Type Ia supernovae are among the most energetic explosions
in the universe. They originate from the thermonuclear explosions of
carbon-oxygen white dwarfs (WDs). Models of normal Type Ia supernovae involve a
sufficiently massive WD accreting material from a companion star. This
eventually triggers unstable thermonuclear burning within the WD which
transitions to a detonation, subsequently consuming and completely unbinding
the WD in a violent supernova explosion. The explosive thermonuclear burning
fuses a large fraction of the original carbon-oxygen WD into intermediate-mass
elements (IME’s) and iron group elements (IGE’s).
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Simulation of a thermonuclear flame plume bursting through
the surface of a white dwarf. Credit: Flash Centre for Computational Science,
University of Chicago.
In a study by George C. Jordan et al. (2012), a variant of
the Type Ia supernovae is proposed. Here, the thermonuclear burning is too weak
to transition to a detonation and does not completely unbind the WD in a
supernova explosion. Such an event is known as a failed-detonation supernova
and it leaves behind a bound remnant of the original WD. A failed-detonation
supernova is characterised by low ejecta expansion velocities, low luminosities
and low ejecta-mass. Although the explosive thermonuclear burning process
during a failed-detonation supernova can generate more than 100 per cent of the
WD’s binding energy, the WD does not become completely unbound because the
energy produced is partitioned in such a way that a large bound remnant of the
original WD remains. For example, a fair amount of the energy goes into
launching just a small portion of the original WD’s mass as high velocity
ejecta.
A failed-detonation supernova asymmetrically ejects material
from the WD. This “kicks” the WD to velocities of a few hundred km/s. Such a
velocity is high enough to fling the WD from its binary system, leading to a
hypervelocity WD. Although some ejecta are produced, much of the burned and
partially-burned material from the thermonuclear burning falls back, enriching
the remnant WD with IME’s and IGE’s. These heavy elements segregate to the
core, forming a WD with a heavy/iron-rich core. A number of studies have termed
such peculiar objects as iron-core WDs. In particular, a study by Isern J. et
al. (1991) proposes yet another mechanism for forming iron-core white dwarfs. It
involves oxygen-neon-magnesium (ONeMg) cores that result from the evolution of
stars with 8 to 12 times the Sun’s mass. Depending on a number of conditions,
the ONeMg cores can undergo explosive thermonuclear burning to form iron-core
WDs.
References:
- George C. Jordan et al. (2012), “Failed-Detonation
Supernovae: Sub-Luminous Low-Velocity Ia Supernovae and Their Kicked Remnant
White Dwarfs with Iron-Rich Cores”, arXiv:1208.5069 [astro-ph.HE]
- Isern J., Canal R., & Labay J. (1991), “The outcome of
explosive ignition of ONeMg cores - Supernovae, neutron stars, or ‘iron’ white
dwarfs?”, ApJ, 372, L83