Vertical stratification of iron in atmospheres of blue horizontal-branch stars

The aim of this study is to search for observational evidence of vertical iron stratification in the atmosphere of fourteen blue horizontal-branch (BHB) stars. We have found from our numerical simulations that five BHB stars: B22, B186 in the globular cluster NGC 288, WF2-820, WF2-2692 in M13 and B203 in M15 show clear signatures of the vertical stratification of iron whose abundance increases toward the lower atmosphere. Two other BHB stars (B334 in M15 and B176 in M92) also show possible iron stratification in their atmosphere. A dependence of the slope of iron stratification on the effective temperature was also discovered. It is found that the vertical stratification of iron is strongest in BHB stars with Teff around 11,500K. The slope of iron abundance decreases as Teff increases and becomes negligible for the BHB stars with Teff= 14,000K. These results support the hypothesis regarding the efficiency of atomic diffusion in the stellar atmospheres of BHB stars with Teff > 11,500K.Comment: 6 pages, 2 figures, 3 table

Stratification of the elements in the atmospheres of blue horizontal-branch stars

Blue horizontal-branch (BHB) stars with $T_{\rm eff}$ approximately larger than 11500 K show several observational anomalies. In globular clusters, they exhibit low rotational velocities, abundance anomalies (as compared to cluster abundances), photometric jumps and gaps and spectroscopic gravities lower than predicted by canonical models. It is commonly believed that the low rotational velocities of these stars permit atomic diffusion to be efficient in their atmosphere thereby causing the observed anomalies. Recent detections of vertical stratification of iron (and some other chemical elements) in several BHB stars concur with this framework. In this paper, improved model atmospheres that include the vertical stratification of the elements are applied to BHB stars to verify if they can explain their observational anomalies. The results from theoretical model atmospheres are consistent with the photometric jumps and gaps observed for BHB stars in globular clusters. It is found that iron stratification in the theoretical models and that obtained from observations have similar tendancies. Our results also show that the spectroscopic gravities obtained while using chemically homogeneous model atmospheres to fit observations are underestimated. These results significantly strengthen the belief that atomic diffusion is responsible for these BHB-star anomalies.Comment: 6 pages, 6 figure

Recovery of the global magnetic field configuration of 78 Virginis from Stokes IQUV line profiles

The surface magnetic field configuration of the Ap star HD 118022 (78 Vir) has been reconstructed in the framework of the magnetic charge distribution (MCD) method from the analysis of Stokes $IQUV$ spectra obtained using the MuSiCoS spectropolarimeter at Pic du Midi Observatory. Magnetically-sensitive Fe~{\sc ii} lines were primarily employed in the analysis, supposing that iron is evenly distributed over the stellar surface. We show that the Stokes $IQUV$ profile shapes and variations of 78 Vir can be approximately fit assuming a global magnetic field configuration described by a slightly decentered, inclined magnetic dipole of polar surface intensity approximately 3.3~kG. The derived inclinations of the stellar rotational axis to the line of sight i=24\pm 5\degr as well as to the magnetic dipole axis \beta=124\pm5\degr are in good agreement with previous estimations by other authors, whereas the sky-projected position angle\thanks{$\Omega$ increases clockwise from the axis to the North Celestial Pole and relates to the azimuth angle $\Theta$ specified by Landolfi at al.~(\cite{Landolfi+93}) as \Omega=360\degr-\Theta.} of the stellar rotation axis \Omega\sim110\degr is reported here for the first time. In addition, several lines of Cr~{\sc ii} and Ti~{\sc ii} were studied, yielding evidence for non-uniform surface distributions of these elements, and magnetic field results similar to those derived from Fe.Comment: Accepted by A&A, 16 page

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

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.Comment: 7 pages, 5 figures, accepted for publication in MNRA

Impact of the stellar oblation effect on estimation of the magnetic dipole strength

The surface magnetic field structure of an ellipsoidal star is modelled in the frame of the magnetic charge description (MCD) approach. It is shown that the stellar oblation effect can lead to the essential overestimation of the magnetic dipole strength value obtained from the mean crossover effect (up to 12%) and quadratic magnetic field (up to 8%) in comparison with its theoretical value obtained for the case of the spherically symmetric star. Taking into account the gravity-darkening phenomenon is argued that this overestimation increases with the growth of the effective gravity difference at the equator and poles of the star. The data of the mean longitudinal magnetic field provide the most correct estimation of the magnetic dipole strength value in the ellipsoidal star

Search for vertical stratification of metals in atmospheres of blue horizontal-branch stars

The observed abundance peculiarities of many chemical species relative to the expected cluster metallicity in blue horizontal-branch (BHB) stars presumably appear as a result of atomic diffusion in the photosphere. The slow rotation (typically $v\sin{i}<$ 10 km s$^{-1}$) of BHB stars with effective temperatures $T_{\rm eff}>$ 11,500 K supports this idea since the diffusion mechanism is only effective in a stable stellar atmosphere. In this work we search for observational evidence of vertical chemical stratification in the atmospheres of six hot BHB stars: B84, B267 and B279 in M15 and WF2-2541, WF4-3085 and WF4-3485 in M13. We undertake an abundance stratification analysis of the stellar atmospheres of the aforementioned stars, based on acquired Keck HIRES spectra. We have found from our numerical simulations that three stars (B267, B279 and WF2-2541) show clear signatures of the vertical stratification of iron whose abundance increases toward the lower atmosphere, while the other two stars (B84 and WF4-3485) do not. For WF4-3085 the iron stratification results are inconclusive. B267 also shows a signature of titanium stratification. Our estimates for radial velocity, $v\sin{i}$ and overall iron, titanium and phosphorus abundances agree with previously published data for these stars after taking the measurement errors into account. The results support the hypothesis regarding the efficiency of atomic diffusion in the stellar atmospheres of BHB stars with $T_{\rm eff}>$ 11,500 K.Comment: 8 pages, 12 figures, accepted for publication in Astronomy and Astrophysic