Magnetic Field of the Ap Star 33 Lib: A Study in Different Spectral Lines

Abstract

Ap stars are chemically peculiar, main-sequence stars with atmospheres having an anomalous chemical composition (compared to the solar one). This abnormal chemical composition is thought to result from diffusion of chemical elements under the influence of radiation pressure and gravitational settling. Depending on the prevailing process, a chemical element either sinks and is accumulated in deeper layers or floats up to the upper layers of the stellar atmosphere. In addition to nonuniform depth distributions of elements, peculiar stars feature spots on their surface enriched or depleted with certain chemical elements. Ap stars have strong global magnetic fields with intensities ranging from several hundred millitesla to several tesla. These fields, as a rule, have a simple dipole configuration that remains stable over time intervals of several decades at the least. One of the unexplored effects observed in chemically peculiar stars is the significant difference in magnetic fields measured using different spectral lines. The present study is focused on measurements of the longitudinal magnetic field of the chemically peculiar Ap star 33 Lib in N = 180 spectral lines. High-resolution circularly polarized spectra collected over four nights in 2006 with the 3.6 m CFHT ESPaDOnS spectrograph were taken from the CADC open database. The magnetic field was calculated based on the Zeeman effect using the single-line method. It was found that the longitudinal magnetic field of 33 Lib averaged over all nights is 〈Be〉 = 274.9 ± 2.7 mT. Statistically significant differences between the magnetic-field magnitudes determined for different spectral lines (including those of the same chemical element) were also found. The weakest magnetic field was measured in the cores of hydrogen lines Hα and Hβ and in Y and Pr lines. The dependence of the magnetic-field magnitude on parameters of spectral lines revealed that weak lines with small Lande factors had the strongest magnetic field. This may be attributed to the nonuniformity of the magnetic field in the stellar atmosphere and the distribution of chemical elements over the surface (and/or with depth) or to both these factors.