Multi-element Doppler imaging of the CP2 star HD 3980

Nesvacil, N.; Lüftinger, T.; Shulyak, D.; Obbrugger, M.; Weiß, W. W.; Драке, Н. А.; Hubrig, S.; Ryabchikova, T.; Kochukhov, O.; Piskunov, Nikolai; Polosukhina, N. S.

doi:10.1051/0004-6361/201117097

Cited by 27 publications

(31 citation statements)

References 34 publications

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“…For instance, in a number of studies of such stars, iron peak elements appear enhanced along the magnetic equator (e.g. Nesvacil et al 2012). We note that the assumption of the presence of concentration of these elements in the region of the magnetic equator, similar to the iron peak element distribution appearing in Ap stars, would explain the amplitude and the shape of the observed Zeeman features displaying crossover effect.…”

Section: Discussionmentioning

confidence: 53%

Chemical spots on the surface of the strongly magnetic Herbig Ae star HD 101412

et al. 2016

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Due to the knowledge of the rotation period and the presence of a rather strong surface magnetic field, the sharp-lined young Herbig Ae star HD 101412 with a rotation period of 42 d has become one of the most well-studied targets among the Herbig Ae stars. High-resolution HARPS polarimetric spectra of HD 101412 were recently obtained on seven different epochs. Our study of the spectral variability over the part of the rotation cycle covered by HARPS observations reveals that the line profiles of the elements Mg, Si, Ca, Ti, Cr, Mn, Fe, and Sr are clearly variable while He exhibits variability that is opposite to the behaviour of the other elements studied. Since classical Ap stars usually show a relationship between the magnetic field geometry and the distribution of element spots, we used in our magnetic field measurements different line samples belonging to the three elements with the most numerous spectral lines, Ti, Cr, and Fe. Over the time interval covered by the available spectra, the longitudinal magnetic field changes sign from negative to positive polarity. The distribution of field values obtained using Ti, Cr, and Fe lines is, however, completely different compared to the magnetic field values determined in previous low-resolution FORS 2 measurements, where hydrogen Balmer lines are the main contributors to the magnetic field measurements, indicating the presence of concentration of the studied iron-peak elements in the region of the magnetic equator. Further, we discuss the potential role of contamination by the surrounding warm circumstellar matter in the appearance of Zeeman features obtained using Ti lines.

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Section: Discussionmentioning

confidence: 53%

Chemical spots on the surface of the strongly magnetic Herbig Ae star HD 101412

et al. 2016

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“…Piskunov & Kochukhov (2002), which is widely used for the reconstruction of the temperature, abundance, and magnetic field maps of stellar surfaces (e.g. Lüftinger et al 2010;Nesvacil et al 2012;). The essential difference of the modified algorithm from the original one is that it additionally adjusts the value of the damping parameter within each iteration step, keeping the Hessian matrix unchanged.…”

Section: Least-squares Deconvolution: Theoretical Backgroundmentioning

confidence: 99%

Denoising spectroscopic data by means of the improved least-squares deconvolution method

