Constraints on the explosion mechanism and progenitors of type Ia
  supernovae
  Observations of SN 2011fe at early times reveal an evolution analogous to a
fireball model of constant color. In contrast, our unmixed delayed detonations
of Chandrasekhar-mass white dwarfs (DDC series) exhibit a faster brightening
concomitant with a shift in color to the blue. In this paper, we study the
origin of these discrepancies. We find that strong chemical mixing largely
resolves the photometric mismatch at early times, but it leads to an enhanced
line broadening that contrasts, for example, with the markedly narrow SiII6355A
line of SN 2011fe. We also explore an alternative configuration with
pulsational-delayed detonations (PDDEL model series). Because of the pulsation,
PDDEL models retain more unburnt carbon, have little mass at high velocity, and
have a much hotter outer ejecta after the explosion. The pulsation does not
influence the inner ejecta, so PDDEL and DDC models exhibit similar radiative
properties beyond maximum. However, at early times, PDDEL models show bluer
optical colors and a higher luminosity, even for weak mixing. Their early-time
radiation is derived primarily from the initial shock-deposited energy in the
outer ejecta rather than radioactive decay heating. Furthermore, PDDEL models
show short-lived CII lines, reminiscent of SN 2013dy. They typically exhibit
lines that are weaker, narrower, and of near-constant width, reminiscent of SN
2011fe. In addition to multi-dimensional effects, varying configurations for
such ``pulsations" offer a source of spectral diversity amongst SNe Ia. PDDEL
and DDC models also provide one explanation for low- and high-velocity gradient
SNe Ia.
