In a conventional type Ia supernova, the
deflagration flame will transition to a detonation flame as it enters the lower
density outer layers of the white dwarf. The detonation flame then consumes the
entire white dwarf and the energy release from thermonuclear burning causes the
whole star to explode violently as a type Ia supernova. The difference between
a deflagration flame and a detonation flame is that the latter propagates
faster than the local speed of sound in the white dwarf. As a result, a detonation
flame is able to consume the entire white dwarf since the material in front of
the detonation flame is unable to “see” the approaching flame front.
However, it is possible to have a failed-detonation
scenario where the deflagration flame fails to transition to a detonation flame.
This may explain a peculiar subset of type Ia supernovae that are characterised
by low ejecta velocities, low luminosities and low ejecta masses. A failed-detonation
type Ia supernova occurs when enough mass is burnt during the deflagration
phase such that the conditions necessary for the deflagration flame to
transition to a detonation flame cannot be achieved and the white dwarf fails
to detonate. In this scenario, thermonuclear burning during the deflagration
phase delivers energy to the white dwarf, causing the star to expand and then
contract. Because too much energy is delivered to the white dwarf, it is unable
to attain high enough densities and temperatures to launch a detonation flame during
maximum contraction.
For a failed-detonation type Ia
supernova, the white dwarf will remain intact as the deflagration is too weak
to completely unbind it. However, the white dwarf will now have a lower mass as
the failed-detonation event is expected to produce a few tenths of a solar mass
of ejecta. The thermonuclear fusion processes occurring within the deflagration
flame results in ejecta that is rich in intermediate-mass elements (magnesium,
silicon and sulphur) and iron-group elements (iron, cobalt and nickel). A
significant proportion of the heavy elements are expected to fall back to the
white dwarf and gravitationally settle to form an iron/heavy-core at its centre.
The end result is an iron/heavy-core C-O white dwarf.
Due to the highly asymmetric nature of
the outburst, the white dwarf will receive a kick velocity of a few 100 km/s. Even
so, the large orbital velocities found in most binary star systems suggest that
even a kick velocity of a few 100 km/s may be insufficient to unbind the
binary. However, for binary systems consisting of a white dwarf accreting
matter from an evolved star such as a red giant, the natal kick velocity is
likely to unbind the system because of the large binary separation between the
white dwarf and the red giant. It is also possible for the natal kick from the asymmetric
outburst to launch the white dwarf towards its companion star and this should
produce very interesting results.
Reference:
George Jordan IV, et al., 2012, “Failed-Detonation
Supernovae: Sub-Luminous Low-Velocity Ia Supernovae and Their Remnant-Kicked
Iron-Core White Dwarfs”, arXiv:1208.5069v1 [astro-ph.HE]