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HD76431 is a slow rotating post-HB star that shows an underabundance ofhelium by 0.5 dex relative to the solar value. These observational factssuggest that atomic diffusion could be active in its atmosphere. We have usedthe MMT and Bok spectra to estimate the atmospheric parameters of the targetstar using the model atmospheres and synthetic spectra calculated with TLUSTYand SYNSPEC. The derived values of the effective temperature, surface gravity,helium abundance are consistent with those obtained by Ramspeck et al. (2001b).It appears that NLTE effect are not important for HD76431. We have used StokesI spectra from ESPaDOnS at CFHT to perform an abundance analysis and a searchfor observational evidence of vertical stratification of the abundance ofcertain elements. The results of our abundance analysis are in good agreementwith previously published data with respect to average abundances. Ournumerical simulations show that carbon and nitrogen reveal signatures ofvertical abundance stratification in the atmosphere of HD76431. It appears thatthe carbon abundance increases toward the deeper atmospheric layers. Nitrogenalso shows a similar behaviour, but in deeper atmospheric layers we obtain asignificant dispersion for the estimates of its abundance. To our knowledge,this is the first demonstration of vertical abundance stratification of metalsin a post-HB star and up to now it is the hottest star to show suchstratification features. We also report the detection of two SiIII and oneTiIII emission lines in the spectra of HD76431 that were not detected inprevious studies.

Spectral analysis and abundances of the post-HB star HD 76431