Proton threshold states in 27Si and the destruction of 26Al at low stellar temperatures

Abstract Using the 27 Al( 3 He, t) 27 Si reaction, we have located and measured accurate excitation energies for seven states in 27 Si between the 26 Al + p threshold and the lowest observed 26 Al ( p , γ) 27 Si resonance. By examining the mirror states in 27 Al via the 27 Mg( 3 He, d) 27 Al and 26 Mg (α, t ) 27 Al reactions, we find that in the 26 AI + p channel there can be at most only one s-wave resonance that lies below the lowest directly observed resonance; this possible l = 0 resonance most probably corresponds to either the 7589, 7557 or 7533 keV state in 27 Si. The remaining six states are most probably formed via l ⩾ 2 proton capture. Upper limits for the reduced proton widths of these states have been used to estimate an upper limit for the 26 Al ( p , γ) 27 Si thermonuclear reaction rate.