Late Stages of Stellar Evolution and Planetary Nebulae
Widely known by the general community are the spectacular deaths of stars by explosions as Supernovas. But only a very small fraction of stars do have enough initial mass to do so. The vast majority of stars have initial masses in the range of a few tenths of the mass of the sun up to 8 solar masses. They undergo during the last 10% of their lifetime various stages of evolution of red giant stars. During the two final stages, the so called asymptotic giant branch (AGB) and thereafter ejection of a nebular shell, called planetary nebula (PN), the stars lose most of their mass. Even an initially 8 solar mass star goes down to so called white dwarf with only 0.6 solar masses during only about 1% of its total lifetime. Thus this stage is important for the feedback of stellar material into outer space. This material contains heavy elements produced during the life of a star by nucleosynthesis – and thus for the whole enrichment of elements in the universe.
While we understand most of the evolution during the AGB, certain phases are not clear. During the last few million years short periodic explosive burning of Helium occurs (Helium flashes). These flashes are causing the enrichment of elements like Neon and Silicon. But also how the procedure of ejecting the atmosphere of the star as a PN starts is not yet understood.
The rapid development of improved telescopes, detectors and computing facilities and the extensive morphological inventory, has put astronomers in the position of a forced understanding of the variety of shapes in which PNe appear. However, interpreting the three-dimensional structures of PNe requires a variety of additional investigations: chemical analysis, determination of the densities and temperatures within the nebulae, measurements and interpretation of expansion velocities, determination of the distances to the nebulae. We carry out all these procedures, which are sometimes time consuming, but also challenging, to confirm our spatio-kinematical modelling of PNe.
A special subtopic are the ‘reborn’ stars. The majority of central stars of PNe are burning hydrogen until they turn into white dwarfs (WD) when nuclear burning ceases. The theory of late stages of giant stars (asymptotic giant branch = AGB) and post-AGB evolution predicts, that the post-AGB evolution to a hot WD makes up for about 1/10 of the AGB star’s Helium flash cycle time, the latter being about 100 000 years. Thus there is roughly a 10% chance that a final flash of the helium-burning shell occurs during the remnant’s transit towards the WD stage. The release of gravothermal energy by the flashing helium shell forces the already very compact star to expand back to giant dimensions: the so-called born-again scenario. It is still not possible to predict accurate numbers concerning the probability of a born-again phase. The working group investigates the known objects in this phase and attempts to predict the behavior by use of numerical hydrodynamic model calculations. These stars produce and also eject an extremely large fraction of carbon. Thus they are important for the ‘biological’ evolution of the universe.
Faculty member active in this area: Stefan Kimeswenger