Abstract
Abstract We present the first realistic 3D simulations of flame front instabilities during type I X-ray bursts. The unperturbed front is characterized by the balance between the pressure gradient and the Coriolis force of a spinning neutron star (ν = 450 Hz in our case). This balance leads to a fast horizontal velocity field parallel to the flame front. This flow is strongly sheared in the vertical direction. When we perturb the front an instability quickly corrugates the front. We identify this instability as the baroclinic instability. Most importantly, the flame is not disrupted by the instability and there are two major consequences: the overall flame propagation speed is ∼10 times faster than in the unperturbed case and distinct flame vortices appear. The speedup is due to the corrugation of the front and the dynamics of the vortices. These vortices may also be linked to the oscillations observed in the light curves of the bursts.
Highlights
The type I X-ray bursts are thermonuclear explosions that burn the material that neutron stars accrete from their companion in low-mass X-ray binaries
Depending on the composition and the mass accretion rate, the heating from the nuclear burning may become uncompensated by the cooling processes, and the temperature rapidly increases leading to a thermonuclear runaway: the ignition of type I bursts (Fujimoto et al 1981; Bildsten 1998; Strohmayer & Bildsten 2006; Galloway & Keek 2017)
In this paper we study the full development of front instabilities in 3D
Summary
The type I X-ray bursts are thermonuclear explosions that burn the material that neutron stars accrete from their companion in low-mass X-ray binaries. The Coriolis force balances the pressure gradient (geostrophic balance), generating a horizontal flow parallel to the flame front This flow follows the thermal wind profile (e.g., Pedlosky 1987) showing a strong vertical shear. The flame propagation is driven mostly by conduction across the hot–cold fluid interface at the front The latter is almost parallel to the horizontal, since the Rossby radius is of order 5 ́ 104 cm, while the layer height is of order 102 cm, and the large conducting surface guarantees a high speed in good agreement with the observations. Baroclinic instability has been previously discussed within the burst framework by, e.g., Fujimoto (1988) and Cumming & Bildsten (2000) These authors addressed angular momentum and mixing in differentially rotating atmospheres, in particular following the expansion of the burning fluid.
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