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Willamo, T., Hackman, T., Lehtinen, J. J., Kapyla, M. J., Ilyin, I., Henry, G. W., . . . Piskunov, N. (2019). Long-term spot monitoring of the young solar analogue V889 Herculis. Astronomy and Astrophysics, 622, Article ID A170.
Open this publication in new window or tab >>Long-term spot monitoring of the young solar analogue V889 Herculis
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2019 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 622, article id A170Article in journal (Refereed) Published
Abstract [en]

Context: Starspots are important manifestations of stellar magnetic activity. By studying their behaviour in young solar analogues, we can unravel the properties of their magnetic cycles. This gives crucial information of the underlying dynamo process. Comparisons with the solar cycle enable us to infer knowledge about how the solar dynamo has evolved during the Sun's lifetime.

Aims: Here we study the correlation between photometric brightness variations, spottedness, and mean temperature in V889 Her, a young solar analogue. Our data covers 18 years of spectroscopic and 25 years of photometric observations.

Methods: We use Doppler imaging to derive temperature maps from high-resolution spectra. We use the Continuous Period Search method to retrieve mean V-magnitudes from photometric data.

Results: Our Doppler imaging maps show a persistent polar spot structure varying in strength. This structure is centred slightly off the rotational pole. The mean temperature derived from the maps shows an overall decreasing trend, as does the photometric mean brightness, until it reaches its minimum around 2017. The filling factor of cool spots, however, shows only a weak tendency to anti-correlate with the decreasing mean brightness.

Conclusions: We interpret V889 Her to have entered into a grand maximum in its activity. The clear relation between the mean temperature of the Doppler imaging surface maps and the mean magnitude supports the reliability of the Doppler images. The lack of correlation between the mean magnitude and the spottedness may indicate that bright features in the Doppler images are real.

Place, publisher, year, edition, pages
EDP SCIENCES S A, 2019
Keywords
stars: activity, starspots, stars: solar-type, stars: individual: HD 171488
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-378369 (URN)10.1051/0004-6361/201834562 (DOI)000458749900004 ()
Funder
Knut and Alice Wallenberg FoundationSwedish Research CouncilSwedish National Space Board
Available from: 2019-03-07 Created: 2019-03-07 Last updated: 2019-03-07Bibliographically approved
Kochukhov, O., Shultz, M. & Neiner, C. (2019). Magnetic field topologies of the bright, weak-field Ap stars θ Aurigae and ε Ursae Majoris. Astronomy and Astrophysics, 621, 1-18, Article ID A47.
Open this publication in new window or tab >>Magnetic field topologies of the bright, weak-field Ap stars θ Aurigae and ε Ursae Majoris
2019 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 621, p. 1-18, article id A47Article in journal (Refereed) Published
Abstract [en]

Context. The brightest magnetic chemically peculiar stars theta Aur and epsilon UMa were targeted by numerous studies of their photometric and spectroscopic variability. Detailed maps of chemical abundance spots were repeatedly derived for both stars. However, owing to the weakness of their surface magnetic fields, very little information on the magnetic field geometries of these stars is available.

Aims. In this study we aim to determine detailed magnetic field topologies of theta Aur and epsilon UMa based on modern, high-resolution spectropolarimetric observations.

Methods. Both targets were observed in all four Stokes parameters using the Narval and ESPaDOnS spectropolarimeters. A multi-line technique of least-squares deconvolution was employed to detect polarisation signatures in spectral lines. These signatures were modelled with a Zeeman-Doppler imaging code.

Results. We succeed in detecting variable circular and linear polarisation signatures for theta Aur. Only circular polarisation was detected for epsilon UMa. We obtain new sets of high-precision longitudinal magnetic field measurements using mean circular polarisation metal line profiles as well as hydrogen line cores, which are consistent with historical data. Magnetic inversions revealed distorted dipolar geometries in both stars. The Fe and Cr abundance distributions, reconstructed simultaneously with magnetic mapping, do not show a clear correlation with the local magnetic field properties, with the exception of a relative element underabundance in the horizontal field regions along the magnetic equators.

