Abundances and chemical stratification analysis in the atmosphere of Cr-type Ap star HD 204411

Mashonkina

et al. 2009

A&A

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Context. High-resolution spectra of some chemically peculiar stars clearly demonstrate the presence of strong abundance gradients in their atmospheres. However, these inhomogeneities are usually ignored in the standard scheme of model atmosphere calculations, breaking the consistency between model structure and spectroscopically derived abundance pattern. Aims. In this paper we present the first empirical self-consistent stellar atmosphere model of the roAp star HD 24712 with stratification of chemical elements included, and which is derived directly from the observed profiles of spectral lines without time-consuming simulations of physical mechanisms responsible for these anomalies. Methods. We used the LLmodels stellar model atmosphere code and DDAFIT minimization tool for analysis of chemical element stratification and construction of a self-consistent atmospheric model. Empirical determination of Pr and Nd stratification in the atmosphere of HD 24712 is based on NLTE line formation for Pr ii/iii and Nd ii/iii with the use of the DETAIL code.Results. Based on an iterative procedure of stratification analysis and subsequent re-calculation of model atmosphere structure, we constructed a self-consistent model of HD 24712, i.e. the model whose temperature-pressure structure is consistent with the results of the stratification analysis. It is shown that stratification of chemical elements leads to considerable changes in model structure compared to the non-stratified homogeneous case. We find that accumulation of rare earth elements (REE) allows for the inverse temperature gradient to be present in the upper atmosphere of the star with a maximum temperature increase of about 600 K. Conclusions.

Section: Effect Of Ree Stratificationmentioning

confidence: 69%

Section: Stratification Analysismentioning

confidence: 99%

Model atmospheres of chemically peculiar stars

Shulyak

Mashonkina

et al. 2009

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“…7. Stratification calculations were made using a step function approximation for element distribution (see Ryabchikova et al 2005). This figure also shows maximal error boxes for the transition zone width (derived for optical lines) and abundance in the deeper atmospheric layers (derived for mult.…”

Section: Comparison With the Line Profiles Observed In The Spectra Ofmentioning

confidence: 99%

The influence of Stark broadening on Cr II spectral line shapes in stellar atmospheres

Dimitrijević

Simić

et al. 2007

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Aims. We consider the effect of Stark broadening on the shapes of Cr ii spectral lines observed in stellar atmospheres of the middle part of the main sequence. Methods. Stark broadening parameters were calculated by the semiclassical perturbation approach. For stellar spectra synthesis, the improved version synth3 of the code synth for synthetic spectrum calculations was used. Results. Stark broadening parameters for Cr ii spectral lines of seven multiplets belonging to 4s − 4p transitions were calculated. New calculated Stark parameters were applied to the analysis of Cr ii line profiles observed in the spectrum of Cr-rich star HD 133792.Conclusions. We found that Stark broadening mechanism is very important and should be taken into account, especially in the study of Cr abundance stratification.

“…The high accuracy of observations and modelling techniques now allows, for example, stretching the realm of abundance analysis to faster rotators than was possible in the past, but also provides the possibility of learning more about the structure of the stellar atmospheres and the ongoing physical processes, especially when spectral synthesis fails to reproduce the observations. For instance, observed discrepancies between observed and synthetic spectra have allowed us to discover that the Article published by EDP Sciences signature of chemical stratification is ubiquitous in the spectra of some chemically peculiar stars Wade et al 2001;Ryabchikova et al 2003) and to perform accurate modelling of this stratification in the atmospheres of Ap stars (for instance, Ryabchikova et al 2005;Kochukhov et al 2006). However, our inability to reproduce observations frequently stems for a very simple cause: that atomic data for individual spectral lines are incorrect.…”

Section: Introductionmentioning

confidence: 99%

The chemical abundance analysis of normal early A- and late B-type stars

Fossati

Bagnulo³

et al. 2009

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Context. Modern spectroscopy of early-type stars often aims at studying complex physical phenomena such as stellar pulsation, the peculiarity of the composition of the photosphere, chemical stratification, the presence of a magnetic field, and its interplay with the stellar atmosphere and the circumstellar environment. Comparatively less attention is paid to identifying and studying the "normal" A-and B-type stars and testing how the basic atomic parameters and standard spectral analysis allow one to fit the observations. By contrast, this kind of study is paramount for eventually allowing one to correctly quantify the impact of the various physical processes that occur inside the atmospheres of A-and B-type stars. Aims. We wish to establish whether the chemical composition of the solar photosphere can be regarded as a reference for early Aand late B-type stars. Methods. We have obtained optical high-resolution, high signal-to-noise ratio spectra of three slowly rotating early-type stars (HD 145788, 21 Peg and π Cet) that show no obvious sign of chemical peculiarity, and performed a very accurate LTE abundance analysis of up to 38 ions of 26 elements (for 21 Peg), using a vast amount of spectral lines visible in the spectral region covered by our spectra. Results. We provide an exhaustive description of the abundance characteristics of the three analysed stars with a critical review of the line parameters used to derive the abundances. We compiled a table of atomic data for more than 1100 measured lines that may be used in the future as a reference. The abundances we obtained for He, C, Al, S, V, Cr, Mn, Fe, Ni, Sr, Y, and Zr are compatible with the solar ones derived with recent 3D radiative-hydrodynamical simulations of the solar photosphere. The abundances of the remaining studied elements show some degree of discrepancy compared to the solar photosphere. Those of N, Na, Mg, Si, Ca, Ti, and Nd may well be ascribed to non-LTE effects; for P, Cl, Sc and Co, non-LTE effects are totally unknown; O, Ne, Ar, and Ba show discrepancies that cannot be ascribed to non-LTE effects. The discrepancies obtained for O (in two stars) and Ne agree with very recent non-LTE abundance analysis of early B-type stars in the solar neighbourhood.