F. LeBlanc

Radiative Accelerations for Evolutionary Model Calculations

Monochromatic opacities from the OPAL database have been used to calculate radiative accelerations for the 21 included chemical species. The 104 frequencies used are sufficient to calculate the radiative accelerations of many elements for T > 105 K, using frequency sampling. This temperature limit is higher for less abundant elements. As the abundances of Fe, He, or O are varied, the radiative acceleration of other elements changes, since abundant elements modify the frequency dependence of the radiative flux and the Rosseland opacity. Accurate radiative accelerations for a given element can only be obtained by allowing the abundances of the species that contribute most to the Rosseland opacity to vary during the evolution and recalculating the radiative accelerations and the Rosseland opacity during the evolution. There are physical phenomena that cannot be included in the calculations if one uses only the OPAL data. For instance, one should correct for the momentum given to the electron in a photoionization. Such effects are evaluated using atomic data from Opacity Project, and correction factors are given.

Stellar model atmospheres with abundance stratification

Context. Atomic diffusion is believed to be an important physical process in the atmospheres of several types of stars. Stellar atmospheres, including the stratification of the elements due to diffusion, are then needed to properly compare theoretical results to observations for such stars. Aims. This paper aims to estimate the effect of vertical abundance stratification on the atmospheric structure of stars and its potential importance regarding observational anomalies for various types of stars. Methods. Simulations using a modified version of the PHOENIX atmosphere code will be described, while taking vertical abundance stratification into account. Results. Our results show that large abundance gradients can exist in the atmospheres of Ap and blue horizontal branch stars. Stratification can also lead to relatively large atmospheric structural changes. The effect of elemental stratification on the atmospheric structure might well be able to explain the well-known core-wing anomaly of the Balmer lines observed for cool Ap stars.

INFLUENCE OF THERMOHALINE CONVECTION ON DIFFUSION-INDUCED IRON ACCUMULATION IN A STARS

Atomic diffusion may lead to heavy-element accumulation inside stars in certain specific layers. Iron accumulation in the Z-bump opacity region has been invoked by several authors to quantitatively account for abundance anomalies observed in some stars, or to account for stellar oscillations through the induced κ-mechanism. These authors, however, never took into account the fact that such an accumulation creates an inverse μ-gradient, unstable for thermohaline convection. Here, we present results for A-F stars, where abundance variations are computed with and without this process. We show that iron accumulation is still present when thermohaline convection is taken into account, but much reduced compared to when this physical process is neglected. The consequences of thermohaline convection for A-type stars as well as for other types of stars are presented.

Diffusion in the Atmospheres of Blue Horizontal-Branch Stars

We investigate the effects of diffusion in the atmospheres of hot horizontal-branch stars using a model atmosphere code including diffusion self-consistently. Equilibrium stratifications (i.e., for which the diffusion velocity equals zero in each layer) are computed for models of effective temperatures between 10,000 and 25,000 K. The stratified models provide much better agreement with many observational features [jump in the (u, u-y) color-magnitude diagram, gaps, lower spectroscopic gravities] in comparison with classical horizontal-branch models. The observed abundance anomalies are also consistent with the amounts that can be supported in the atmospheres.

Opacity Sampling in Radiative Acceleration Calculations

The accuracy requirements on atomic data for the calculation of stellar evolution with atomic diffusion are determined. In particular, the density of frequency grids needed for precise radiative acceleration (grad) calculations via the sampling method are presented. In order to minimize the number of frequency points needed for precise grad calculations, frequency grids that are more refined in the regions of the spectrum where the radiative flux is large are suggested. The following number of frequency points are needed for opacity table calculations to be used in stellar evolutionary codes including diffusion: 50,000 points for 4.20 ≤ log T ≤ 4.5, 30,000 points for 4.5 5.5. These opacity tables would render possible the study atomic diffusion in the exterior regions of certain chemically peculiar stars such as Ap or HgMn stars. In the sampling method, correction factors can be applied after the basic integrations over sampled spectra to include such effects as ion velocity averaging, redistribution of momentum among ions, and electron recoil during photoionization; these corrections are evaluated and illustrated for a few typical stellar models. Silicon is used as an example to show that the corrections are important mainly for T < 50,000 K. These corrections are used in stellar evolution calculations based on OPAL monochromatic opacity tables.

