Three-dimensional magnetic and abundance mapping of the cool Ap star HD 24712 II. Two-dimensional Magnetic Doppler Imaging in all four Stokes parameters

摘要

Aims: We present a magnetic Doppler imaging study from all Stokes parametersof the cool, chemically peculiar star HD 24712. This is the very first suchanalysis performed at a resolving power exceeding 10^5. Methods: The analysis is performed on the basis of phase-resolvedobservations of line profiles in all four Stokes parameters obtained with theHARPSpol instrument attached at the 3.6-m ESO telescope. We use the magneticDoppler imaging code, INVERS10, which allows us to derive the magnetic fieldgeometry and surface chemical abundance distributions simultaneously. Results: We report magnetic maps of HD 24712 recovered from a selection ofFeI, FeII, NdIII, and NaI lines with strong polarization signals in all Stokesparameters. Our magnetic maps successfully reproduce most of the detailsavailable from our observation data. We used these magnetic field maps toproduce abundance distribution map of Ca. This new analysis shows that thesurface magnetic field of HD 24712 has a dominant dipolar component with a weakcontribution from higher-order harmonics. The surface abundance distributionsof Fe and Ca show enhancements near the magnetic equator with an underabundantpatch at the visible (positive) magnetic pole; Nd is highly abundant around thepositive magnetic pole. The Na abundance map shows a high overabundance aroundthe negative magnetic pole. Conclusions: Based on our investigation and similar recent four Stokesparameter magnetic mapping studies, we present a tentative evidence for thehypothesis that Ap stars with dipole-like fields are older than stars withmagnetic fields that have more small-scale structures. We find that ourabundance maps are inconsistent with recent theoretical calculations of atomicdiffusion in presence of magnetic fields.