Thursday, May 1, 2014

Formation of Iron-Core White Dwarfs

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).

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.

- 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