We argue that detonations of sub-Chandrasekhar mass white dwarfs can lead to bright explosions with light curves and spectra similar to those of observed Type Ia supernovae. Given that binary systems containing accreting sub-Chandrasekhar mass white dwarfs should be common, this suggests that a non-negligible fraction of the observed Type Ia supernova rate may arise from sub-Chandrasekhar mass explosions, if they can be ignited. We discuss aspects of how such explosions might be realized in nature and both merits and challenges associated with invoking sub-Chandrasekhar mass explosion models to account for observed Type Ia supernovae.

The Supernova Working Group was re-established at the IAU XXV General Assembly in Sydney, 21 July 2003, sponsored by Commissions 28 (Galaxies) and 47 (Cosmology). Here we report on some of its activities since 2005.

As far as coordination and planning of observational activities are concerned, the main need for WG-SN activities seem to lie in the field of suitable follow-ups to the many on-going and planned search programs (in particular for SNe Ia's; LOTOSS, NGSS, SNfactory, SNLS, ESSENCE, GOODS, PanStarrs, . . .). With the new robotic 2m-class telescopes (e.g., the Las Cumbres Observatory Global Telescope Network and other robotic telescopes) photometric follow-ups do not seem a major problem, but for most of the searches spectroscopic follow-up requires 4m-class telescopes at least which will become rare in the future. Possible ways out were discussed.

The year 2006 marks the 1000th anniversary of the supernova of 1006 C.E., the brightest supernova in all of recorded human history. This is also a time of great excitement in the supernova community: Observations from space observatories including Hubble, Chandra, XMM-Newton, and Spitzer, together with ones from powerful new ground-based telescopes and instruments, are revealing supernova remnants in the Galaxy and beyond in unprecedented detail. Fully three-dimensional computational codes and simulations running on powerful new machines are providing insight into the physics of supernovae freed from the simplifying assumptions that have restricted past understanding. Automated supernova searches are discovering hundreds of new supernovae every year, some at redshifts of 1 or beyond. And supernovae have revolutionized cosmology through the discovery of an accelerating universe, and they hold promise for deepening our understanding of the ‘dark energy’ that drives the acceleration.

The Supernova Working Group (SNWG) was re-established at the General Assembly in Sidney on July 21, 2003, sponsored by Commissions 28 (Galaxies) and 47 (Cosmology). Here we report on some of its activities since 2003.

The present status of our understanding of core-collapse and of thermonuclear supernovae is reviewed. It will be argued that the failure of numerical simulations of the collapse of massive stars to produce explosions is probably caused by our incomplete knowledge of the (micro-) physics involved. In contrast, for thermonuclear (type Ia) supernovae the basic physics seems to be well under control and, therefore, it is not surprising that model predictions and observations are in good agreement.

Present stellar evolution codes predict that stars with He-core masses above approximately 2 M⊙, corresponding to main sequence masses of at least 8 M⊙ burn carbon non-violently. After hydrostatic core carbon burning all those stars contain O-Ne-Mg cores but their further evolution is strongly dependent on the stellar entropy and thus on the main sequence and the core mass. If the He-core mass is below 3 M⊙ the O-Ne-Mg core grows due to carbon-burning in a shell and the crucial question is, whether or not it grows beyond the critical mass for Neignition (≅1.37 M⊙). Stars with He-cores less massive than about 2.4 M⊙ will never ignite Ne, but due to electron-captures, mainly on Ne and Mg, their cores will contract until O-burning begins. Since the matter of the O-Ne-Mg core is weakly degenerate O-burning propagates as a (subsonic) deflagration front and incinerates a certain fraction of the core into a nuclear statistical equilibrium (NSE) composition of iron-group elements (Nomoto, 1984). If, on the other hand, the mass of the O-Ne-Mg core is slightly larger than 1.37 M⊙ Ne and O burn in a shell from about 0.6 M⊙ to 1.4 M⊙, but again the outcome is a NSE-composition (Wilson et al., 1985). In both cases the core-mass finally exceeds the Chandrasekhar limit because electron captures on free protons and heavy nuclei lower the electron concentration and consequently also the effective Chandrasekhar mass. The cores, therefore, continue to contract and finally collapse to neutron star densities with iron-core masses between 0.7 and 1.4 M⊙.