The high ratio of⁴⁴Ti/⁵⁶Ni in Cassiopeia a and the axisymmetric collapse-driven supernova explosion

The large abundance ratio of ⁴⁴Ti/⁵⁶Ni in Cassiopeia A is puzzling. In fact, the ratio seems to be larger than the theoretical constraint derived by Woosley & Hoffman. However, this constraint is obtained on the assumption that the explosion is spherically symmetric, whereas Cas A is famous for the asymmetric form of the remnant. Recently, Nagataki et al. calculated the explosive nucleosynthesis of axisymmetrically deformed collapse-driven supernova. They reported that the ratio of ⁴⁴Ti/⁵⁶Ni was enhanced by the stronger alpha-rich freezeout in the polar region. In this Letter, we apply these results to Cas A and examine whether this effect can explain the large amount of ⁴¹Ti and the large ratio of ⁴⁴Ti/⁵⁶Ni. We demonstrate that the conventional, spherically symmetric explosion model cannot explain the ⁴⁴Ti mass produced in Cas A if its lifetime is shorter than ∼80 yr and the intervening space is transparent to the gamma-ray line from the decay of ⁴⁴Ti. On the other hand, we show that the axisymmetric explosion models can solve the problem. We expect the same effect from a three-dimensionally asymmetric explosion, since the stronger alpha-rich freezeout will also occur in that case in the region where the larger energy is deposited.