Non-LTE line formation for Pr ii  and Pr iii  in A and Ap stars

Mashonkina, L.; Ryabchikova, T. A.; Ryabtsev, A. N.; Kildiyarova, R R

doi:10.1051/0004-6361:200810258

Cited by 33 publications

(49 citation statements)

References 53 publications

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“…As shown for Nd by Mashonkina et al (2005) and for Pr by Mashonkina et al (2009), the main non-LTE effect for their first ions in the model atmosphere representing the atmosphere of HD 24712 is overionization caused by a super-thermal radiation of non-local origin close to the thresholds of the lowexcitation levels. The departures from LTE for the lines of the first and the second ions are of the opposite sign and they are significant if the element is concentrated in the uppermost atmospheric layers where collisions are inefficient in establishing thermodynamic equilibrium.…”

Section: Nlte Calculationsmentioning

confidence: 84%

“…For Pr and Nd, we performed a NLTE stratification analysis as described in Mashonkina et al (2009) and Mashonkina et al (2005) using a trial-and-error method and the observed equivalent widths of the lines of the first and second ions.…”

Section: Stratification Analysismentioning

confidence: 99%

“…The present investigation of Pr ii/iii and Nd ii/iii is based on the NLTE methods treated in our earlier studies (Mashonkina et al 2009(Mashonkina et al , 2005, where atomic data and the problems of line formation were considered in detail. Briefly, comprehensive model atoms of praseodymium and neodymium include the measured and the predicted energy levels, in total, 6708 levels of Pr ii and Pr iii and 2258 levels of Nd ii and Nd iii.…”

Section: Nlte Calculationsmentioning

confidence: 99%

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Model atmospheres of chemically peculiar stars

Shulyak

Ryabchikova

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.

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Section: Nlte Calculationsmentioning

confidence: 84%

Section: Stratification Analysismentioning

confidence: 99%

Section: Nlte Calculationsmentioning

confidence: 99%

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Model atmospheres of chemically peculiar stars

Shulyak

Ryabchikova

Mashonkina

et al. 2009

A&A

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“…The fact that Mashonkina et al (2005Mashonkina et al ( , 2009) find important non-LTE effects in formation of lines of Pr and Nd ii and iii in magnetic Ap stars with T eff as low as 7250 K suggests that non-LTE effects in Si could potentially be important well below the conventional limit of T eff ≈ 15 000 K.…”

Section: Non-lte Effects In B-type Starsmentioning

confidence: 99%

Abundances determined using Si ii and Si iii in B-type stars: evidence for stratification

Bailey

Landstreet

2013

A&A

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Context. It is becoming clear that determination of the abundance of Si using lines of Si ii and Si iii can lead to quite discordant results in mid to late B-type stars. The difference between the Si abundances derived from the two ion states can exceed one dex in some cases. Aims. We have carried out a study intended to clarify which kinds of B stars exhibit this discrepancy, to try to identify regularities in the phenomenon, and to explore possible explanations such as abundance stratification by comparing models to observed spectra. Methods. We used spectra from the ESPaDOnS spectropolarimeter and FEROS spectrograph, supplemented with spectra from the ESO and ELODIE archives, of magnetic Bp, HgMn, and normal B-type stars ranging in effective temperature from about 10 500 to 15 000 K. Using these spectra, we derived abundances using the spectrum synthesis program zeeman, which can take the influence of magnetic fields into account. For each star, accurate abundances of Si ii, Si iii, Ti, Cr, and Fe were derived from two separate ∼100 Å windows. Si ii abundances were deduced from multiple lines, and Si iii abundances were found using λλ 4552, 4567, and 4574.Results. All magnetic Bp stars in our sample show a discordance between the derived abundances of the first and second ions of silicon, with the latter being between 0.6−1.7 dex higher. The same behaviour is observed in the non-magnetic stars but to a much smaller extent: Si iii is enhanced by between 0.3−0.8 dex compared to Si ii. We do not detect the discrepancy in three stars, HD 22 136 (normal), HD 57 608 (HgMn) and HD 27 295 (HgMn); these are the only stars in our sample for which the microturbulence parameter is significantly different from zero, and which therefore probably have convection occurring in their atmospheres. Conclusions. We find that vertical stratification of silicon in the atmospheres of B-type stars may provide an explanation of this phenomenon, but our detailed stratification models do not completely explain the discrepancies, which may, in part, be due to non-LTE effects.

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“…For five species, Sr ii, Zr ii, Ba ii, Pr ii, and Eu ii, we performed NLTE calculations using the methods described in our earlier studies (Belyakova & Mashonkina 1997;Velichko et al 2010;Mashonkina et al 1999Mashonkina et al , 2009Mashonkina & Gehren 2000) and determined the NLTE element abundances. They are presented in Table 8 In contrast to Sr ii and Ba ii, the term structure of the other NLTE species is produced by multiple electronic configurations and consists of hundreds and thousands of energy levels.…”

Section: Nlte Effectsmentioning

confidence: 99%

The Hamburg/ESOR-process enhanced star survey (HERES)

Mashonkina

Christlieb

Barklem

et al. 2010

A&A

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AstrophysicsThe Hamburg/ESO R-process enhanced star survey (HERES) V ABSTRACTAims. We present a detailed abundance analysis of a strongly r-process enhanced giant star discovered in the HERES project, HE 2327−5642, for which [Fe/H] = −2.78, [r/Fe] = +0.99. Methods. We determined the stellar parameters and element abundances by analyzing the high-quality VLT/UVES spectra. The surface gravity was calculated from the non-local thermodynamic equilibrium (NLTE) ionization balance between Fe i and Fe ii, and Ca i and Ca ii.Results. Accurate abundances for a total of 40 elements and for 23 neutron-capture elements beyond Sr and up to Th were determined in HE 2327−5642. For every chemical species, the dispersion in the single line measurements around the mean does not exceed 0.11 dex. The heavy element abundance pattern of HE 2327−5642 is in excellent agreement with those previously derived for other strongly r-process enhanced stars, such as CS 22892-052, CS 31082-001, and HE 1219-0312. Elements in the range from Ba to Hf match the scaled Solar r-process pattern very well. No firm conclusion can be drawn about the relationship between the fisrt neutroncapture peak elements, Sr to Pd, in HE 2327−5642 and the Solar r-process, due to the uncertainty in the Solar r-process. A clear distinction in Sr/Eu abundance ratios was found between the halo stars of different europium enhancement. The strongly r-process enhanced stars contain a low Sr/Eu abundance ratio at [Sr/Eu] = −0.92 ± 0.13, while the stars with 0 < [Eu/Fe] < 1 and [Eu/Fe] < 0 have 0.36 dex and 0.93 dex higher Sr/Eu values, respectively. Radioactive dating for HE 2327−5642 with the observed thorium and rare-earth element abundance pairs results in an average age of 13.3 Gyr, when based on the high-entropy wind calculations, and 5.9 Gyr, when using the Solar r-residuals. We propose that HE 2327−5642 is a radial-velocity variable based on our high-resolution spectra covering ∼4.3 years.

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Non-LTE line formation for Pr ii and Pr iii in A and Ap stars

Cited by 33 publications

References 53 publications

Model atmospheres of chemically peculiar stars

Model atmospheres of chemically peculiar stars

Abundances determined using Si ii and Si iii in B-type stars: evidence for stratification

The Hamburg/ESOR-process enhanced star survey (HERES)

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