The Physics of Type Ia Supernova Explosions: A new European Research and Training Network

European astronomers and researchers at the Max-Planck-Institute for Astrophysics have joined in a EU funded Research and Training Network to perform systematic and detailed observations of nearby Type Ia supernovae. They aim at a better understanding of the nature of these stellar explosions and the use of very distant supernovae for determining the structure of our Universe.

Some stars end their lives with a powerful explosion which can destroy them completely. Usually, these explosions are accompanied by a dramatic increase of the light output from what was the star. If one such event occurs sufficiently close to us, it can be observed even with the naked eye. Only a few such events were ever so bright, and they were called `novae' (`new' stars) by ancient astronomers.

We now differentiate between mild (in relative terms!) surface explosions (Novae) and the very bright explosions that mark the sudden end of a star's life, and call the latter, rather un-inventively perhaps, `Supernovae'.

This is however a well-deserved name. For a few weeks, a single supernova (SN) can emit almost as much light as a whole galaxy, even though a galaxy contains about a billion stars (Fig.1).

When looked at closely, SNe are not all the same. We now know that their properties depend on the properties of the star that exploded. The one common feature is that their light output depends on the synthesis of ⁵⁶Ni in the nuclear reactions that accompany or lead to the explosion. ⁵⁶Ni is an unstable nucleus, and it decays into ⁵⁶Co, which then decays into stable ⁵⁶Fe. These decays are accompanied by the emission of gamma-rays, which eventually become the SN light.

It is fortunate that the brightest among all SNe, those called Type Ia's, also appear to be the most homogeneous group: their light curves are all very similar, and so are their spectra at all phases.

The spectra (Fig.2) show strong lines of Fe, confirming that SNe Ia produce much ⁵⁶Ni. Lines of intermediate-mass elements such as Si and Ca are also present, while the lighter elements, H and He, which are the basic constituents of stars, are completely absent from the spectra. Actually, this is the defining property of SNe Ia. This is commonly taken to suggest that SNe Ia are the results of the explosion of very old stars, which have lost their outer, H-rich envelopes, and have evolved to the state of White Dwarfs. These are essentially the inner cores of relatively low-mass stars, which are left behind to cool after the star has exhausted its energy source.

Clearly, if the luminosity of SNe Ia is roughly constant, we should easily be able to determine their distances comparing the observed flux with the flux at the source. Because they are so bright, SNe Ia can be used to probe very large distances, so that not only can we determine the size of the local universe, but also question the shape and nature of the universe as a whole, which is known as Cosmology.

Unfortunately, things are not that easy, and all SNe Ia are not exactly equal. They can however be placed on a relative brightness scale based on some small variations of the shapes of their light curves. Using this method, astronomers have been able to measure distances for SNe Ia out to a redshift of about 1.

The surprising result was that the universe seems to be accelerating its expansion, which is the opposite of what the most favoured cosmological theories predicted. This result has important consequences for all of physics, because the acceleration would be caused by either a cosmological constant (a term in the cosmological equations initially introduced by Einstein, who however did not believe in it because of its implications - "... my biggest blunder...") or by a yet unknown form of `dark energy' with negative pressure.

Currently, our understanding of the supernova physics is the major systematic uncertainty of this result. The distant supernovae exploded at a time when our solar system was just forming. There is no guarantee that these distant explosions are the same as the nearby ones on which the empirical relations are based. Only once we understand the physics of the explosions will we be able to assess whether they can be used as distance indicators reliably, and whether we have to search for new physics beyond the standard models of particle physics and cosmology.

In order to improve our knowledge of these key objects through an advance in both observations and modelling, a group of European astronomers led by Wolfgang Hillebrandt at MPA has organized as a team and proposed for a Research and Training Network, which has been funded by the EU and has recently started. The idea is that accurate observations and modelling of relatively nearby SNe is the only way that we can presently follow to understand their nature and the causes of their range of properties.

Participants to the Network include German, British, Italian, French, Spanish and Swedish institutes. Researchers involved have begun the programme by successfully applying for joint observing time on most major European telescopes, thus maximizing and optimizing the amount of telescope time allocated for SN studies.

In the short time since its beginning, the RTN has already collected very accurate data on two nearby SNe Ia, SNe 2002bo and 2002er. Both of these SNe appear to be rather standard. A spectrum of SN 2002bo near maximum is shown in Fig.2. It is possible to identify many of the lines in the spectrum, which is dominated by elements such as Fe, Si, S, Ca. The photometry of SN 2002er (Fig.3) is perhaps the most accurate coverage of the evolution of a SN Ia available to date. It is through data such as these that we can hope to make significant progress in our understanding of Type Ia SNe. We expect that the elements visible in the spectrum change as the SN evolves. The material ejected with the SN thins out with time, as it expands, and deeper and deeper layers are therefore revealed. The structure of these layers can show how SNe Ia explode, something we only have indirect and vague knowledge on so far.

At the same time, advances in multi-dimensional modelling of thermonuclear explosions in White Dwarfs will give accurate predictions (see HIGHLIGHT of June 2000), through the use of Radiative Transfer codes, of the observational consequences of these theoretical models. We are in an ideal position to make a decisive step in our knowledge, and will be ready to treat observations of distant SNe Ia as soon as they will be of a quality comparable to that of nearby ones.

Paolo Mazzali, Wolfgang Hillebrandt

Further information:
The SNIa Research Training Network
Thermonuclear Flames in Type Ia Supernova Explosions - A Microscopic View
Type Ia Supernova Simulations