Conclusions. Our study provides the first ever detailed surface magnetic field maps for broad-line, weak-field chemically peculiar stars, showing that their field topologies are qualitatively similar to those found in stronger field stars. The Fe and Cr chemical abundance maps reconstructed for theta Aur and epsilon UMa are at odds with the predictions of current theoretical atomic diffusion calculations.

Keywords
stars: atmospheres, stars: chemically peculiar, stars: magnetic field, starspots, stars: individual: theta Aur, stars: individual: epsilon UMa
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-374427 (URN)10.1051/0004-6361/201834279 (DOI)000454877000001 ()
Funder
Swedish Research Council, 621-2014-5720Swedish National Space Board, 185/14Swedish National Space Board, 137/17
Available from: 2019-01-28 Created: 2019-01-28 Last updated: 2019-01-28Bibliographically approved
Villebrun, F., Alecian, E., Hussain, G., Bouvier, J., Folsom, C. P., Lebreton, Y., . . . Petit, P. (2019). Magnetic fields of intermediate-mass T Tauri stars: I. Magnetic detections and fundamental stellar parameters. Astronomy and Astrophysics, 622, Article ID A72.
Open this publication in new window or tab >>Magnetic fields of intermediate-mass T Tauri stars: I. Magnetic detections and fundamental stellar parameters
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2019 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 622, article id A72Article in journal (Refereed) Published
Abstract [en]

Context. The origin of the fossil magnetic fields detected in 5 to 10% of intermediate-mass main sequence stars is still highly debated.

Aims. We want to bring observational constraints to a large population of intermediate-mass pre-main sequence (PMS) stars in order to test the theory that convective-dynamo fields generated during the PMS phases of stellar evolution can occasionally relax into fossil fields on the main sequence.

Methods. Using distance estimations, photometric measurements, and spectropolarimetric data from HARPSpol and ESPaDOnS of 38 intermediate-mass PMS stars, we determined fundamental stellar parameters (T-eff, L and v sin i) and measured surface magnetic field characteristics (including detection limits for non-detections, and longitudinal fields and basic topologies for positive detections). Using PMS evolutionary models, we determined the mass, radius, and internal structure of these stars. We compared different PMS models to check that our determinations were not model-dependant. We then compared the magnetic characteristics of our sample accounting for their stellar parameters and internal structures.

Results. We detect magnetic fields in about half of our sample. About 90% of the magnetic stars have outer convective envelopes larger than similar to 25% of the stellar radii, and heavier than similar to 2% of the stellar mass. Going to higher mass, we find that the magnetic incidence in intermediate-mass stars drops very quickly, within a timescale on the order of few times 0.1 Myr. Finally, we propose that intermediate-mass T Tauri stars with large convective envelopes, close to the fully convective limit, have complex fields and that their dipole component strengths may decrease as the sizes of their convective envelopes decrease, similar to lower-mass T Tauri stars.

Keywords
stars: activity, stars: fundamental parameters, stars: magnetic field, stars: pre-main sequence, stars: variables: T Tauri, Herbig Ae/Be
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-377341 (URN)10.1051/0004-6361/201833545 (DOI)000457147300001 ()
Available from: 2019-02-22 Created: 2019-02-22 Last updated: 2019-02-22Bibliographically approved
Lavail, A., Kochukhov, O. & Wade, G. A. (2018). A sudden change of the global magnetic field of the active M dwarf AD Leo revealed by full Stokes spectropolarimetric observations. Monthly notices of the Royal Astronomical Society, 479(4), 4836-4843
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: 2018-10-12Bibliographically approved
Quitral-Manosalva, P., Cunha, M. S. & Kochukhov, O. (2018). A theoretical tool for the study of radial velocities in the atmospheres of roAp stars. Monthly notices of the Royal Astronomical Society, 480(2), 1676-1688
Open this publication in new window or tab >>A theoretical tool for the study of radial velocities in the atmospheres of roAp stars
2018 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 480, no 2, p. 1676-1688Article in journal (Refereed) Published
Abstract [en]

