Discovery of photospheric argon in very hot central stars of planetary nebulae and white dwarfs

Context. We report the first discovery of argon in hot evolved stars and white dwarfs. We have identified the Ar  1063.55 Å line in some of the hottest known (Teff = 95 000−110 000 K) central stars of planetary nebulae and (pre-) white dwarfs of various spectral type. Aims. We determine the argon abundance and compare it to theoretical predictions from stellar evolution theory as well as from diffusion calculations. Methods. We analyze high-resolution spectra taken with the Far Ultraviolet Spectroscopic Explorer. We use non-LTE line-blanketed model atmospheres and perform line-formation calculations to compute synthetic argon line profiles. Results. We find a solar argon abundance in the H-rich central star NGC 1360 and in the H-deficient PG1159 star PG 1424+535. This confirms stellar evolution modeling that predicts that the argon abundance remains almost unaffected by nucleosynthesis. For the DAO-type central star NGC 7293 and the hot DA white dwarfs PG 0948+534 and RE J1738+669 we find argon abundances that are up to three orders of magnitude smaller than predictions of calculations assuming equilibrium of radiative levitation and gravitational settling. For the hot DO white dwarf PG 1034+001 the theoretical overprediction amounts to one dex. Conclusions. Our results confirm predictions from stellar nucleosynthesis calculations for the argon abundance in AGB stars. The argon abundance found in hot white dwarfs, however, is another drastic example that the current state of equilibrium theory for trace elements fails to explain the observations quantitatively.