Abundance analyses of cool extreme helium stars

Abstract

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, BD+ 1°4381 (FQ Aqr), LS IV -14° 109, BD -1° 3438 (NO Ser) and LS IV -1° 002 (V2244 Oph), are presented. All these stars show evidence of H- and He-burning 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 CN- and ON-cycled. Most of the EHes have only CN-cycled material in their atmospheres. There is an indication that the CN- and 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.