The Site of the Helium Flash

Calculations have been made, using the Henyey method, of evolutionary sequences of stars approaching and at the time of the helium flash. Any hydrodynamical effects have been neglected. These calculations have been made for both Population I and Population II compositions. The effects of nitrogen burning and neutrino energy losses have been considered by constructing sequences both with and without these processes. For the Population I composition $({X}_\text{CNO}\,=\,0.02)$ the flash is in a shell if there are neutrino losses whereas for Population II $({X}_\text{CNO}\,=\,0.001)$ the flash is initially in a shell but is soon shifted to the centre, even though there are neutrino losses. As a result, if there are neutrino losses, the character of the flash is different in the two stellar populations. This may result in different evolution subsequently and in different H–R diagrams. The boundary between the two cases is at an initial composition in the range $({X}_\text{CNO}\,=\,0.002)\,\text{to}\,0.003$. This shift of the flash to the centre for Population II stars is a property of the nitrogen burning; it does not occur if nitrogen burning is neglected. Neglect of neutrinos puts the flash at the centre. Observed luminosities of stars at the peak of the red giant branch of globular clusters are consistent with theoretical predictions if these stars are making their first ascent of the giant branch.