The Physics of Deflagration-to-Detonation Transition in Type Ia Supernovae

BACKGROUND: The scenario currently best capable of explaining the observational properties of normal bright type Ia supernovae (SNIa), which are of primary importance for cosmology, is the delayed detonation model of the explosion of a white dwarf star with the mass near the Chandrasekhar limit in a single-degenerate binary system. In this model, the explosion starts as a subsonic deflagration that later transitions to a supersonic detonation (deflagration-to-detonation transition, or DDT). Significant progress has been made over the years both experimentally and numerically in elucidating the physics of DDT in terrestrial confined systems. It remains unclear, however, whether and how a detonation can be formed in an unpressurized, unconfined system such as the interior of a WD. Modern large-scale multidimensional models of SNIa cannot capture the DDT process and, thus, are forced to make two crucial assumptions, namely (a) that DDT does occur at some point, and (b) when and where it occurs. As a result, delayed detonation is a parameterized model that must be "tuned" in order to obtain the proper match with the observations. This substantially hinders the possibility of investigating potential sources of systematic errors in the calibration of normal bright SNIa as standard candles. Recently we have carried out a systematic study of the high-speed turbulence-flame interaction through first-principles direct numerical simulations (DNS) using reaction models similar to those describing terrestrial chemical flames. Our analysis has shown that at sufficiently high turbulent intensities, subsonic turbulent flames in unconfined environments, such as the WD interior, are indeed inherently susceptible to DDT. The associated mechanism is based on the unsteady evolution of turbulent flames faster than the Chapman-Jouguet deflagrations. This process is qualitatively different from the traditional spontaneous reaction wave model and does not require the formation of distributed flames. These results provide the first direct ab initio demonstration of DDT in turbulent reactive flows. They show that DDT is indeed possible in unconfined media and provide a detailed physical description of this process. OBJECTIVES: Here we propose to perform the detailed and systematic analysis of the new spontaneous DDT mechanism to demonstrate its applicability in SNIa explosions and to determine precise conditions required for the onset of DDT. Culmination of this effort will be the first DNS-validated subgrid-scale DDT model capable of accurately predicting the time and location of detonation initiation and suitable for use in large-scale SNIa simulations. METHODS: All key stages of the new DDT mechanism will be studied using high-resolution direct numerical simulations of turbulence interaction with both chemical and thermonuclear flames. These will be carried out with fixed grid and adaptive mesh refinement numerical codes that have previously been extensively used in studies of both terrestrial and astrophysical combustion. The results will be incorporated as a subgrid model in large-scale 3D fluid dynamics calculations of SNIa. SIGNIFICANCE: Analysis performed in the course of this work will remove the parameterization of the single-degenerate delayed detonation model on DDT conditions. This, in turn, will open the possibility for meaningful comparison of the observational signatures of this explosion scenario with the photometric, spectroscopic, and polarimetric signatures of SNIa and, thus, for identifying and describing potential sources of systematic errors in SNIa calibration as cosmological standard candles. Substantial improvement of the accuracy of such calibration will be crucial for the success of current and future NASA missions aimed at studying the nature of dark energy.