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High-resolution spectropolarimetric observations provide simultaneousinformation about stellar magnetic field topologies and three-dimensionaldistributions of chemical elements. Here we present analysis of a unique fullStokes vector spectropolarimetric data set, acquired for the cool magnetic Apstar HD 24712. The goal of our work is to examine circular and linearpolarization signatures inside spectral lines and to study variation of thestellar spectrum and magnetic observables as a function of rotational phase. HD24712 was observed with the HARPSpol instrument at the 3.6-m ESO telescope overa period of 2010-2011. The resulting spectra have S/N ratio of 300-600 andresolving power exceeding 1. The multiline technique of least-squaresdeconvolution (LSD) was applied to combine information from the spectral linesof Fe-peak and rare-earth elements. We used the HARPSPol spectra of HD 24712 tostudy the morphology of the Stokes profile shapes in individual spectral linesand in LSD Stokes profiles corresponding to different line masks. From the LSDStokes V profiles we measured the longitudinal component of the magnetic field,<Bz>, with an accuracy of 5-10 G. We also determined the net linearpolarization from the LSD Stokes Q and U profiles. We determined an improvedrotational period of the star, P_rot = 12.45812 +/- 0.00019d. We measured <Bz>from the cores of Halpha and Hbeta lines. The analysis of <Bz> measurementsshowed no evidence for a significant radial magnetic field gradient in theatmosphere of HD 24712. We used our <Bz> and net linear polarizationmeasurements to determine parameters of the dipolar magnetic field topology. Wefound that magnetic observables can be reasonably well reproduced by thedipolar model. We discovered rotational modulation of the Halpha core andrelated it a non-uniform surface distribution of rare-earth elements.

Three-dimensional magnetic and abundance mapping of the cool Ap star HD 24712 I. Spectropolarimetric observations in all four Stokes parameters