A HYDRODYNAMICAL MODEL OF FG-SAGITTAE PULSATIONS

Pulsations of the planetary nebula core, FG Sge, have been investigated by numerical methods of radiative hydrodynamics. Parameters of the initial model are as follows: mass M= 1 o luminosity L= 3200 Lo (Mbol = -4.0 mag); effective temperature Teff = 5500 K. The formation of the cold extended envelope during the growth of pulsation amplitude has been obtained. Interaction of model pulsations with the extended atmosphere causes powerful, quasi-periodic shock waves responsible for the mass loss. It is shown that the value of mass loss strongly depends upon the numerical method used for the calculation of shock wave propagation. We discuss the influence of hydrogen molecule dissociation and hydrogen atom ionization on the propagation of the shock wave through the stellar atmosphere and on the estimation of mass loss. The difference between the observed pulsation period P0 = 90 day and the period computed for our model Pc = 30 day is discussed. It is shown that this discrepancy disappears for a model with the parameters: M= 0.8 Mo, L= 1.6 × 104 Lo. © 1981 Royal Astronomical Society.