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

Abstract

HD 76431 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 HD 76431. 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 HD 76431. 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 Si III and one Ti III emission lines in the spectra of HD 76431 that were not detected in previous studies.

NLTE spectroscopic analysis of the3He anomaly in subluminous B-type stars

“…In this way, the optical depth at which the change in helium abundance occurs can also be identified. As an example, stratification analyses have been carried out successfully for carbon and nitrogen in the post-HB star HD 76431 by Khalack et al (2014) and for nitrogen, sulfur, titanium, and in particular for iron, in BHB stars by Khalack et al (2007Khalack et al ( , 2008Khalack et al ( , 2010 and LeBlanc et al (2010). The strategy to derive the stratified helium abundance profile throughout the photosphere with the hybrid LTE/NLTE approach was reported in Maza et al (2014a), who tested it successfully on κ Cancri.…”

Project VeSElkA: abundance analysis of chemical species in HD 41076 and HD 148330

“…This approach has been successfully employed to search for the signatures of vertical abundance stratification in the atmospheres Notes: a results for two separate spectra (see Table 1) of several BHB stars (Khalack et al 2008;Khalack et al 2010;LeBlanc et al 2010), one post-HB star (Khalack et al 2014), and several HgMn stars (Thiam et al 2010). A number of the upper main sequence CP stars observed in the frame of the VeSElkA project (Khalack & LeBlanc 2015a;Khalack & LeBlanc 2015b) have been studied using the aforementioned approach.…”

“…In the first approach we can use the high-resolution (R=60000) and high signal-to-noise ratio (S/N¿250) spectra to derive from the fitting procedure values of the free model parameters with a precision sufficiently high to track the abundance variation with optical depth (Khalack et al 2014;Khalack & Poitras 2015;LeBlanc et al 2015). The spectra with lower resolution (R=45000) and smaller S/N ratio also can be used, but the fitting procedure will results in a lower precision of abundance estimates (Khalack et al 2007; Khalack et al 2008; Khalack et al 2010).…”

“…This condition was imposed because a star with slow axial rotation has a higher probability to possess a hydrodynamically stable atmosphere where the atomic diffusion mechanism can produce vertical stratification of element abundances. Small value of Vsin i results in a number of narrow and mostly unblended line profiles in stellar spectra that we can employ for our abundance analysis (Khalack et al 2013;Khalack et al 2014).…”

“…Fundamental stellar parameters employed in the second approach and the velocities obtained by averaging the data from all (total number is given in the brackets) analysed line profiles.Notes: a results for two separate spectra (see Table1) of several BHB stars(Khalack et al 2008;Khalack et al 2010;LeBlanc et al 2010), one post-HB star(Khalack et al 2014), and several HgMn stars(Thiam et al 2010). A number of the upper main sequence CP stars observed in the frame of the VeSElkA project(Khalack & LeBlanc 2015a;Khalack & LeBlanc 2015b) …”