Fusion and reactions of $\alpha$+$^8$Be in the Hoyle resonance and associated resonances region
The fusion of $\alpha$ and $^8$Be to produce a $^{12}$C nucleus is a crucial process in nucleosynthesis. In the laboratory, this process can only be studied theoretically as a $^8$Be target or projectile cannot be prepared experimentally. We use the potential scattering theory in the coupled-channel formalism to study such a process in terms of the collision between the $\alpha$ particle on a deformed $^8$Be nucleus, both on resonance and off resonance in the Hoyle resonance and associated resonances region. The experimental $^{12}$C energy levels and widths constrain the nuclear potential to suggest the need to include a parity-dependent surface potential component that is more attractive for even-$L$ positive-parity partial waves than for odd-$L$ negative-parity partial waves. As a consequence, the radial dependence of the total potentials for the set of \{0$^+$, 2$^+$, 4$^+$\} resonances of ${}^{12}$C exhibit a double-hump behavior, possessing two local energy minima and a doublet of each of the ${}^{12}$C \{0$^+$, 2$^+$, 4$^+$\} resonances in the Hoyle and associated resonances region. We examine the approximate agreement of the theoretical results with experiment and suggest the search for the as-yet unobserved lower-energy 2${}^+_2$ and 4${}_1^+$ resonances to test the double-hump potential description. In addition, for practical astrophysical applications, we evaluate and estimate the astrophysical $S(E_{\rm c.m.})$-factor for the $\alpha$+$^8$Be $\to$ $^{12}$C$(0^{+*})$ $\to$ $^{12}$C$(2_1^+)$ + $\gamma$ reaction for $E_{\rm c.m.}$ $<$ 1.0 MeV.