Although delayed detonation models of thermonuclear explosions of white
dwarfs seem promising for reproducing Type Ia supernovae, the transition of the
flame propagation mode from subsonic deflagration to supersonic detonation
remains hypothetical. A potential instant for this transition to occur is the
onset of the distributed burning regime, i.e. the moment when turbulence first
affects the internal flame structure. Some studies of the burning microphysics
indicate that a deflagration-to-detonation transition may be possible here,
provided the turbulent intensities are strong enough. Consequently, the
magnitude of turbulent velocity fluctuations generated by the deflagration
flame is analyzed at the onset of the distributed burning regime in several
three-dimensional simulations of deflagrations in thermonuclear supernovae. It
is shown that the corresponding probability density functions fall off towards
high turbulent velocity fluctuations much more slowly than a Gaussian
distribution. Thus, values claimed to be necessary for triggering a detonation
are likely to be found in sufficiently large patches of the flame. Although the
microphysical evolution of the burning is not followed and a successful
deflagration-to-detonation transition cannot be guaranteed from simulations
presented here, the results still indicate that such events may be possible in
Type Ia supernova explosions.Comment: 6 pages, 2 figures, to appear in ApJ 668, 1103 (2007
