Magnetic field of the Ap star 33 Lib: Study in different spectral lines

1Butkovskaya, VV, 1Plachinda, SI
1Main Astronomical Observatory of the National Academy of Sciences of Ukraine, Kyiv, Ukraine
Kinemat. fiz. nebesnyh tel (Online) 2019, 35(2):65-80
Start Page: Physics of Stars and Interstellar Medium
Language: Russian

Ap stars are chemically peculiar main sequence stars whose atmospheres have an anomalous chemical composition compared to the Sun. The abnormal chemical composition of the peculiar stars is due to the diffusion of chemical elements in their atmospheres under the joint action of the radiation pressure and the gravitational settling. Depending on which of the processes prevails, the chemical element either «sinks» and accumulates in the deeper layers, or it floats and accumulates in the upper layers of the stellar atmosphere. In addition to the non-uniform distribution of chemical elements with depth, there are inhomogeneities of chemical elements on the surface of peculiar stars in the form of spots enriched or depleted in one or another chemical element. Ap stars have strong global magnetic fields with an intensity from a few hundred to tens of thousands of Gauss. These fields, as a rule, have a simple dipole configuration that is stable in time at least at time intervals of several decades. One of the unexplored effects observed in chemically peculiar stars is the fact that the magnetic field measured using different spectral lines can differ significantly from each other. In the paper, we present the results of measuring the longitudinal magnetic field of a chemically peculiar Ap star 33 Lib in different spectral lines (N = 180). High-resolution circularly polarized spectra collected over 4 nights in 2006 with the 3.6 m CFHT ESPaDOnS were taken from the open CADC database. The magnetic field was calculated using Single Line method by measuring Zeeman splitting of individual spectral lines. It was established that the averaged over all nights longitudinal magnetic field is e> = 274.9 ± 2.7 mT. It was found, that there are spectral lines, including spectral lines of the same chemical element, which demonstrate significantly different magnetic field. The weakest magnetic field is measured in the cores of the hydrogen lines Hα and Hβ, as well as in the Y and Pr lines. The study of the dependence of the magnitude of the magnetic field on the parameters of the spectral lines has showed that strongest magnetic field values are measured in the weak lines with small Lande factors. The reason for this difference in the measured magnetic field values can be both the inhomogeneous magnetic field in the star’s atmosphere, and the inhomogeneous distribution of chemical elements over the surface and/or with depth, or all of these factors simultaneously.

Keywords: magnetic field, peculiar stars, spectropolarimetry, stellar atmospheres

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