Hydrogen deficient post-AGB stars

Background

Most H-deficient post-Asymptotic Giant Branch (AGB) stars are likely to be related to each other. From the Wolf-Rayet (WR) central stars of Planetary Nebulae (PNe), to the R Coronae Borealis (RCB) stars, to non-DA white dwarfs (WD) such as the pulsating PG1159 stars.... the zoo of hydrogen-less evolved, intermediate-mass stars is vast. The mechanism that rids a star of its hydrogen when the star is on the AGB, has been mostly related to the AGB pulsational phase at which the star leaves the AGB and to a lesser extent to binary interactions. I have worked on several of these star classes and of late I am becoming increasingly convinced that binary interactions are at the source of the H-deficiency.

My Role

My PhD thesis was on H-deficient Wolf-Rayet central stars of PN ([WC] CSPNe), which constitute about 20% of the population. After studying in depth four of them relating the chanracteristics of the stars, modelled with a WR stellar atmosphere model, to the PN, analyzed with a nebular code such as Cloudy (De Marco et al. 1997, 1998, 2001, De Marco & Crowther 1998, 1999, Crowther et al. 1998, Ercolano et al. 2004) I took an interest in one of them, because of the discovery, on HST spectra, of an edge-on disk (De Marco et al. 2002), possibly pointing to a binary origin to the class and leading to another paper (De Marco & Soker 2002). More recently, I have worked on high resolution imaging of these objects, with an intent to determine the presence of disks, using the VLTI (Chesneau et al. 2006, Lagadec et al. 2006).

This interest lead me to the study of born-again stars, which have experienced a helium shell flash when they were already on the post-AGB, or even the WD cooling track. We reviewd the properties of V605 Aql, one of the 5 known born-again stars in Clayton & De Marco (1997), and I continued to work on the fast changes of these stars, in particuar those of Sakurai's object, a born-again star that outbursted in 1995 (Jacoby et al. 1998, Kerber et al. 2002, Clayton et al. 2006). Recently, we found that the ejecta of two born-again stars, which show a [WC] spectrum, have ejecta that are oxygen-rich, rather than carbon-rich as predicted by the born-again, final helium shell flash scenario. The ejecta abundances are more like those of ONeMg novae than those expected from a last helium shell flash. This is really puzzling (Wesson et al. 2008).

The connection between [WC] stars and born-again is reinforced, and made more complicated by the fact that they share properties with yet another class of H-deficient stars: the R Coronae Borealis stars (RCB stars). The [WC] star that I discovered having a disk, has the light variability proerties of an RCB star (Jones et al. 1999). Some of the hottest RCB stars are a real mix of these three classes (De Marco et al. 2002)

Last Update: June 2008