Proton-threshold states in 28Si

Abstract

The 27Al( 3He, d) 28Si reaction has been used to locate candidates for resonances in the 27Al + p system residing near the proton-capture threshold in the energy region characteristic of quiescent stellar hydrogen burning. Two such states are observed at excitation energies E x = 11.658 MeV( J π = 2 +) and 11.671 MeV ( J π = 1 −). A comparison of the cross sections for the 27Al( 3He, d) 28Si and the 27Al(α, t) 28Si reactions implies angular-momentum transfers of l = 2 and l = 3, respectively, for the two states of interest. Using this result, an astrophysically significant upper limit on the thermonuclear reaction rate has been calculated for the 27Al(p, γ) 28Si reaction which is found to be too slow to affect the 27Al abundance in red giants.