Pictures of dying stars: Surprising discovery opens up new insights into the explosion mechanisms of supernovae

A team of astronomers led by Paolo Mazzali, Max-Planck-Institute for Astrophysics (MPA), are providing hints that a recent supernova may be an intermediate between normal supernovae and gamma-ray bursts. According to them the star that exploded has collapsed into a black hole, producing a weak jet, typical of gamma-ray bursts. Mazzali and his colleagues thus negated an earlier interpretation published in Nature, which considered SN2008D as normal supernova.

Fig. 1: Composite images of NGC2770 obtained at different epochs. In the first image SN2007uy is visible as a bright spot. This supernova was being observed with Swift when SN2008D exploded. The second image shows both SN2007uy and SN2008D (the bright spot near the northern tip of the galaxy) 2 days after the discovery of SN2008D. In the third image both supernovae are still visible, but are fading noticeably.

Fig. 2: The clip is based on a galaxy + supernovae image. The brightness of the two supernovae has been modified artificially to reproduce the actual time evolution of the luminosity of the two supernovae.

The violent explosion that marks the end of the life of stars with mass larger than about 8 times the mass of the Sun is one of the most spectacular events that can be observed in the Universe. Stars can only support themselves against their own gravity by burning their interior through nuclear reactions that produce the energy that makes them shine.

When the nuclear fuel is exhausted and the core has been turned to iron, the star collapses, forming a compact object, a neutron star or a black hole. A shock wave is formed that causes the outer layers of the star to be expelled at high velocity. Energy is produced as part of these layers are nuclearly burned by the shock, causing a very bright but short-lived event called a core-collapse supernova.

The earliest signature of the explosion is the release of energetic neutrinos, which present technology only allows us to detect if the SN is in our Galaxy or in one of its nearest neighbours. However, a galaxy typically harbours only a few Supernovae per century, so this is an unlikely event.

Most SNe are detected as their optical light makes them stand out with respect to all other stars in the galaxy to which they belong. This occurs typically a few days after the explosion.

There is however a transient signal that should make it possible to discover SNe very soon after the core of the star has collapsed. Very massive stars sometimes produce a burst in the gamma- or X-rays (Gamma-ray burst, GRB). Such massive stars should produce black holes. If the star rotates sufficiently rapidly a powerful jet, which is formed as material falls into the black hole, causes the star to explode. The GRB is thought to be the visible signature of the emergence of the jet from the surface of the star. The SNe that ensue are characterised by an extremely large kinetic energy, so that they have been nicknamed "hypernovae".

Normal core-collapse SNe, on the other hand, explode because of the propagation through the outer stellar layers of a shock which is born when the neutron star is formed. The emergence of the shock that drives the explosion is also expected to be marked by a very short-lived transient in the X-rays, Ultraviolet and in optical light. These events are harder to detect serendipitously than GRBs because X-ray, UV and optical telescopes do not have the large field of view typical of gamma-ray detectors.

Recently, the NASA X- and gamma-ray satellite Swift has detected an X-ray transient that has later been associated to a SN. Researchers at the Max-Planck Institute for Astrophysics (MPA), the Italian National Institute for Astrophysics (INAF), and at various other institutions have observed the SN and have analysed the results. The team was led by Paolo Mazzali of MPA and INAF's Padova Observatory. The SN, called 2008D, is located in the galaxy NGC2770, about 100 million light years from the Earth.

"What made this event new and interesting", says Mazzali, "is that the X-ray signal was very weak and soft, very different from a GRB and more in line with what is expected from the breakout of a normal shock." So, after the SN was discovered, the team rapidly observed it from Asiago Observatory in Northern Italy and established that is was a SN of Type Ic. "These are SNe that come from stars that have lost their hydrogen and helium-rich outermost layers before exploding", says Mazzali. "This is the type of SN that is always seen in association with GRBs, which made it even more interesting". This type of SN is likely to originate from a star that was affected by a close companion in a binary system.

The team set up an observational campaign to follow the SN using both ESO and national telescopes, collecting a large quantity of data. The early behaviour of the SN indicated that it was a highly energetic event, although not quite as powerful as GRB-SNe. After a few days, however, the spectra of the SN began to transform. In particular Helium lines appeared, showing that the progenitor star was not stripped as deeply as GRB-SNe. From then on, the supernova had a typical narrow-line Type Ib (He-rich) spectrum.

Over the years, Mazzali and his group have developed theoretical models to analyse the properties of SNe. When applied to SN2008D, their models indicated that the progenitor star was originally as massive as 30 solar masses, but that at the time of the explosion the star had a mass of only 8-10 solar masses. The rest had been lost via stellar winds and binary interaction. The likely result of the collapse of such a massive star is a black hole. "Since the masses and energies involved are smaller than in every known GRB-SN, we think that the collapse gave rise to a weak jet, and that the presence of the Helium layer made it even more difficult for the jet to remain collimated, so that when it emerged from the stellar surface the signal was weak."

An American group has suggested that this event was a typical occurrence of breakout of the shock formed at the birth of the neutron star in a normal core-collapse SN. However, "the unlikely event of detecting a Type Ib SN, the high energy of the explosion, the likely formation of a black hole, and the asymmetries that we are detecting in the material expelled by the explosion, all point to an exceptional event, where the explosion is driven by a jet. As our X-ray and gamma-ray instruments become more advanced, we are slowly uncovering the very diverse properties of stellar explosions. The bright GRBs were the easiest to discover, and now we are seeing variations on a theme that link these special events to more normal ones."

These are however very important discoveries, as they continue to paint a picture of how massive star end their lives, producing compact remnants, injecting heavy elements back into the gas from which new stars will be formed, and uncovering the variety of phenomena that occur in the violent Universe.

Paolo A. Mazzali