Abundance analyses of cool extreme helium stars

Extreme helium stars (EHes) with effective temperatures from 8000 to 13 000 K are among the coolest EHes and overlap the hotter R CrB stars in effective temperature. The cool EHes may represent an evolutionary link between the hot EHes and the R CrB stars. Abundance analyses of four cool EHes, BD118 4381 (FQ Aqr), LS IV 2148 109, BD 218 3438 (NO Ser) and LS IV 218 002 (V2244 Oph), are presented. All these stars show evidence of Hand Heburning at earlier stages of their evolution. To test for an evolutionary connection, the chemical compositions of cool EHes are compared with those of hot EHes and R CrB stars. Relative to Fe, the N abundance of these stars is intermediate between those of hot EHes and R CrB stars. For the R CrB stars, the metallicity M derived from the mean of Si and S appears to be more consistent with the kinematics than that derived from Fe. When metallicity M derived from Si and S replaces Fe, the observed N abundances of EHes and R CrB stars fall at or below the upper limit corresponding to thorough conversion of initial C and O to N. There is an apparent difference between the composition of R CrB stars and EHes, the former having systematically higher [N/M] ratios. The material present in the atmospheres of many R CrB stars is heavily CNand ON-cycled. Most of the EHes have only CN-cycled material in their atmospheres. There is an indication that the CNand ON-cycled N in EHes was partially converted to Ne by a-captures. If EHes are to evolve to R CrB stars, fresh C in EHes has to be converted to N; the atmospheres of EHes have just sufficient hydrogen to raise the N abundance to the level of R CrB stars. If Ne is found to be normal in R CrB stars, the proposal that EHes evolve to R CrB stars fails. The idea that R CrB stars evolve to EHes is ruled out; the N abundance in R CrB stars has to be reduced to the level of EHes, as the C/He, which is observed to be uniform across EHes, has to be maintained. Hence the inferred [N/M], C/He and [Ne/M] ratios, and the H-abundances of these two groups indicate that the EHes and the R CrB stars may not be on the same evolutionary path. The atmospheres of H-deficient stars probably consist of three ingredients: a residue of normal H-rich material, substantial amounts of H-poor CN(O)-cycled material, and C(and O-) rich material from gas exposed to He-burning. This composition could be a result of final He-shell flash in a single post-AGB star (FF scenario), or a merger of two white dwarfs (DD scenario). Although the FF scenario accounts for Sakurai’s object and other stars (e.g., the H-poor central stars of planetary nebulae), present theoretical calculations imply higher C/He and O/He ratios than are observed in EHes and R CrB stars. Quantitative predictions are lacking for the DD scenario.