Tkachenko

Reeth

Tsymbal

et al. 2013

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Context. The MOST, CoRoT, and Kepler space missions have led to the discovery of a large number of intriguing, and in some cases unique, objects among which are pulsating stars, stars hosting exoplanets, binaries, etc. Although the space missions have delivered photometric data of unprecedented quality, these data are lacking any spectral information and we are still in need of ground-based spectroscopic and/or multicolour photometric follow-up observations for a solid interpretation. Aims. The faintness of most of the observed stars and the required high signal-to-noise ratio (S/N) of spectroscopic data both imply the need to use large telescopes, access to which is limited. In this paper, we look for an alternative, and aim for the development of a technique that allows the denoising of the originally low S/N (typically, below 80) spectroscopic data, making observations of faint targets with small telescopes possible and effective. Methods. We present a generalization of the original least-squares deconvolution (LSD) method by implementing a multicomponent average profile and a line strengths correction algorithm. We tested the method on simulated and real spectra of single and binary stars, among which are two intrinsically variable objects. Results. The method was successfully tested on the high-resolution spectra of Vega and a Kepler star, KIC 04749989. Application to the two pulsating stars, 20 Cvn and HD 189631, showed that the technique is also applicable to intrinsically variable stars: the results of frequency analysis and mode identification from the LSD model spectra for both objects are in good agreement with the findings from literature. Depending on the S/N of the original data and spectral characteristics of a star, the gain in S/N in the LSD model spectrum typically ranges from 5 to 15 times. Conclusions. The technique introduced in this paper allows an effective denoising of the originally low S/N spectroscopic data. The high S/N spectra obtained this way can be used to determine fundamental parameters and chemical composition of the stars. The restored LSD model spectra contain all the information on line profile variations present in the original spectra of pulsating stars, for example. The method is applicable to both high-(>30 000) and low-(<30 000) resolution spectra, although the information that can be extracted from the latter is limited by the resolving power itself.

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“…using multiple lines for a given element whenever possible, and treating line blends with detailed radiative transfer calculations in realistic model atmospheres (e.g. Kochukhov et al 2004b;Lüftinger et al 2010a;Nesvacil et al 2012).…”

Section: Introductionmentioning

confidence: 99%

Doppler imaging of chemical spots on magnetic Ap/Bp stars

Kochukhov

2016

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Context. Doppler imaging (DI) is a powerful spectroscopic inversion technique that enables conversion of a line profile time series into a two-dimensional map of the stellar surface inhomogeneities. DI has been repeatedly applied to reconstruct chemical spot topologies of magnetic Ap/Bp stars with the goal of understanding variability of these objects and gaining an insight into the physical processes responsible for spot formation. Aims. In this paper we investigate the accuracy of chemical abundance DI and assess the impact of several different systematic errors on the reconstructed spot maps. Methods. We have simulated spectroscopic observational data for two different Fe spot distributions with a surface abundance contrast of 1.5 dex in the presence of a moderately strong dipolar magnetic field. We then reconstructed chemical maps using different sets of spectral lines and making different assumptions about line formation in the inversion calculations. Results. Our numerical experiments demonstrate that a modern DI code successfully recovers the input chemical spot distributions comprised of multiple circular spots at different latitudes or an element overabundance belt at the magnetic equator. For the optimal reconstruction based on half a dozen spectral intervals, the average reconstruction errors do not exceed ∼0.10 dex. The errors increase to about 0.15 dex when abundance distributions are recovered from a few and/or blended spectral lines. Ignoring a 2.5 kG dipolar magnetic field in chemical abundance DI leads to an average relative error of 0.2 dex and maximum errors of 0.3 dex. Similar errors are encountered if a DI inversion is carried out neglecting a non-uniform continuum brightness distribution and variation of the local atmospheric structure. None of the considered systematic effects lead to major spurious features in the recovered abundance maps.Conclusions. This series of numerical DI simulations proves that inversions based on one or two spectral lines with simplifying assumptions of the non-magnetic radiative transfer and a single model atmosphere are generally reliable provided that the stellar magnetic field is not much stronger than 2-3 kG and the recovered spot map has a contrast of at least ∼0.3 dex. In the light of these findings, we assess magnetic field strengths of several dozen Ap/Bp stars previously studied with DI methods, concluding that the vast majority of the published chemical spot maps are unaffected by the systematic errors addressed in this paper.

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Multi-element Doppler imaging of the CP2 star HD 3980

Cited by 27 publications

References 34 publications

Chemical spots on the surface of the strongly magnetic Herbig Ae star HD 101412

Chemical spots on the surface of the strongly magnetic Herbig Ae star HD 101412

Denoising spectroscopic data by means of the improved least-squares deconvolution method

Doppler imaging of chemical spots on magnetic Ap/Bp stars

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