The extreme hydrogen-deficiency of the R Coronae Borealis (R CrB) stars together with overabundances of elements produced at the time of helium burning suggest that these stars are in late evolutionary stages. A removal of essentially all of the hydrogen-rich envelope prior to leaving the asymptotic giant branch (AGB) is very unlikely in the context of single star evolution for low- and intermediate-mass stars. Furthermore, due to the short time-scale following evolution off the AGB and the lack of significant pre-planetary nebula material, the stars cannot readily be identified with a phase immediately after departing from the AGB. Instead, the R CrB stars are likely to be re-born stars, i.e., stars that evolved off the AGB to the white dwarf cooling regime but now show brief re-appearances as luminous giants.
Two proposals for re-born stars have garnered most attention: the "final flash" (Renzini 1979) and the "double degenerate" (Webbink 1984) models. In the former a final He-shell flash in a post-AGB star descending the white dwarf cooling track briefly expands the stellar envelope to giant dimensions once again. This He-shell flash quickly depletes the envelope of hydrogen to create an R CrB-like supergiant. Examples of such stars may be the recently discovered Sakurai's object (Asplund et al. 1997b) and FG Sge (Gonzalez et al. 1998). The second model involves a merger of a He white dwarf and a C-O white dwarf. Close white dwarf binaries such as WD 2331+290 and WD 0957-666 may merge within a Hubble time and produce hydrogen-deficient giants (Iben et al. 1997). The pros and cons of each scenario are discussed in Iben et al. (1996) and Schönberner (1996). Possible progenitors and descendants to the R CrB stars are the hydrogen-deficient carbon (HdC) stars (Warner 1967) and the extreme helium (EHe) stars (e.g. Jeffery 1996), respectively, which may subsequently evolve into the helium sub-dwarf O () stars.
The two suggested scenarios will probably produce different atmospheric chemical compositions. Estimates of the elemental abundances are therefore essential for placing the stars in the correct context of stellar evolution. In two companion articles the compositions of 17 R CrB stars and two EHe stars (Lambert et al. 1998) and Sakurai's object (Asplund et al. 1997b) have been determined. V854 Cen is discussed separately being a `peculiar' (!) R CrB star with two special characteristics: a relatively high hydrogen abundance and a propensity to undergo frequent declines. In the present paper the composition of V854 Cen is analysed. Similarities and differences with the compositions of R CrB stars and Sakurai's object are investigated and discussed in light of the proposed evolutionary scenarios.
© European Southern Observatory (ESO) 1998
Online publication: March 23, 1998