EVOLUTION FROM AGB STAR TO PLANETARY NEBULA

Abstract

ABSTRACT Stellar evolution through the asymptotic giant branch (AGB) and planetary nebula (PN) phases is examined with particular emphasis on stellar mass loss. The Mount Stromlo Stellar Structure Code has been used to produce 22 stellar evolutionary sequences, complete from the main sequence to the white dwarf regime. The models correspond to initial masses of 0.89, 0.945, 1.0, 1.5, 2.0, 2.5, 3.5, and 5.0 M\\s, and metallicities 0.016, 0.008, 0.004, and 0.001. Mass loss during the AGB phase has been incorporated into the calculations following the empirical relationship between mass loss rate and pulsation period defined by Galactic AGB stars. The calculations show that the overall time-dependent behaviour of the mass loss rate is significantly modulated during any helium shell flash cycle, and that the AGB phase is terminated by severe mass loss over the last 2--3 helium shell flash cycles. The maximum AGB luminosities predicted are in excellent agreement with those observed for Magellanic Cloud AGB stars. The AGB calculations also indicate that one is likely to observe normal M-, S-, or C-type AGB stars with hollow circumstellar shells. Time-dependent hydrodynamic models of the winds around such stars show that circumstellar matter can flow back towards the star during the luminosity dip experienced immediately following a flash event. Multiple shell planetary nebulae (PNe) may also be the result of this type of behaviour, though the hydrodynamic calculations have not been extended this far as yet. In 21 cases, no effort was made to control the phase of the helium shell flash cycle at which the AGB stage was terminated. An additional 5 cases were constructed by varying the AGB mass loss rate by ~10% over the last stages of AGB evolution, thus varying the phase. The fraction of Hydrogen to Helium burning nuclei suggests that ~40% of all nuclei are preferentially burning Helium when they leave the AGB@. Calculations were extended through the PNN regime, incorporating a semi-empirical mass loss rate which was consistent with radiation pressure driven wind theory to within the large scatter of observational mass loss rates. Spectrophotometric observations were obtained in the range 3400--7900A for a total of 80 PNe in the Magellanic Clouds. Subsequent photoionisation modelling will allow these objects to be placed onto a Hertzsprung-Russell (H--R) diagram. From the modelling data already available in the literature for Magellanic Cloud PNe, the majority appear to conform with the properties of He burning PNN tracks. This is in contrast to Galactic PNe which have been assumed to be powered by H burning nuclei.