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Magnetic fields of cool stars from near-infrared spectropolarimetry
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.ORCID iD: 0000-0001-8477-5265
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Description
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 [en]
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: urn:nbn:se:uu:diva-406608ISBN: 978-91-513-0930-9 (print)OAI: oai:DiVA.org:uu-406608DiVA, id: diva2:1421018
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
List of papers
1. Magnetic fields of intermediate mass T Tauri stars
Open this publication in new window or tab >>Magnetic fields of intermediate mass T Tauri stars
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2017 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 608, article id A77Article in journal (Refereed) Published
Abstract [en]

Aims: In this paper, we aim to measure the strength of the surface magnetic fields for a sample of five intermediate mass T Tauri stars and one low mass T Tauri star from late-F to mid-K spectral types. While magnetic fields of T Tauri stars at the low mass range have been extensively characterized, our work complements previous studies towards the intermediate mass range; this complementary study is key to evaluate how magnetic fields evolve during the transition from a convective to a radiative core.

Methods: We studied the Zeeman broadening of magnetically sensitive spectral lines in the H-band spectra obtained with the CRIRES high-resolution near-infrared spectrometer. These data are modelled using magnetic spectral synthesis and model atmospheres. Additional constraints on non-magnetic line broadening mechanisms are obtained from modelling molecular lines in the K band or atomic lines in the optical wavelength region.

Results: We detect and measure mean surface magnetic fields for five of the six stars in our sample: CHXR 28, COUP 107, V2062 Oph, V1149 Sco, and Par 2441. Magnetic field strengths inferred from the most magnetically sensitive diagnostic line range from 0.8 to 1.8 kG. We also estimate a magnetic field strength of 1.9 kG for COUP 107 from an alternative diagnostic. The magnetic field on YLW 19 is the weakest in our sample and is marginally detected, with a strength of 0.8 kG.

Conclusions: We populate an uncharted area of the pre-main-sequence HR diagram with mean magnetic field measurements from high-resolution near-infrared spectra. Our sample of intermediate mass T Tauri stars in general exhibits weaker magnetic fields than their lower mass counterparts. Our measurements will be used in combination with other spectropolarimetric studies of intermediate mass and lower mass T Tauri stars to provide input into pre-main-sequence stellar evolutionary models.

Keywords
stars: pre-main sequence, stars: magnetic field, line: profiles
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-345062 (URN)10.1051/0004-6361/201731889 (DOI)000417620400004 ()
Funder
Knut and Alice Wallenberg FoundationSwedish Research CouncilSwedish National Space Board
Available from: 2018-03-08 Created: 2018-03-08 Last updated: 2020-04-01Bibliographically approved
2. A sudden change of the global magnetic field of the active M dwarf AD Leo revealed by full Stokes spectropolarimetric observations
Open this publication in new window or tab >>A sudden change of the global magnetic field of the active M dwarf AD Leo revealed by full Stokes spectropolarimetric observations
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.

National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-360239 (URN)10.1093/mnras/sty1825 (DOI)000441383700040 ()
Funder
Swedish National Space BoardSwedish Research CouncilKnut and Alice Wallenberg Foundation
Available from: 2018-09-11 Created: 2018-09-11 Last updated: 2020-04-01Bibliographically approved
3. Characterising the surface magnetic fields of T Tauri stars with high-resolution near-infrared spectroscopy
Open this publication in new window or tab >>Characterising the surface magnetic fields of T Tauri stars with high-resolution near-infrared spectroscopy
2019 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 630, article id A99Article in journal (Refereed) Published
Abstract [en]

Aims: In this paper, we aim to characterise the surface magnetic fields of a sample of eight T Tauri stars from high-resolution near-infrared spectroscopy. Some stars in our sample are known to be magnetic from previous spectroscopic or spectropolarimetric studies. Our goals are firstly to apply Zeeman broadening modelling to T Tauri stars with high-resolution data, secondly to expand the sample of stars with measured surface magnetic field strengths, thirdly to investigate possible rotational or long-term magnetic variability by comparing spectral time series of given targets, and fourthly to compare the magnetic field modulus < B > tracing small-scale magnetic fields to those of large-scale magnetic fields derived by Stokes V Zeeman Doppler Imaging (ZDI) studies.

Methods: We modelled the Zeeman broadening of magnetically sensitive spectral lines in the near-infrared K-band from high-resolution spectra by using magnetic spectrum synthesis based on realistic model atmospheres and by using different descriptions of the surface magnetic field. We developped a Bayesian framework that selects the complexity of the magnetic field prescription based on the information contained in the data.

Results: We obtain individual magnetic field measurements for each star in our sample using four different models. We find that the Bayesian Model 4 performs best in the range of magnetic fields measured on the sample (from 1.5 kG to 4.4 kG). We do not detect a strong rotational variation of < B > with a mean peak-to-peak variation of 0.3 kG. Our confidence intervals are of the same order of magnitude, which suggests that the Zeeman broadening is produced by a small-scale magnetic field homogeneously distributed over stellar surfaces. A comparison of our results with mean large-scale magnetic field measurements from Stokes V ZDI show different fractions of mean field strength being recovered, from 25-42% for relatively simple poloidal axisymmetric field topologies to 2-11% for more complex fields.

Place, publisher, year, edition, pages
EDP SCIENCES S A, 2019
Keywords
stars: pre-main sequence, stars: magnetic field, line: profiles
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-395836 (URN)10.1051/0004-6361/201935695 (DOI)000487978300006 ()
Funder
Knut and Alice Wallenberg FoundationSwedish Research Council, 621-2014-5720Swedish National Space Board, 185/14Swedish National Space Board, 137/17
Available from: 2019-10-25 Created: 2019-10-25 Last updated: 2020-04-01Bibliographically approved
4. The large-scale magnetic field of the eccentric pre-main-sequence binary system V1878 Ori
Open this publication in new window or tab >>The large-scale magnetic field of the eccentric pre-main-sequence binary system V1878 Ori
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(English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966Article in journal (Refereed) Submitted
Keywords
binaries: spectroscopic, stars: magnetic field, stars: individual: V1878 Ori, techniques: spectroscopic, techniques: polarimetric
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:uu:diva-407473 (URN)
Available from: 2020-04-01 Created: 2020-04-01 Last updated: 2020-04-05

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