2008ApJ...686..337W

This study investigates the expansion properties of nickel bubbles in Type Ia supernovae due to radioactive heating from the ⁵⁶Ni ⟶ ⁵⁶Co ⟶ ⁵⁶Fe decay sequence, under the spherically symmetric approximation. An exponentially declining medium is considered as the ejecta substrate, allowing for the density gradient expected in a Type Ia supernova. The heating gives rise to an inflated Ni bubble, which induces a forward shock that compresses the outer ambient gas into a shell. In the adiabatic case (in which no radiative energy diffuses across the shell) the structure of the shell can be approximately described by a self-similar solution that is determined by the expansion rate and ambient density gradient of the shell. In the radiative case, the shell expansion weakens but the frozen-in velocity remains comparable. The thickness of the shell is ∼1% of the radius of the bubble, and the density contrast across the shell reaches ≳100 in a narrow region that is limited by numerical resolution. The inferred ejecta clump (shell component) is denser than that given by the model using a uniform ejecta substrate. For the ejecta knots present at the edges of Tycho's remnant, explosion conditions similar to those of successful explosion models are found, including the deflagration W7 and the delayed detonation yield with sufficient velocities and timely clump-remnant interaction; the luminous helium detonations and the low-energy Chandrasekhar-mass explosions are unfavorable. The clump speed can be increased as the initial density contrast of ⁵⁶Ni, 0.5≲ω<1, is reduced, because of a realistic elemental distribution. In all cases, the initiation of the clump-remnant interaction is expected in under 2000 yr after the supernova explodes.