35. Stellar Constitution (Constitution des Étoiles)

Abstract

Paczynski discussed the final evolution of Population I low- and moderate-mass stars, using a numerical technique designed to deal with the dynamical instabilities found; a semi-convective region appears in the model, similar to the results of Uus. He also described the evolution of helium stars which reached the R CrB part of the HR diagram only if neutrino emission was included. J. P. Cox described a survey (with King) of the Cepheid instability strip, using linear and nonlinear calculations, aimed at determining Cepheid masses. The left-hand edge of the observed strip is well-described if the masses are those given by evolution without mass-loss. Von Sengbush showed that the evolution to the giant branch of a 4 M⊙ star with X = 0.36 was very similar to that of a 7 M⊙ star with X = 0.6. Christy surveyed the non-linear aspects of pulsation over a range of models, including Cepheids and extending to RV Tau stars. Masses of about 0.6 times the evolutionary mass seemed indicated for Cepheids. The disagreement with Cox could be due to uncertainties in colour/temperature conversions or in the opacities. Giannone (with Giannuzzi) reported Case B evolution of close binaries with mass exchange. For the cases studied, the mass of the final white dwarf is in the range 0.2 M⊙ to 0.4 M⊙, depending on the initial separation and mass ratio. Work on post-heliumflash models (in collaboration with Castellani and Renzini) shows the presence of a semi-convective region which increases the time-scale of evolution. Demarque showed that the assumptions of relativistic and non-relativistic degeneracy have quite different effects on post-helium flash evolution. It also appears to be necessary for horizontal branch stars to have a range of masses. Refsdal showed that moderate mass loss suppressed the loops in the HR diagram in the central-helium-burning stage of evolution. Schwarzschild remarked that his models also possessed semi-convection zones. He suggested that details of horizontal branch models might be very sensitive to opacity. Massevich reported on late evolution with stellar wind mass loss, which may be important for certain Teff and g, and on binary evolution with mass exchange. Mass loss from the whole system speeds up the evolution.