uu.seUppsala University Publications
Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Magnetic fields and chemical maps of Ap stars from four Stokes parameter observations
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.ORCID iD: 0000-0001-6231-4628
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.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1349
Keyword [en]
chemically peculiar stars, magnetic fields, spectropolarimetry, Doppler imaging, stellar atmospheres
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy with specialization in Astrophysics; Astronomy
Identifiers
URN: urn:nbn:se:uu:diva-278535ISBN: 978-91-554-9492-6 (print)OAI: oai:DiVA.org:uu-278535DiVA: diva2:907455
Public defence
2016-04-15, Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2016-03-21 Created: 2016-02-24 Last updated: 2016-04-04
List of papers
1. Three-dimensional magnetic and abundance mapping of the cool Ap star HD 24712: I. Spectropolarimetric observations in all four Stokes parameters
Open this publication in new window or tab >>Three-dimensional magnetic and abundance mapping of the cool Ap star HD 24712: I. Spectropolarimetric observations in all four Stokes parameters
Show others...
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.

Keyword
stars: chemically peculiar, stars: individual: HD 24712, stars: magnetic field, polarization
National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-214053 (URN)10.1051/0004-6361/201220950 (DOI)000326574000008 ()
Available from: 2014-01-07 Created: 2014-01-07 Last updated: 2017-09-07Bibliographically approved
2. Three-dimensional magnetic and abundance mapping of the cool Ap star HD 24712 II. Two-dimensional magnetic Doppler imaging in all four Stokes parameters
Open this publication in new window or tab >>Three-dimensional magnetic and abundance mapping of the cool Ap star HD 24712 II. Two-dimensional magnetic Doppler imaging in all four Stokes parameters
2015 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 573, A123Article in journal (Refereed) Published
Abstract [en]

Aims. We present a magnetic Doppler imaging study from all Stokes parameters of the cool, chemically peculiar star HD24712. This is the very first such analysis performed at a resolving power exceeding 105. Methods. The analysis is performed on the basis of phase-resolved observations of line profiles in all four Stokes parameters obtained with the HARPSpol instrument attached at the 3.6 m ESO telescope. We used the magnetic Doppler imaging code INVERS 10, which allowed us to derive the magnetic field geometry and surface chemical abundance distributions simultaneously. Results. We report magnetic maps of HD24712 recovered from a selection of Fe I, Fe II, Nd III, and Na i lines with strong polarization signals in all Stokes parameters. Our magnetic maps successfully reproduce most of the details available from our observation data. We used these magnetic field maps to produce abundance distribution map of Ca. This new analysis shows that the surface magnetic field of HD24712 has a dominant dipolar component with a weak contribution from higher-order harmonics. The surface abundance distributions of Fe and Ca show enhancements near the magnetic equator with an underabundant patch at the visible (positive) magnetic pole; Nd is highly abundant around the positive magnetic pole. The Na abundance map shows a high overabundance around the negative magnetic pole. Conclusions. Based on our investigation and similar recent magnetic mapping studies that used four Stokes parameters, we present tentative evidence for the hypothesis that Ap stars with dipole-like fields are older than stars with magnetic fields that have more small-scale structures. We find that our abundance maps are inconsistent with recent theoretical calculations of atomic diffusion in presence of magnetic fields.

Keyword
stars: chemically peculiar, stars: atmospheres, stars: abundances, stars: individual: HD24712, stars: magnetic field
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-251539 (URN)10.1051/0004-6361/201424559 (DOI)000350806000023 ()
Available from: 2015-04-20 Created: 2015-04-20 Last updated: 2017-09-07Bibliographically approved
3. Three-dimensional magnetic and abundance mapping of the cool Ap star HD 24712: III.  Three-dimensional abundance distribution of iron and calcium
Open this publication in new window or tab >>Three-dimensional magnetic and abundance mapping of the cool Ap star HD 24712: III.  Three-dimensional abundance distribution of iron and calcium
2016 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746Article in journal (Other academic) Submitted
Abstract [en]

Context. The presence of abundance inhomogeneities in the atmospheres of chemically peculiar Ap stars presents us with an outstand- ing opportunity to investigate three-dimensional abundance distributions of chemical elements with Doppler imaging techniques, and confront these observations with the theory of atomic diffusion in the presence of a magnetic field. Aims. In this paper we present the first three-dimensional abundance distribution of Fe and Ca in the atmosphere of the cool, magnetic, chemically peculiar star HD 24712.

Methods. We used a modified version of the magnetic Doppler imaging code invers10 to derive the abundance distribution of Fe and Ca in three directions in a self-consistent way without any simplifying assumptions about the magnetic field structure of HD 24712. The analysis was performed on the basis of phase-resolved observations obtained with the HARPSpol instrument at the 3.6 m ESO telescope.

Results. The resulting three-dimensional abundance distributions of Fe and Ca successfully reproduce the observed line profiles of all spectral lines used in the inversion. We reveal a strong correlation between the horizontal and vertical abundance distribution of Fe derived from two and three-dimensional magnetic Doppler imaging, higher effective local abundance corresponds to the stratification profiles in which the transition region is shifted higher in the atmosphere. However, we could not confirm the presence of a similar correlation for Ca. 

Keyword
chemically peculiar stars, stellar atmospheres, abundances, HD 24712, magnetic field
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:uu:diva-278534 (URN)
Available from: 2016-02-24 Created: 2016-02-24 Last updated: 2017-09-08Bibliographically approved
4. Magnetic Doppler imaging of the chemically peculiar star HD 125248
Open this publication in new window or tab >>Magnetic Doppler imaging of the chemically peculiar star HD 125248
2016 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 588, A138Article in journal (Refereed) Published
Abstract [en]

Context. Intermediate-mass, chemically peculiar stars with strong magnetic fields provide an excellent opportunity to study the topology of their surface magnetic fields and the interplay between magnetic geometries and abundance inhomogeneities in the atmospheres of these stars. Aims. We reconstruct detailed maps of the surface magnetic field and abundance distributions for the magnetic Ap star HD125248. Methods. We performed the analysis based on phase-resolved, four Stokes parameter spectropolarimetric observations obtained with the HARPSpol instrument. These data were interpreted with the help of magnetic Doppler imaging techniques and model atmospheres taking the effects of strong magnetic fields and nonsolar chemical composition into account. Results. We improved the atmospheric parameters of the star, T-eff = 9850 +/- 250K and log g = 4.05 +/- 0.10. We performed detailed abundance analysis, which confirmed that HD125248 has abundances typical of other Ap stars, and discovered significant vertical stratification effects for the FeII and CrII ions. We computed LSD Stokes profiles using several line masks corresponding to Fe-peak and rare earth elements, and studied their behavior with rotational phase. Combining previous longitudinal field measurements with our own observations, we improved the rotational period of the star Prot = 9.29558 +/- 0.00006 d. Magnetic Doppler imaging of HD125248 showed that its magnetic field is mostly poloidal and quasi-dipolar with two large spots of different polarity and field strength. The chemical maps of Fe, Cr, Ce, Nd, Gd, and Ti show abundance contrasts of 0.9-3.5 dex. Among these elements, the Fe abundance map does not show high-contrast features. Cr is overabundant around the negative magnetic pole and has 3.5 dex abundance range. The rare earth elements and Ti are overabundant near the positive magnetic pole. Conclusions. The magnetic field of HD125248 has strong deviations from the classical oblique dipole field geometry. A comparison of the magnetic field topology of HD125248 with the results derived for other stars using four Stokes magnetic Doppler imaging suggests evidence that the field topology becomes simpler with increasing age. The abundance maps show weak correlation with magnetic field geometry, but they do not agree with the theoretical atomic diffusion calculations, which predict element accumulation in the horizontal field regions.

Place, publisher, year, edition, pages
Paris: , 2016
Keyword
chemically peculiar stars, abundances, atmospheres, magnetic field, HD 125248
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:uu:diva-277374 (URN)10.1051/0004-6361/201527719 (DOI)000373207800150 ()
Funder
Göran Gustafsson Foundation for Research in Natural Sciences and MedicineKnut and Alice Wallenberg FoundationSwedish Research CouncilSwedish National Infrastructure for Computing (SNIC)
Available from: 2016-02-19 Created: 2016-02-19 Last updated: 2017-11-30Bibliographically approved

Open Access in DiVA

fulltext(1274 kB)120 downloads
File information
File name FULLTEXT01.pdfFile size 1274 kBChecksum SHA-512
e53ee85701e84dfcfb93ccb253c4f1d2d560a9bc70a8ea17f3bd9a6b3b05a825c0fc73963662e1418c3995a3780febf49ae37d28f2292b1c4898b65a00d72238
Type fulltextMimetype application/pdf
Buy this publication >>

Authority records BETA

Rusomarov, Naum

Search in DiVA

By author/editor
Rusomarov, Naum
By organisation
Department of Physics and Astronomy
Astronomy, Astrophysics and Cosmology

Search outside of DiVA

GoogleGoogle Scholar
Total: 120 downloads
The number of downloads is the sum of all downloads of full texts. It may include eg previous versions that are now no longer available

isbn
urn-nbn

Altmetric score

isbn
urn-nbn
Total: 955 hits
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf