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Magnetism, chemical spots, and stratification in the HgMn star ϕ Phoenicis
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.
Sterrekundig Instituut, Universiteit Utrecht.
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2012 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 539, A142- p.Article in journal (Refereed) Published
Abstract [en]

Context. Mercury-manganese (HgMn) stars have been considered as non-magnetic and non-variable chemically peculiar (CP) starsfor a long time. However, recent discoveries of the variability in spectral line profiles have suggested an inhomogeneous surfacedistribution of chemical elements in some HgMn stars. From the studies of other CP stars it is known that magnetic field plays a keyrole in the formation of surface spots. All attempts to find magnetic fields in HgMn stars have yielded negative results.Aims. In this study, we investigate the possible presence of a magnetic field in ϕ Phe (HD 11753) and reconstruct surface distributionof chemical elements that show variability in spectral lines.We also test a hypothesis that a magnetic field is concentrated in chemicalspots and look into the possibility that some chemical elements are stratified with depth in the stellar atmosphere.Methods. Our analysis is based on high-quality spectropolarimetric time-series observations, covering a full rotational period ofthe star. Spectra were obtained with the HARPSpol at the ESO 3.6-m telescope. To increase the sensitivity of the magnetic fieldsearch, we employed the least-squares deconvolution (LSD) technique. Using Doppler imaging code INVERS10, we reconstructedsurface chemical distributions by utilising information from multiple spectral lines. The vertical stratification of chemical elementswas calculated with the DDAFit program.Results. Combining information from all suitable spectral lines, we set an upper limit of 4 G on the mean longitudinal magnetic field.For chemical spots, an upper limit on the longitudinal field varies between 8 and 15 G. We confirmed the variability of Y, Sr, and Tiand detected variability in Cr lines. Stratification analysis showed that Y and Ti are not concentrated in the uppermost atmosphericlayers.Conclusions. Our spectropolarimetric observations rule out the presence of a strong, globally-organised magnetic field in ϕ Phe.This implies an alternative mechanism of spot formation, which could be related to a non-equilibrium atomic diffusion. However, thetypical time scales of the variation in stratification predicted by the recent time-dependent diffusion models exceed significantly thespot evolution time-scale reported for ϕ Phe.

Place, publisher, year, edition, pages
2012. Vol. 539, A142- p.
Keyword [en]
stars: chemically peculiar, stars: individual: Phi Phe, stars: variables: general
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy with specialization in Astrophysics
Identifiers
URN: urn:nbn:se:uu:diva-160291DOI: 10.1051/0004-6361/201118167ISI: 000303262000149OAI: oai:DiVA.org:uu-160291DiVA: diva2:450340
Available from: 2011-10-20 Created: 2011-10-20 Last updated: 2017-12-08Bibliographically approved
In thesis
1. Magnetic Fields and Chemical Spots in HgMn Stars
Open this publication in new window or tab >>Magnetic Fields and Chemical Spots in HgMn Stars
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Mercury-manganese (HgMn) stars belong to the class of chemically peculiar (CP) stars. It was recently discovered that some HgMn stars have spots of chemical elements on their surfaces. According to conventional picture of CP stars, magnetic field facilitates the formation and long term stability of chemical spots by controlling stratification of elements in stellar atmosphere. However, previous attempts to find magnetic field in HgMn stars set an upper limit on its strength at the level of about 20-100 Gauss. Observational evidence suggested that even weaker magnetic fields can be responsible for the formation of chemical spots. The main goal of our work was to verify this possibility.

The search for weak magnetic fields requires the use of least-squares deconvolution (LSD) technique.  This method combines information from many spectral lines providing a mean line profile with increased signal-to-noise ratio. Up to now there was no extensive comparison of the LSD profile with real spectral lines. We showed that the LSD profile of the intensity spectrum does not behave like a real spectral line as a function of chemical composition. However, for circular polarization, LSD profile resembles the profile of a spectral line with mean atomic parameters.

We performed a comprehensive search for magnetic field in 47 HgMn stars and their companions, based on high-quality spectropolarimetric data obtained with the HARPSpol polarimeter at the ESO 3.6-m telescope. With the help of LSD technique, an upper limit on the mean longitudinal magnetic field was brought down to 2-10 G for most stars. We concluded that magnetic field is not responsible for the spot formation in HgMn stars.

We obtained full rotational phase coverage for the HgMn stars φ Phe and 66 Eri. This enabled us to investigate line profile variability, reconstruct surface maps of chemical elements, and perform a search for magnetic field with very high sensitivity. For φ Phe we derived surface maps of Y, Sr, Ti, Cr, and obtained an upper limit of 4 G on the field strength. We also found marginal indication of vertical stratification of Y and Ti. No magnetic field was detected in both components of 66 Eri, with an upper limit of 10-24 G. We discovered chemical spots of Y, Sr, Ba, and Ti, in the primary star. We demonstrated a relation between the binary orbit and the morphology of these spots.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2011. 46 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 873
Keyword
Chemically peculiar stars; magnetic fields; spectropolarimetry; Doppler imaging
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy with specialization in Astrophysics
Identifiers
urn:nbn:se:uu:diva-160308 (URN)978-91-554-8199-5 (ISBN)
Public defence
2011-12-08, Häggsalen, Ångström Laboratory, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
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Available from: 2011-11-09 Created: 2011-10-20 Last updated: 2012-10-16

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Makaganiuk, VitaliiKochukhov, OlegPiskunov, NikolaiStempels, Henricus C.

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