AGB Stars and Planetary Nebulae
(D. Gonzalez Delgado, K. Justtanont, G. Mellema, H. Olofsson, F.L. Schöier)

The research at Stockholm Observatory centres on the properties of the AGB mass loss and the formation and evolution of planetary nebulae (PNe). On the observational side a number of projects have been concluded, and some new have been started. Detailed observations of the CO radio line emission from two stars, U Cam and TT Cyg, both seem to have gone through a brief period of very intense mass loss, have been performed using the IRAM interferometer on Plateau de Bure, France. This study of episodic mass loss of AGB-stars is now extended to include imaging of the shells in resonance line scattered stellar light. Observations at the ESO 3.6m telescope and the NOT have been done, and the first results on the carbon stars R Scl and U Ant are prepared for publication. In another interferometric study, using the Cal.tech. interferometer at Owens Valley, evidence for a Keplerian disk around an apparently normal AGB-star, RV Boo, was found. In order to interpret our CO$_{2}$ emission results obtained with ISO, we have observed OH masers in order to verify the suggestion that CO$_{2}$ is formed by the reaction of OH and CO in the inner part of the circumstellar envelope. A preliminary result shows that OH masers exist in the region where CO$_{2}$ is emitting, thus giving weight to the proposed formation path. A major project aimed at a census of AGB-stars in Local Group galaxies have been started on the NOT. Images in four filters are used to identify the AGB-stars and to classify them chemically. A probable post-AGB object, HD101584, was found to have a remarkable molecular envelope, where high-velocity features outline a spatially resolved bipolar outflow, while the central region appears complicated, with the possible existence of disks at two spatial scales.

On the interpretational side also a number of projects have been concluded, and some new have been started. Using near- and mid-infrared ISO observations, we detect several molecular absorption and emission bands, e.g., H$_{2}$O, CO, OH and CO$_{2}$. The models give estimates of the temperature of the absorbing/emitting gas and the column density of the gas. Heating mechanisms in post-AGB pbjects and PNe have been investigated using ISO LWS observations. In AFGL 2688 where there is evidence for a fast wind interacting with the remnant AGB envelope, shocks are used to desbribe the gas heating while in NGC 7027, the strong far-UV radiation is thought to be heating the gas. In the intermediate object, AFGL 618, both mechanisms may be operating. A molecular excitation and radiative transfer code based on the Monte Carlo method, which include the energy balance equation and line overlaps, has been developed and applied in two studies. In one of them, radio and ISO $^{12}$CO and $^{13}$CO line data on the carbon star IRAS15194-5155 could be modelled assuming a constant mass loss rate and $^{12}$CO/$^{13}$CO during at least a few thousand years. This star may be massive enough that it has experienced hot bottom burning, but not massive enough to convert carbon to nitrogen, and hence it remains a carbon star, but with a low $^{12}$C/$^{13}$C-ratio. In the other, interferometric and single-dish circumstellar HCN and CN emission from carbon stars have been modelled. It was found that the predictions of a photodissociation model are qualitatively, and in most cases also quantitatively, correct. For both molecules the inclusion of line overlap in the radiative transfer analysis is important.

Some AGB stars manage to lose their entire hydrogen envelope and become the so-called Wolf-Rayet (WR) type central stars of Planetary Nebulae (PN). These stars are normally characterised by heavy mass loss (up to $10^{-5}$ M$_\odot$ yr$^{-1}$) at high velocities (1000 km s$^{-1}$). Because of the absence of hydrogen, the thermal behaviour of the winds can differ from the usual. We investigated the effects the WR abundances can have on the formation of the PN shells. It is expected that the nebular expansion velocities will be higher, the shell more turbulent, and the so-called momentum-conserving will last longer. We also continued the study of the properties of the cometary knots in the Helix nebula (NGC 7392). We used HST observations to estimate the intensity of the ionizing flux from the star, as well as the intensity of the diffuse ionizing flux of scattered photons inside the nebula. It was found that the values provided by the knots are consistent with previously determined values, and that the diffuse ionizing flux is substantially less than the popular `On The Spot' estimate and it is indicative of a non-closed nebular shell.