Birth and early evolution of a planetary nebula

The final expulsion of gas by a star as it forms a planetary nebula --- the ionized shell of gas often observed surrounding a young white dwarf --- is one of the most poorly understood stages of stellar evolution. Such nebulae form extremely rapidly (about 100 years for the ionization) and so the formation process is inherently difficult to observe. Particularly puzzling is how a spherical star can produce a highly asymmetric nebula with collimated outflows. Here we report optical observations of the Stingray Nebula which has become an ionized planetary nebula within the past few decades. We find that the collimated outflows are already evident, and we have identified the nebular structure that focuses the outflows. We have also found a companion star, reinforcing previous suspicions that binary companions play an important role in shaping planetary nebulae and changing the direction of successive outflows.Comment: 9 pages + 3 figures. To appear in Nature, 2 April 199

The population of close double white dwarfs in the Galaxy

We present a new model for the Galactic population of close double white dwarfs. The model accounts for the suggestion of the avoidance of a substantial spiral-in during mass transfer between a giant and a main-sequence star of comparable mass and for detailed cooling models. It agrees well with the observations of the local sample of white dwarfs if the initial binary fraction is close to 50% and an ad hoc assumption is made that white dwarfs with mass less than about 0.3 solar mass cool faster than the models suggest. About 1000 white dwarfs brighter than V=15 have to be surveyed for detection of a pair which has total mass greater than the Chandrasekhar mass and will merge within 10 Gyr.Comment: 15 pages, 7 figures, to appear in Proc. ``The influence of binaries on stellar population studies'', Brussels, August 2000 (Kluwer, D. Vanbeveren ed.

Planetary nebulae: abundances and abundance gradients

In this work, a review is given of some recent results and problems involved in the determination of chemical abundances of galactic planetary nebulae, particularly regarding disk and bulge objects