Investigations of “normal” planetary nebulae (PNe)

The rapid development of improved 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 have carried out all these procedures which are sometimes time consuming but also challenging to confirm our spatio-kinematical modeling of PNe.



Novae and peculiar erruptive Variables


Novae – especially recurrent Novae with several outburst known - represent an extreme astrophysical laboratory. Novae are double stars at which one evolved solid partner (white dwarf or neutron star) accretes mass of his companion. We carry out both, visual observations in Chile and at our own facilities, and numerical calculations to determine the structure of these objects. However, Novae are also indicators of extragalactic populations. We intend to use them to study the inter galactic stellar population.


Peculiar eruptive variables (e.g. V838 Mon):

V838 Mon – originally classified as Nova – attracted the astrophysical community after its second outburst, which was discovered at the observatory here in Innsbruck. The spectral evolution of V838 Mon was as peculiar as its light curve. In spite of large ejection velocities at the outburst onset (~500 km/sec), the expanding ejecta never reached optically thin conditions. It remained optically thick and got cooler and cooler with time, initially mimicking a K giant, then making a complete excursion along the whole sequence of M giant spectra down to M10, and finally entering the realm of L-type supergiants, a spectral type never seen before anywhere in the Universe and characterized by temperatures so low that were previously measured only in brown dwarfs.
Little consensus has been reached so far on the nature and causes of the outburst of V838 Mon. The interpretations published in the literature cover a wide range of possibilities such as the swallowing of giant planets, merging of the components of a binary star, surface helium flash in a highly evolved and very massive star and a highly degenerate hydrogen flash in a low mass, cool and very slowly accreting white dwarf.


The “born-again” PNe

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 thermal-pulse cycle time, the latter being about 100 000 years. Thus there is roughly a 10% chance that a final thermal pulse 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 the born-again. The working group investigates the know objects of the class and attempts to predict the behavior by use of numerical hydrodynamic model calculations.


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