or black-hole. That torus of material should be accreted on a timescale consistent with those observed in GRBs, can produce appropriate amounts of energy and could plausibly be expected to create an energetic jet, as we see relativistic jets from less extreme accreting systems (e.g. microquasars).

Artist's Impression of a Hypernova-Produced Gamma-Ray Burst

Taken from a good NASA webpage, we suggest that the best explanation for the hypernova phenomenon requires a rapidly rotating progenitor star and hence probably a binary system, so the picture above is lacking a companion star. On the same webpage there is also an animation of a hypernova (4.3 MB). The centre of the massive star collapses into a black-hole, and large amounts of energy is liberated when further material is accreted by that black hole, producing jets that punch their way out of the stellar envelope. Note that the details of the jet formation mechanism are still unknown. A gamma-ray burst is observed when looking down the highly relativistic jets.

The identification of some type Ibc supernovae with gamma-ray bursts (e.g. SN 2003dh with GRB 030329, SN 1998bw with GRB 980425) has strengthened the GRB-hypernova model, but it seems that one major misconception still exists: possibly due to the terminology, "hypernovae" are associated with especially massive stars. We emphasise that, although a massive star is necessary it is not sufficient a criterion to guarantee a GRB: rapid rotation of the progenitor is a second requirement, which we suggest is most naturally achieved via a binary evolution channel (see Podsiadlowski et al., 2004). Within the department we are modelling GRB, both their progenitor populations and their explosion mechanisms.

Kicks From Core-Collapse Supernovae

SN 1987A

SN 1987A was the first naked-eye supernova since Kepler's supernova in 1604 and is a highly anomalous supernova. Contrary to what had been predicted, the progenitor was a blue supergiant instead of a red supergiant, and is surrounded by a spectacular, but very complex, nebula consisting of three bright rings (seen most clearly with images from the Hubble space telescope) and has various chemical anomalies in its envelope. At the moment, these anomalies can be best explained if the progenitor originally was in a binary and has merged with its companion in the not-too-distant past. We are actively modelling all the detailed physical processes involved in the merging of two massive stars, in particular the dynamical evolution of the system in the merger phase, the associated anomalous nucleosynthesis and the dynamical ejection of part of the envelope, presumably producing the triple-ring nebula around the supernova.

Morris and Podsiadlowski (astro-ph/0502288) have performed hydrodynamic calculations of mass ejection during the merger of massive stars and have been successful in reproducing the characteristics of the distinctive nebula around SN 1987A, providing further support for a binary merger explanation being responsible for the progenitor.