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

TL;DR

This study analyzes the atmospheric composition of the post-HB star HD 76431, revealing vertical stratification of metals and detecting new emission lines, thus providing novel insights into its atmospheric structure and diffusion processes.

Contribution

First demonstration of vertical metal abundance stratification in a post-HB star and identification of new emission lines in HD 76431.

Findings

Carbon and nitrogen show vertical abundance stratification.

Detected SiIII and TiIII emission lines not previously observed.

HD 76431 is the hottest star with metal stratification evidence.

Abstract

HD76431 is a slow rotating post-HB star that shows an underabundance of helium by 0.5 dex relative to the solar value. These observational facts suggest that atomic diffusion could be active in its atmosphere. We have used the MMT and Bok spectra to estimate the atmospheric parameters of the target star using the model atmospheres and synthetic spectra calculated with TLUSTY and 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 Stokes I spectra from ESPaDOnS at CFHT to perform an abundance analysis and a search for observational evidence of vertical stratification of the abundance of certain elements. The results of our abundance analysis are in good agreement with previously published data with respect to average abundances. Our numerical simulations show that carbon and nitrogen reveal signatures of vertical abundance stratification in the atmosphere of HD76431. It appears that the carbon abundance increases toward the deeper atmospheric layers. Nitrogen also shows a similar behaviour, but in deeper atmospheric layers we obtain a significant dispersion for the estimates of its abundance. To our knowledge, this is the first demonstration of vertical abundance stratification of metals in a post-HB star and up to now it is the hottest star to show such stratification features. We also report the detection of two SiIII and one TiIII emission lines in the spectra of HD76431 that were not detected in previous studies.