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We report the first discovery of argon in hot evolved stars and white dwarfs.We have identified the ArVII 1063.55A line in some of the hottest known(Teff=95000-110000 K) central stars of planetary nebulae and (pre-) whitedwarfs of various spectral type. We determine the argon abundance and compareit to theoretical predictions from stellar evolution theory as well as fromdiffusion calculations. We analyze high-resolution spectra taken with the FarUltraviolet Spectroscopic Explorer. We use non-LTE line-blanketed modelatmospheres and perform line-formation calculations to compute synthetic argonline profiles. We find a solar argon abundance in the H-rich central starNGC1360 and in the H-deficient PG1159 star PG1424+535. This confirms stellarevolution modeling that predicts that the argon abundance remains almostunaffected by nucleosynthesis. For the DAO-type central star NGC7293 and thehot DA white dwarfs PG0948+534 and REJ1738+669 we find argon abundances thatare up to three orders of magnitude smaller than predictions of calculationsassuming equilibrium of radiative levitation and gravitational settling. Forthe hot DO white dwarf PG1034+001 the theoretical overprediction amounts to onedex. Our results confirm predictions from stellar nucleosynthesis calculationsfor the argon abundance in AGB stars. The argon abundance found in hot whitedwarfs, however, is another drastic example that the current state ofequilibrium theory for trace elements fails to explain the observationsquantitatively.Comment: Accepted for publication in A&

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