A quantitative NLTE analysis of visual and ultraviolet spectra of four helium-rich subdwarf O stars

Hot subdwarfs represent a poorly understood late phase of stellar evolution. While binary evolution plays an important role for B-subdwarfs (sdB), the origin of the He dominated O-subdwarfs (He-sdO) is unknown. We search for chemical signatures of their genesis by means of quantitative analyses of visual and FUV spectra. Using Tlusty/Synspec to compute line blanketed NLTE model atmospheres, Teff/log g and abundances have been derived for 4 prototypical He-sdO stars. Final models included H,He,C,N,O,Ne,Mg,Al,Si,P,S,Fe&Ni. Because of the enrichment of either N or C, models including these elements at the appropriate high abundance provide sufficiently accurate approximations to the temperature stratification of full models. No indications for binarity were found, neither radial velocity variations nor photometric evidence. All stars have atmospheres almost free of H and Teff =42000K to 47000K, and log g= 5.4 to 5.7. CD-31 4800 displays the signatures of CNO burning, while heavier elements are subsolar by ~0.4 dex, except Ne & Si being ~solar. The C-rich He-sdOs show a slightly subsolar metallicity accompanied by N-enrichment and O-deficiency. Ne is enriched up to a factor of ten with respect to the sun in LS IV+10 9. Stellar masses scatter around the canonical helium core flash mass. The abundance pattern of CD-31 4800 is consistent with predictions of models for cold mergers of He-WDs except for the low O abundance observed. Models for composite mergers predict abundance pattern dissimilar to those determined for the C-rich stars. [CW83]0904-02, though, may be a candidate for a composite He-WD merger, as it rotates and is more massive than the other stars. New evolutionary models for the hot flasher scenario predict abundance patterns similar to those determined for the C-rich stars. Hence, C-rich He-sdO may result from late He flashes with deep-mixing episodes. (abbreviated)
Comment: 32 pages,, 17 Figures, A&A accepted August 20th, 2018