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Three-dimensional magnetic and abundance mapping of the cool Ap star HD 24712: I. Spectropolarimetric observations in all four Stokes parameters
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
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2013 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 558, A8- p.Article in journal (Refereed) Published
Abstract [en]

Context. High-resolution spectropolarimetric observations provide simultaneous information about stellar magnetic field topologies and three-dimensional distributions of chemical elements. High-quality spectra in the Stokes IQUV parameters are currently available for very few early-type magnetic chemically peculiar stars. Here we present analysis of a unique full Stokes vector spectropolarimetric data set, acquired for the cool magnetic Ap star HD 24712 with a recently commissioned spectropolarimeter. Aims. The goal of our work is to examine the circular and linear polarization signatures inside spectral lines and to study variation of the stellar spectrum and magnetic observables as a function of rotational phase. Methods. HD 24712 was observed with the HARPSpol instrument at the 3.6-m ESO telescope over a period of 2010-2011. We achieved full rotational phase coverage with 43 individual Stokes parameter observations. The resulting spectra have a signal-to-noise ratio of 300600 and resolving power exceeding 10(5). The multiline technique of least-squares deconvolution (LSD) was applied to combine information from the spectral lines of Fe-peak and rare earth elements. Results. We used the HARPSPol spectra of HD 24712 to study the morphology of the Stokes profile shapes in individual spectral lines and in LSD Stokes profiles corresponding to different line masks. From the LSD Stokes V profiles we measured the longitudinal component of the magnetic field, < Bz >, with an accuracy of 510 G. We also determined the net linear polarization from the LSD Stokes Q and U profiles. Combining previous < Bz > measurements with our data allowed us to determine an improved rotational period of the star, P-rot = 12.45812 +/- 0.00019 d. We also measured the longitudinal magnetic field from the cores of H alpha and H beta lines. The analysis of < Bz > measurements showed no evidence for a significant radial magnetic field gradient in the atmosphere of HD 24712. We used our < Bz > and net linear polarization measurements to determine parameters of the dipolar magnetic field topology. We found that magnetic observables can be reasonably well reproduced by the dipolar model, although significant discrepancies remain at certain rotational phases. We discovered rotational modulation of the H alpha core and related it to a non-uniform surface distribution of rare earth elements.

Place, publisher, year, edition, pages
2013. Vol. 558, A8- p.
Keyword [en]
stars: chemically peculiar, stars: individual: HD 24712, stars: magnetic field, polarization
National Category
Natural Sciences
URN: urn:nbn:se:uu:diva-214053DOI: 10.1051/0004-6361/201220950ISI: 000326574000008OAI: oai:DiVA.org:uu-214053DiVA: diva2:683968
Available from: 2014-01-07 Created: 2014-01-07 Last updated: 2016-03-22Bibliographically approved
In thesis
1. Magnetic fields and chemical maps of Ap stars from four Stokes parameter observations
Open this publication in new window or tab >>Magnetic fields and chemical maps of Ap stars from four Stokes parameter observations
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Our knowledge of stellar magnetic fields relies almost entirely on circular polarization observations, which has historically limited our understanding of the stellar magnetic field topologies. Recently, it has become possible to obtain phase-resolved high-resolution spectropolarimetric observations in all four Stokes parameters for early-type magnetic stars. Interpretation of such observations with the Magnetic Doppler imaging technique has uncovered a new, previously unknown, level of complexity of surface stellar magnetic fields. This new insight is critical for understanding the origin, evolution and structure of magnetic fields in early-type stars.

In this study we observed the magnetic, chemically peculiar Ap stars HD 24712 (DO Eri, HR 1217) and HD 125248 (CS Vir, HR 5355) in all four Stokes parameters with the HARPSpol spectropolarimeter at the ESO 3.6-m telescope. The resulting spectra have high signal-to-noise ratio and superb resolving power, by far surpassing the quality of any existing stellar Stokes parameter observations.

We studied variation of the spectrum and magnetic observables of HD 24712 as a function of rotational phase (paper I). In the subsequent magnetic Doppler imaging investigation of this star, we interpreted the phase-resolved Stokes line profile observations (paper II). This analysis showed that HD 24712, unlike more massive Ap stars studied in all four Stokes parameters, has a dominant dipolar field component with a negligible contribution of small-scale magnetic structures. Simultaneously with magnetic mapping we derived surface abundance distributions of Fe, Nd, Na, and Ca.

Building upon the technique of Magnetic Doppler imaging, we developed the first three-dimensional abundance inversion code and applied it to reconstruct the abundance distributions of Fe and Ca in three dimensions in the atmosphere of HD 24712 (paper III).

We also performed Magnetic Doppler imaging analysis of the spectropolarimetric observations of HD 125248 (paper IV). The reconstructed detailed maps of the surface abundance distribution and magnetic field topology of HD 125248 revealed a magnetic field with significant deviations from the canonical dipolar field geometry, and strong surface abundance inhomogeneities for Cr and several rare earth elements.

We assessed our inversion results in the context of magnetic Doppler imaging studies of other magnetic, chemically peculiar Ap stars and latest theoretical research on the evolution and stability of magnetic fields in radiative stellar interiors. Our analysis suggests that old or less massive Ap stars have predominantly dipolar magnetic fields while more massive or younger stars exhibit more complicated field topologies. We also compared our three-dimensional chemical abundance maps of HD 24712 to the predictions of theoretical atomic diffusion calculations in magnetized stellar atmospheres, generally finding a lack of agreement between theory and observations.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2016. 39 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1349
chemically peculiar stars, magnetic fields, spectropolarimetry, Doppler imaging, stellar atmospheres
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy with specialization in Astrophysics; Astronomy
urn:nbn:se:uu:diva-278535 (URN)978-91-554-9492-6 (ISBN)
Public defence
2016-04-15, Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Available from: 2016-03-21 Created: 2016-02-24 Last updated: 2016-04-04

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