Astron. Astrophys. 332, 651-660 (1998)

6. Conclusions

The derived chemical composition for V854 Cen suggests that it belongs to the minority group of the R CrB stars, though less extreme than the other three minority members. One is tempted to term it a minority-majority transition object. As an R CrB star, V854 Cen is not only one of the stars with the lowest Fe/C ratio but also one of the least H-deficient. V854 Cen accentuates the anti-correlation between the H and Fe abundances found for the R CrB and EHe stars (Heber 1986; Lambert et al. 1998). Like most R CrB stars, V854 Cen shows indications of a mild neutron exposure.

Perhaps, the most interesting result is the abundance similarities between V854 Cen and the final He-shell flash candidate Sakurai's object (Asplund et al. 1997b). In fact, V854 Cen resembles Sakurai's object more closely than any of the other R CrB stars. This might suggest that the two stars also share the same evolutionary background. Most of the abundance differences could if so be attributed to less amount of H-envelope ingestion in connection with the final flash for V854 Cen.

If V854 Cen is indeed a final flash object, a search in optical or IR for a fossil shell from a previous planetary nebula stage could be rewarding. In this context we note that nebular emission lines of [O I ], [N II ], [S II ] and H , have already been observed for the star during a decline (Rao & Lambert 1993). A connection between V854 Cen and Sakurai's object would suggest that the final flash scenario is a viable channel to form R CrB stars. Unfortunately, the status of V854 Cen regarding class assignment as a minority or majority member is unclear. It is therefore difficult to associate either group, if any, with being final flash objects. An alternative interpretation is that all R CrB stars have been formed through the same mechanism but e.g. dust-gas separation have introduced the differences in abundances between the two groups, though such dust depletion must then have proceeded quite differently than in the ISM. Further theoretical predictions for the two proposed models regarding the resulting chemical compositions for different initial conditions are clearly desirable in order to discriminate between them, as well as work on the expected results of a dust depletion in H-deficient and C-rich environments. A continued monitoring of possible abundance changes in Sakurai's object is clearly also a priority.

© European Southern Observatory (ESO) 1998

Online publication: March 23, 1998
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