A self-consistent empirical model atmosphere, abundance and stratification analysis of the benchmark roAp star<i>α</i> Circini

Kochukhov, O.; Shulyak, D.; Ryabchikova, T.

doi:10.1051/0004-6361/200911653

Cited by 39 publications

(43 citation statements)

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“…This effect has been further studied by Kochukhov et al (2009) and others cited in the Introduction. It now seems quite clear that stratification does occur to a very important extent in the cool magnetic Ap stars, and that it affects many chemical elements, including Si.…”

Section: Stratificationmentioning

confidence: 77%

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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Section: Stratificationmentioning

confidence: 77%

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

Bailey

Landstreet

2013

A&A

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“…The authors used observations from the Sydney University Stellar Interferometer (SUSI) and additional data from visual and UV observations calibrated to absolute units to accurately derive T eff and log(g) of the star. Even thought the interferometry can provide us with accurate radii of stars, the accuracy of T eff determination is still limited by the incompleteness of observations in all spectral ranges (that would give a value of the bolometric flux) and model atmospheres used; see Kochukhov et al, 2009. Application of interferometric techniques to another Ap star β CrB is reported in this conference (JD04).…”

Section: Selected Results From Recent Ground-based Observations Of Cpmentioning

confidence: 97%

“…This allow us to explore the changes in model structure due to stratification that were extracted directly from observations without modelling the processes that could be responsible for the observed inhomogeneities. Such an empirical modelling can be applied to any CP star for which accurate spectroscopic observations exist; see Kochukhov et al, 2009, Shulyak et al, 2009b and talk JD04-i:5 of this meeting. Having a suitable code for computing CP star spectra, it is now possible to verify how Gaia sees CP stars.…”

Section: Gaia Space Mission and Cp Starsmentioning

confidence: 99%

Pushing the limit of instrument capabilities

Shulyak¹,

Weiß²,

Mathys³

et al. 2009

Proc. IAU

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Abstract. Chemically Peculiar (CP) stars have been the subject of systematic research for more than 50 years. With the discovery of pulsation of some of the cool CP stars, and the availability of advanced spectropolarimetric instrumentation and high signal-to-noise, high resolution spectroscopy, a new era of CP star research emerged about 20 years ago. Together with the success in ground-based observations, new space projects are developed that will greatly benefit future investigations of these unique objects. In this contribution we will give an overview of some interesting results obtained recently from ground-based observations and discuss the future outstanding Gaia space mission and its impact on CP star research.

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“…For instance, for α Cir, Kupka et al (1996) determine 7900 K, while a recent determination by Kochukhov et al (2009) gives 7500 K. Indeed, models of atmospheres of Ap stars are extremely complex, and the consequent determination of effective temperature (hence radius, assuming one knows the luminosity) by spectroscopic data is subject to major difficulties. A good way to test the different effective temperatures is to use them as input for excitation models since the excitation region is very sensitive to the temperature.…”

Section: Discussionmentioning

confidence: 99%

The fundamental parameters of the roAp star 10 Aquilae

Perraut

Borgniet

Cunha

et al. 2013

A&A

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Context. Owing to the strong magnetic field and related abnormal surface layers existing in rapidly oscillating Ap (roAp) stars, systematic errors are likely to be present when determining their effective temperatures, which potentially compromises asteroseismic studies of this class of pulsators. Aims. Using the unique angular resolution provided by long-baseline visible interferometry, our goal is to determine accurate angular diameters of a number of roAp targets, so as to derive unbiased effective temperatures (T eff ) and provide a T eff calibration for these stars. Methods. We obtained long-baseline interferometric observations of 10 Aql with the visible spectrograph VEGA at the combined focus of the CHARA array. We derived the limb-darkened diameter of this roAp star from our visibility measurements. Based on photometric and spectroscopic data available in the literature, we estimated the star's bolometric flux and used it, in combination with its parallax and angular diameter, to determine the star's luminosity and effective temperature. Results. We determined a limb-darkened angular diameter of 0.275 ± 0.009 mas and deduced a linear radius of R = 2.32 ± 0.09 R . For the bolometric flux we considered two datasets, leading to an effective temperature of T eff = 7800 ± 170 K and a luminosity of L/L = 18 ± 1 or T eff = 8000 ± 210 K and L/L = 19 ± 2. Finally we used these fundamental parameters together with the large frequency separation determined by asteroseismic observations to constrain the mass and the age of 10 Aql, using the CESAM stellar evolution code. Assuming a solar chemical composition and ignoring all kinds of diffusion and settling of elements, we obtained a mass M/M ∼ 1.92 and an age of ∼780 Gy or a mass M/M ∼ 1.95 and an age of ∼740 Gy, depending on the derived value of the bolometric flux. Conclusions. For the first time, thanks to the unique capabilities of VEGA, we managed to determine an accurate angular diameter for a star smaller than 0.3 mas and to derive its fundamental parameters. In particular, by only combining our interferometric data and the bolometric flux, we derived an effective temperature that can be compared to those derived from atmosphere models. Such fundamental parameters can help for testing the mechanism responsible for the excitation of the oscillations observed in the magnetic pulsating stars.

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A self-consistent empirical model atmosphere, abundance and stratification analysis of the benchmark roAp starα Circini

Cited by 39 publications

References 50 publications

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

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

Pushing the limit of instrument capabilities

The fundamental parameters of the roAp star 10 Aquilae

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