The doublydegenerate scenario

A double CO white dwarf system is supposed to lose orbital angular momentum due to gravitational-wave radiation. Once the less-massive white dwarf has filled the Roche lobe, dynamically unstable mass transfer induces complete merging of the

Helen Mrosla

Figure 38.1. Evolution of the nuclear luminosity due to helium burning in helium-accreting white dwarf models with a constant mass-accretion rate of M = 5 x 10-7M0 yr-1, as a function of the increasing white dwarf mass, for models with and without rotation as indicated. The initial white dwarf mass is O.998M0.

Figure 38.1. Evolution of the nuclear luminosity due to helium burning in helium-accreting white dwarf models with a constant mass-accretion rate of M = 5 x 10-7M0 yr-1, as a function of the increasing white dwarf mass, for models with and without rotation as indicated. The initial white dwarf mass is O.998M0.

8 IT

No Rotation

With Rotation

1.00

1.01

mwq/m0

1.04

Figure 38.2. Stability conditions for a helium-shell source in the density-temperature plane. The solid lines separate the thermally unstable region from the stable region, for five relative shell-source thicknesses (i.e. D/rs = 0.0, 0.1, 0.2, 0.3, and 0.4). The dotted contour lines denote the degeneracy parameter (:= f/(kT))\ XHe = 0.662 and XC = 0.286 have been assumed. From Yoon et al. (2004b).

Central Merger -^

Central Merger -^

Figure 38.3. A schematic illustration of the merger of double CO white dwarfs at quasi-static equilibrium.

binary components within a few minutes (e.g. Benz etal. 1990). Authors of previous theoretical studies assumed that a thick disk is produced around the primary white dwarf, and that mass accretion from the disk onto the central object occurs at a rate near the Eddington accretion rate (~10-5 Mq yr-1 ). Formation of a neutron star is the most likely outcome in this case, because off-center carbon ignition due to rapid mass accretion converts the white dwarf into an ONeMg white dwarf, which is susceptible to electron-capture-induced collapse at the Chandrasekhar limit (e.g. Saio & Nomoto 1998).

However, smoothed particle-hydrodynamics calculations indicate that the merger consists of three distinctive regions when it reaches quasi-static equilibrium: a slowly rotating cold core, a rapidly rotating hot envelope, and a centrifugally supported disk (Benz et al. 1990; Segretain et al. 1997; Yoon et al. 2007). Therefore, the merger at quasi-static equilibrium may be better described by a differentially rotating CO giant star surrounded by a Keplerian disk, as illustrated in Figure 38.3, rather than by a CO white dwarf + thick-disk system. Yoon et al. (2007) constructed one-dimensional differentially rotating CO giant stellar models, which mimic the central region of the merger. Their evolutionary calculations show that accretion of matter onto the cold core from the hot envelope is controlled by neutrino cooling at the interface where the temperature is highest, and that off-center carbon ignition can be avoided if the temperature peak at the interface during quasi-static equilibrium does not exceed the critical limit for carbon ignition (i.e. rpeak< ~6 x 108 K), and if the mass accretion from the Keplerian disk onto the envelope occurs slowly enough (M < ~10-5M0 yr- ). This makes it possible for the DD system to be a viable channel to SNe Ia explosion, in contrast to the previous belief that double CO white dwarfs generally undergo accretion-induced collapse.

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