The metal abundances in the atmospheres of hot white dwarfs ( WDs ) entering the cooling sequence are determined by the preceding Asymptotic Giant Branch ( AGB ) evolutionary phase and , subsequently , by the onset of gravitational settling and radiative levitation .
In this paper , we investigate three hot He-rich WDs , which are believed to result from a late He-shell flash .
During such a flash , the He-rich intershell matter is dredged up and dominates the surface chemistry .
Hence , in contrast to the usual H-rich WDs , their spectra allow direct access to s-process element abundances in the intershell that were synthesized during the AGB stage .
In order to look for trans-iron group elements ( atomic number Z > 29 ) , we performed a non-local thermodynamic equilibrium model atmosphere analysis of new ultraviolet spectra taken with the Cosmic Origins Spectrograph aboard the Hubble Space Telescope .
One of our program stars is of PG 1159 spectral type ; this star , PG 1707 $ + $ 427 , has effective temperature T _ { \mathrm { eff } } = 85 000 K , and surface gravity \log g = 7.5 .
The two other stars are DO white dwarfs : WD 0111 $ + $ 002 has T _ { \mathrm { eff } } = 58 000 K and \log g = 7.7 , and PG 0109 $ + $ 111 has T _ { \mathrm { eff } } = 70 000 K and \log g = 8.0 .
These stars trace the onset of element diffusion during early WD evolution .
While zinc is the only trans-iron element we could detect in the PG 1159 star , both DOs exhibit lines from Zn , Ga , Ge , Se ; one additionally exhibits lines from Sr , Sn , Te , and I and the other from As .
Generally , the trans-iron elements are very abundant in the DOs , meaning that radiative levitation must be acting .
Most extreme is the almost six orders of magnitude oversolar abundance of tellurium in PG 0109 $ + $ 111 .
In terms of mass fraction , it is the most abundant metal in the atmosphere .
The two DOs join the hitherto unique hot DO RE 0503 $ - $ 289 , in which 14 trans-iron elements had even been identified .