Stellar explosions give rise to the temporary appearance of bright new stars in the sky. Among these so-called supernovae is a particular class of objects that is characterized by the absence of hydrogen but strong evidence of silicon in their spectra. These "Type Ia" supernovae are of particular interest to modern astrophysics. Their importance stems from the fact that their absolute brightness can be obtained from observations using an empirically derived relationship between their light curve shape and brightness. Since objects of known absolute brightness can be used to measure distances, this makes Type Ia supernovae the most important beacons to measure the expansion rate of the Universe. Despite their importance, however, we still lack a good understanding of the physical origin of these explosions or of the astrophysical systems in which they occur.

In the widely-accepted standard model, Type Ia supernovae result from thermonuclear explosions of a White Dwarf star composed of carbon and oxygen. Isolated White Dwarfs, which represent the final stages of relatively low-mass stars like our Sun, are eternally stable. Having exhausted their nuclear energy supply, these stars simply cool down and fade over billions of years. Many stars, however, are part of a binary system. In such a case it is possible for the two stars to interact with each other and some such interaction is most likely the reason why explosions of White Dwarfs occur. Several evolutionary scenarios for binary star systems that could lead to Type Ia supernova explosions have been proposed but, despite considerable effort, we still do not know which ones are realized in nature.

In the currently most favoured scenario the White Dwarf accretes hydrogen-rich material from a companion star until it grows to a critical mass (the Chandrasekhar mass). At that point, the temperatures and densities in the core of the White Dwarf grow sufficiently high that thermonuclear reactions start and the White Dwarf explodes. Although this scenario is capable to reproduce the observed diversity of Type Ia supernovae (Press Release February 2007), there are severe problems in explaining the observed rate of Type Ia supernovae within this scenario as was recently revealed by MPA researchers (Press Release February 2010).

The mergers of two White Dwarfs in a close binary were suggested as an alternative to trigger Type Ia supernovae. However, it has long been unclear whether these systems could really produce thermonuclear explosions. Only recently a team of MPA researchers has shown (Press Release January 2010) that this is indeed possible. However, they concluded that this progenitor channel will produce most likely a peculiar sub-class of the Type Ia supernovae.

A third option, which has received relatively little attention so far, is that of a White Dwarf which accretes helium-rich material from a companion star. This leads to the build-up of a helium layer on the White Dwarf, which may eventually become unstable and ignite, ultimately leading to an explosion of the underlying White Dwarf. In contrast to the other scenarios outlined above, this would happen when the mass of the White Dwarf is lower than the Chandrasekhar limit. Since low-mass White Dwarfs are more common than White Dwarfs that grow to the Chandrasekhar limit, this model may do much better than the standard model in accounting for the observed rate of Type Ia supernovae. However, it had been largely discarded since the presence of a thick helium-rich layer leads to a composition of the explosion ejecta which differs from that seen in observations. New calculations by Bildsten and Shen (University of California, Santa Barbara) of the properties of the helium shell, however, have indicated that ignition might occur for much thinner shells than previously thought. This opens the door for considering whether this explosion might resemble a Type Ia supernova if the helium shell is thin enough.

In a recent paper (published in the Astrophysical Journal Letters) a team of MPA scientists re-investigated this sub-Chandrasekhar-mass scenario, considering an idealized case where the influence of the layer of accreted helium is negligible. For this they hydrodynamically simulated artificial explosions of a set of "naked" sub-Chandrasekhar-mass carbon-oxygen White Dwarfs. Using radiative transfer simulations they derived synthetic observables for their explosion models and compared them to observations of Type Ia supernovae. ”The light curves and spectra we obtain from these simulations are in astonishingly good agreement with observed properties of Type Ia supernovae,“ says Stuart Sim, the principal investigator of this study. Thus, the MPA scientists conclude that ”explosions of sub-Chandrasekhar-mass White Dwarfs might be a viable model of Type Ia supernovae if the optical display is dominated not by the products of burning in the helium-shell but by the ejecta produced in the explosion of the underlying White Dwarf.“

Friedrich Röpke, a co-investigator of the study, mentions another appealing property of sub-Chandrasekhar-mass explosions: ”Since the mass of the exploding White Dwarf is not fixed to the Chandrasekhar limit, the initial mass of the exploding carbon-oxygen White Dwarf provides a simple physical parameter to account for the range of observed brightnesses of Type Ia supernovae.“ Thus, this scenario has the potential to give a simple physical explanation for the range of properties of Type Ia supernovae. The race is now on to answer the critical question: can real explosions simultaneously meet conditions for igniting the helium shell whilst not giving rise to an outer ejecta layer which is incompatible with observations?

Markus Kromer

Original publications:

S.A. Sim, F.K. Röpke, W. Hillebrandt, M. Kromer, R. Pakmor, M. Fink, A.J. Ruiter, I.R. Seitenzahl, "Detonations in Sub-Chandrasekhar-mass C+O White Dwarfs", The Astrophysical Journal Letters 714 (2010), L52-L57

M. Fink, F. K. Roepke, W. Hillebrandt, I. R. Seitenzahl, S. A. Sim, M. Kromer, "Double-detonation sub-Chandrasekhar supernovae: can minimum helium shell masses detonate the core?", Astronomy & Astrophysics 514 (2010), id.A53