Barium central stars

The four known planetary nebulae with barium central stars: LoTr 5 (Graham et al. 2004), WeBo 1 (Bond et al. 2003), A 70 (Miszalski et al. 2012) and Hen 2-39 (Miszalski et al. 2013).

At the heart of all planetary nebulae is a star hot enough to ionise the gaseous envelope that we see, usually a white dwarf. Curiously, some planetary nebulae only show nuclei that are cool giant or sub-giant stars at optical wavelengths. These stars are too cool to ionise the nebula. So what’s going on? If the star is not a superposition (an unrelated field star), these cool stars are likely to be companions to the white dwarf with an orbital period of a few hundred days or even longer. The most interesting subset of these stars show enriched amounts of carbon and/or slow neutron capture process (s-process) elements (e.g. barium). In planetary nebulae, we are catching these so-called barium stars soon after their formation. The thing is, these stars are not evolved enough to generate these chemical peculiarities, but this is where the white dwarf companion comes in. Earlier in its life, just before it formed the planetary nebula, its strong wind was loaded with carbon and s-process elements, and this was accreted by the companion, which we now see polluted with this chemical signature. The whole ensemble found in these barium stars with planetary nebulae make them particularly powerful laboratories for studying chemical nucleosynthesis and mass transfer in evolved stars. There are however only four examples known. One of these, Hen 2-39, we recently discovered with SALT. Further information:

The [WN]-type Wolf-Rayet central star of IC4663

A Gemini Focus newsletter article (Issue 44, June 2012; click on the image to the left to view it) on our discovery of the first clear-cut case of a [WN] Wolf-Rayet central star of a planetary nebula. The [WN3] nucleus of IC4663 (below) was found serendipitously from Gemini South spectroscopy of the central star. For more information see the short Gemini Web Feature article or the published paper available from here or hereic4663.jpg

The cosmic sprinklers of Fleming1

Some planetary nebulae show pairs of jets. The most spectacular example being Fleming 1 (right), in which we have recently discovered a close binary nucleus consisting of two white dwarfs orbiting each other every 1.2 days. For decades astronomers thought only a binary system could create such jets, but only now have we been able to prove it, more than 100 years after its discovery by Williamina Fleming. For more information see: