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A sudden change of the global magnetic field of the active M dwarf AD Leo revealed by full Stokes spectropolarimetric observations
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.ORCID iD: 0000-0001-8477-5265
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
Department of Physics and Space Science, Royal Military College of Canada, PO Box 17000, Stn Forces, Kingston, K7K 7B4 Ontario, Canada.
2018 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 479, no 4, p. 4836-4843Article in journal (Refereed) Published
Abstract [en]

In this paper we present an analysis of the first high-resolution full Stokes vector spectropolarimetric observations of the active M dwarf AD Leo. Based on observations collected in 2016 with the ESPaDOnS instrument at CFHT, we derived the least-squares deconvolved Stokes profiles and detected linear polarization signatures in spectral lines. At the same time, we discovered that the circular polarisation profiles corresponding to our data set are significantly weaker compared to all archival spectra of AD Leo, which exhibited approximately constant profiles over the time-scale of at least 6 yr until 2012. Magnetic maps obtained using Zeeman Doppler imaging confirm the sudden change in the surface magnetic field. Although the total magnetic field energy decreased by about 20 per cent between 2012 and 2016, the field component responsible for the observed circular polarization signatures corresponds to a stronger field occupying a smaller fraction of the stellar surface in the more recent map. These results represent the first evidence that active M dwarfs with dipole-dominated axisymmetric field topologies can undergo a long-term global magnetic variation.

Place, publisher, year, edition, pages
2018. Vol. 479, no 4, p. 4836-4843
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:uu:diva-360239DOI: 10.1093/mnras/sty1825ISI: 000441383700040OAI: oai:DiVA.org:uu-360239DiVA, id: diva2:1247174
Funder
Swedish National Space BoardSwedish Research CouncilKnut and Alice Wallenberg FoundationAvailable from: 2018-09-11 Created: 2018-09-11 Last updated: 2020-04-01Bibliographically approved
In thesis
1. Magnetic fields of cool stars from near-infrared spectropolarimetry
Open this publication in new window or tab >>Magnetic fields of cool stars from near-infrared spectropolarimetry
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Magnetic fields rule many physical processes in and around stars throughout their lifetime. All cool stars possess a magnetic field, likely generated by dynamo processes. In order to properly understand the evolution of cool stars, we need to understand their magnetism. Stellar magnetic fields can be directly observed through the imprint of the Zeeman effect in intensity and polarized spectra. In intensity spectra (Stokes I), spectral lines are broadened or split into several components by the magnetic field. Modelling this effect in high-resolution spectra allows us to determine the average unsigned magnetic field strength over the stellar surface. The magnetic field also induces circular (Stokes V) and linear polarization (Stokes QU) in spectral lines, according to its orientation. These polarization signals can be used to map the large-scale magnetic field at the surface of the star using tomographic techniques such as Zeeman Doppler imaging (ZDI). 

In this thesis, we investigated pre-main-sequence T Tauri stars and the active M dwarf AD Leo with the goal to understand their magnetic fields. We modelled the Zeeman broadening in high-resolution near-infrared spectra of low-mass and intermediate-mass T Tauri stars and derived their mean magnetic field strengths. In intermediate-mass T Tauri stars, we only found fields weaker than 2-3 kG. However, we found that low-mass T Tauri stars can have a wide range of magnetic field strength from relatively weak fields of 1.5 kG to fields as strong as 4.4 kG, and that their field strengths do not correlate with stellar parameters. Our observations of the M dwarf AD Leo led to the first detection of linear polarization in the spectral lines of an M dwarf. We also discovered that its Stokes V profiles, which were constant over many years, had changed in our observations. We mapped its global magnetic field using ZDI and found that it became concentrated into smaller areas on the stellar surface. Finally, we analyzed Stokes IV observations of the spectroscopic binary V1878 Ori. Both components of this system are intermediate-mass T Tauri stars with very similar properties. We determined stellar parameters by studying orbital motion of the components and comparing their disentangled spectra to theoretical models. We then mapped the global magnetic fields of the two stars simultaneously using ZDI. We found that their magnetic fields have radically different geometries and different strengths.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2020. p. 54
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1927
Keywords
stars: magnetic field, stars: pre-main-sequence, stars: late-type, techniques: spectroscopic, techniques: polarimetric
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:uu:diva-406608 (URN)978-91-513-0930-9 (ISBN)
Public defence
2020-05-26, Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2020-05-05 Created: 2020-04-01 Last updated: 2020-05-05

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Lavail, AlexisKochukhov, Oleg

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