The Lorentz force in atmospheres of CP stars:<i>θ</i> Aurigae

Shulyak, D.; Valyavin, G. G.; Kochukhov, Oleg; Lee, B. C.; Галазутдинов, Г. А.; Kim, K. M.; Han, Inwoo; Burlakova, T. E.; Tsymbal, V.; Lyashko, D.

doi:10.1051/0004-6361:20064998

Cited by 24 publications

(26 citation statements)

References 25 publications

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“…This agrees with the results of multipolar modeling of magnetic topology and implies that possible modification of the hydrostatic equilibrium by the Lorentz force (see Shulyak et al 2007, and references therein) is absent in HD 137509. Thus, all differences in the pressure structure between magnetic and nonmagnetic models that we obtain are only caused by additional opacity in the Zeeman components.…”

Section: Model Atmosphere Calculationssupporting

confidence: 90%

Model atmospheres of magnetic chemically peculiar stars

Shulyak¹,

Kochukhov

Khan

2008

A&A

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Context. In the past few years, we have developed stellar model atmospheres that included effects of anomalous abundances and a strong magnetic field. In particular, the full treatment of anomalous Zeeman splitting and polarized radiative transfer were introduced in the model atmosphere calculations for the first time. The influence of the magnetic field on the model atmosphere structure and various observables were investigated for stars of different fundamental parameters and metallicities. However, these studies were purely theoretical and did not attempt to model real objects. Aims. In this investigation we present results of modeling the atmosphere of one of the most extreme magnetic chemically peculiar stars, HD 137509. This Bp SiCrFe star has a mean surface magnetic field modulus of about 29 kG. Such a strong field, as well as clearly observed abundance peculiarities, make this star an interesting target for applying our newly developed model atmosphere code. Methods. We used the recent version of the line-by-line opacity sampling stellar model atmosphere code LLmodels, which incorporates the full treatment of Zeeman splitting of spectral lines, detailed polarized radiative transfer, and arbitrary abundances. We compared model predictions with photometric and spectroscopic observations of HD 137509, aiming to reach a self-consistency between the abundance pattern derived from high-resolution spectra and abundances used for model atmosphere calculation. Results. Based on magnetic model atmospheres, we redetermined abundances and fundamental parameters of HD 137509 using spectroscopic and photometric observations. This allowed us to obtain better agreement between observed and theoretical parameters compared to non-magnetic models with individual or scaled-solar abundances. Conclusions. We confirm that the magnetic field effects lead to noticeable changes in the model atmosphere structure and should be taken into account in the stellar parameter determination and abundance analysis.

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Section: Model Atmosphere Calculationssupporting

confidence: 90%

Model atmospheres of magnetic chemically peculiar stars

Shulyak¹,

Kochukhov

Khan

2008

A&A

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“…As an additional test for the existence of Lorentz forces due to magnetic fields, we computed the standard deviation σ of the H β line profiles compared to the mean profile from all rotation phases. If the stellar structure is disturbed due to the presence of non force-free magnetic fields, σ should reveal the characteristic fingerprint as the impact of the Lorentz force: the amplitude rises in the wings and drops to the line center (Kroll 1989;Valyavin et al 2004Valyavin et al , 2005Shulyak et al 2007). From Fig.…”

Section: Variability Of the Hydrogen Line Profilesmentioning

confidence: 98%

The helium weak silicon star HR 7224

Lehmann¹,

Tkachenko²,

Tsymbal

et al. 2007

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Aims. We investigate the abundance distributions of silicon and iron on the surface of HR 7224 and check for the presence of magnetic fields. We try further to find an explanation for the observed Balmer line variations, alternative to that by surface temperature gradients given in a previous article. Methods. Based on time series of more than 570 high-resolution spectra we investigate the abundance distribution of silicon and iron using the Doppler Imaging technique applying two independent program codes and using single and multiple components atmosphere models. We further calculate the combined effect of abundance changes of Si, Fe, and He on the H β line profile and compare it with the observations. The mean longitudinal magnetic field strength of HR 7224 is measured spectropolarimetrically. Results. The Doppler Imaging analysis of HR 7224 shows the presence of large silicon and iron spots on the star's surface, large gradients of the abundances of these elements, and hints to enhanced line strengths near the visible pole of the star. The explanation of the observed Balmer line variations by inhomogeneous abundance distributions alone requires unrealistic large gradients of the helium abundance. Magnetic field measurements gave no definite results, a field strength of the order of +400 G is likely. Conclusions. We can exclude the hypothesis that the hydrogen line profile variations of HR 7224 are caused by inhomogeneous abundance distributions of Fe, Si, and He alone or by pressure gradients like in the case of Ap stars. Still the observed variations can be described only by the combined effect of abundance and temperature gradients. In this case, the derived Doppler imaging maps will reflect also the temperature gradients and the surface will be cooler in regions of enhanced line strength like at the visible pole. At present state, the physical origin of possible surface temperature gradients remains unknown.

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“…This program treats atomic line opacity by a direct, line‐by‐line spectrum synthesis and is able to account for individual non‐solar chemical abundance patterns (Khan & Shulyak 2007) and inhomogeneous vertical distribution of elements (Kochukhov, Shulyak & Ryabchikova 2009b; Shulyak et al 2009). The code also includes a provision for treating the effects of a strong magnetic field on the line opacity, including Zeeman splitting and polarized radiative transfer (Kochukhov, Khan & Shulyak 2005; Khan & Shulyak 2006), and a contribution of the Lorentz force to the equation of hydrostatic balance (Shulyak et al 2007). However, as demonstrated by the previous studies, for the moderate 2–4 kG magnetic field of α 2 CVn, Zeeman splitting and polarized radiative transfer can be safely neglected in the model atmosphere calculations.…”

Section: Model Atmospheres and Local Line Profilesmentioning

confidence: 99%

Magnetic Doppler imaging considering atmospheric structure modifications due to local abundances: a luxury or a necessity?

Kochukhov¹,

Wade²,

Shulyak³

2012

Monthly Notices of the Royal Astronomical Society

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Magnetic Doppler imaging is currently the most powerful method of interpreting high‐resolution spectropolarimetric observations of stars. This technique has provided the very first maps of stellar magnetic field topologies reconstructed from time series of full Stokes vector spectra, revealing the presence of small‐scale magnetic fields on the surfaces of Ap stars. These studies were recently criticised by Stift et al., who claimed that magnetic inversions are not robust and are seriously undermined by neglecting a feedback on the Stokes line profiles from the local atmospheric structure in the regions of enhanced metal abundance. We show that Stift et al. misinterpreted published magnetic Doppler imaging results and consistently neglected some of the most fundamental principles behind magnetic mapping. Using state‐of‐the‐art opacity sampling model atmosphere and polarized radiative transfer codes, we demonstrate that the variation of atmospheric structure across the surface of a star with chemical spots affects the local continuum intensity but is negligible for the normalized local Stokes profiles except for the rare situation of a very strong line in an extremely Fe‐rich atmosphere. For the disc‐integrated spectra of an Ap star with extreme abundance variations, we find that the assumption of a mean model atmosphere leads to moderate errors in Stokes I but is negligible for the circular and linear polarization spectra. Employing a new magnetic inversion code, which incorporates the horizontal variation of atmospheric structure induced by chemical spots, we reconstructed new maps of magnetic field and Fe abundance for the bright Ap star α2 CVn. The resulting distribution of chemical spots changes insignificantly compared to the previous modelling based on a single model atmosphere, while the magnetic field geometry does not change at all. This shows that the assertions by Stift et al. are exaggerated as a consequence of unreasonable assumptions and extrapolations, as well as methodological flaws and inconsistencies of their analysis. Our discussion proves that published magnetic inversions based on a mean stellar atmosphere are highly robust and reliable, and that the presence of small‐scale magnetic field structures on the surfaces of Ap stars is indeed real. Incorporating horizontal variations of atmospheric structure in Doppler imaging can marginally improve reconstruction of abundance distributions for stars showing very large iron overabundances. But this costly technique is unnecessary for magnetic mapping with high‐resolution polarization spectra.

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The Lorentz force in atmospheres of CP stars:θ Aurigae

Cited by 24 publications

References 25 publications

Model atmospheres of magnetic chemically peculiar stars

Model atmospheres of magnetic chemically peculiar stars

The helium weak silicon star HR 7224

Magnetic Doppler imaging considering atmospheric structure modifications due to local abundances: a luxury or a necessity?

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