The new Toulouse-Geneva stellar evolution code including radiative accelerations of heavy elements

Context. Atomic diffusion has been recognized as an important process that has to be considered in any computations of stellar models. In solar-type and cooler stars, this process is dominated by gravitational settling, which is now included in most stellar evolution codes. In hotter stars, radiative accelerations compete with gravity and become the dominant ingredient in the diffusion flux for most heavy elements. Introducing radiative accelerations into the computations of stellar models modifies the internal element distribution and may have major consequences on the stellar structure. Coupling these processes with hydrodynamical stellar motions has important consequences that need to be investigated in detail. Aims. We aim to include the computations of radiative accelerations in a stellar evolution code (here the TGEC code) using a simplified method (SVP) so that it may be coupled with sophisticated macroscopic motions. We also compare the results with those of the Montreal code in specific cases for validation and study the consequences of these coupled processes on accurate models of A- and early-type stars. Methods. We implemented radiative accelerations computations into the Toulouse-Geneva stellar evolution code following the semianalytical prescription proposed by Alecian and LeBlanc. This allows more rapid computations than the full description used in the Montreal code. Results. We present results for A-type stellar models computed with this updated version of TGEC and compare them with similar published models obtained with the Montreal evolution code. We discuss the consequences for the coupling with macroscopic motions, including thermohaline convection.

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

Blue horizontal-branch (BHB) stars with T eff approximately larger than 11 500 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 tendencies. 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.

Abundances and search for vertical stratification in the atmospheres of four HgMn stars

Using high resolution, high-S/N archival UVES spectra, we have performed a detailed spectroscopic analysis of 4 chemically peculiar HgMn stars (HD 71066, HD 175640, HD 178065 and HD 221507). Using spectrum synthesis, mean photospheric chemical abundances are derived for 22 ions of 16 elements. We find good agreement betwee n our derived abundances and those published previously by other authors. For the 5 el ements that present a sufficient number of suitable lines, we have attempted to detect vertical chemical stratification by analyzing the dependence of derived abundance as a function of optical depth. For most elements and most stars we find no evidence of chemical stratification w ith typical 3σ upper limits of Δ log Nelem/Ntot ∼ 0.1− 0.2 dex per unit optical depth. However, for Mn in the atmosphere of HD 178065 we find convincing evidence of stratification. Mode ling of the line profiles using a two-step model for the abundance of Mn yields a local abundance varying approximately linearly by ∼ 0.7 dex through the optical depth range log τ5000 = −3.6 to ‐2.8.

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 14 blue horizontal-branch (BHB) stars. We have found from our numerical simulations that five BHB stars: B22, B186 in the globular cluster NGC 288, WF 2−820, WF 2−2692 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 T eff around 11 500 K. The slope of iron abundance decreases as T eff increases and becomes negligible for the BHB stars with T eff ≈ 14 000 K. These results support the hypothesis regarding the efficiency of atomic diffusion in the stellar atmospheres of BHB stars with T eff � 11 500 K.

Search For Vertical Stratification Of Metals In Atmospheres Of Blue Horizontal-Branch Stars

Context. The observed abundance peculiarities of many chemical species relative to the expected cluster metallicity in blue horizontalbranch (BHB) stars presumably appear as a result of atomic diffusion in the photosphere. The slow rotation (typically v sini 11 500 K supports this idea since the diffusion mechanism is only effective in a stable stellar atmosphere. Aims. 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 M 15 and WF2-2541, WF4-3085 and WF4-3485 in M 13. Methods. We undertake an abundance stratification analysis of the stellar atmospheres of the aforementioned stars, based on acquired Keck HIRES spectra. Results. 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 sini 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 Teff > 11 500 K.