Over the last decade, significant amounts of high-spectral and time-resolution spectroscopic data have been acquired for a number of rapidly oscillating Ap stars (roAp). Progress in the understanding of the information held by these data requires the development of theoretical models that can be directly compared with them. In this work, we present a theoretical model for the radial velocities of roAp stars that takes full account of the coupling between the pulsations and the magnetic field. We explore the impact on the radial velocities of changing the position of the observer, the mode frequency, and angular degree, as well as of changing the region of the disc where the elements are concentrated. We find that for integrations over the full disc, in the outermost layers the radial velocity is generally dominated by the acoustic waves, showing a rapid increase in amplitude. The most significant depth-variations in the radial velocity phase are seen for observers directed towards the equator and for even degree modes with frequencies close to, or above the acoustic cutoff. Comparison between the radial velocities obtained for spots of elements located around the magnetic poles and around the magnetic equator, shows that these present distinct amplitude-phase relations, resembling some of the differences seen in the observations. Finally, we discuss the conditions under which one may expect to find false nodes in the pulsation radial velocity of roAp stars.

Place, publisher, year, edition, pages
Oxford University Press (OUP), 2018
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-368683 (URN)10.1093/mnras/sty1935 (DOI)000449614800020 ()
Funder
Knut and Alice Wallenberg FoundationSwedish Research CouncilSwedish National Space Board
Available from: 2018-12-06 Created: 2018-12-06 Last updated: 2019-01-22Bibliographically approved
Shultz, M., Rivinius, T., Wade, G. A., Alecian, E. & Kochukhov, O. (2018). HD 156324: A Tidally Locked Magnetic SB3 With an Orbitally Disrupted Centrifugal Magnetosphere. CONTRIBUTIONS OF THE ASTRONOMICAL OBSERVATORY SKALNATE PLESO, 48(1), 298-299
Open this publication in new window or tab >>HD 156324: A Tidally Locked Magnetic SB3 With an Orbitally Disrupted Centrifugal Magnetosphere
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2018 (English)In: CONTRIBUTIONS OF THE ASTRONOMICAL OBSERVATORY SKALNATE PLESO, ISSN 1335-1842, Vol. 48, no 1, p. 298-299Article in journal (Refereed) Published
Abstract [en]

Period analysis of radial velocity, equivalent width, and magnetic measurements of the SB3 system HD 156324 yield identical results in all cases, indicating the system is tidally locked with orbital and rotational periods of 1.58 d. Its H alpha emission profile exhibits marked morphological departures from the usual pattern observed amongst magnetic B-type stars, which can plausibly be ascribed to tidal disruption of the gravitocentrifugal potential.

Place, publisher, year, edition, pages
SLOVAK ACADEMY SCIENCES ASTRONOMICAL INST, 2018
Keywords
stars: individual: HD 156324, stars: magnetic field, binaries: spectroscopic, binaries (including multiple): close, stars: massive
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-357777 (URN)000432791300051 ()
Available from: 2018-08-22 Created: 2018-08-22 Last updated: 2018-08-22Bibliographically approved
Kochukhov, O., Johnston, C., Alecian, E. & Wade, G. A. (2018). HD66051: the first eclipsing binary hosting an early-type magnetic star. Monthly notices of the Royal Astronomical Society, 478(2), 1749-1762
Open this publication in new window or tab >>HD66051: the first eclipsing binary hosting an early-type magnetic star
2018 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 478, no 2, p. 1749-1762Article in journal (Refereed) Published
Abstract [en]

Early-type magnetic stars are rarely found in close binary systems. No such objects were known in eclipsing binaries prior to this study. Here we investigated the eclipsing, spectro-scopic double-lined binary HD66051, which exhibits out-of-eclipse photometric variations suggestive of surface brightness inhomogeneities typical of early-type magnetic stars. Using a new set of high-resolution spectropolarimetric observations, we discovered a weak magnetic field on the primary and found intrinsic, element-dependent variability in its spectral lines. The magnetic field structure of the primary is dominated by a nearly axisymmetric dipolar component with a polar field strength B-d approximate to 600G and an inclination with respect to the rotation axis of beta(d) = 13 degrees. A weaker quadrupolar component is also likely to be present. We combined the radial velocity measurements derived from our spectra with archival optical photometry to determine fundamental masses (3.16 and 1.75 M-circle dot) and radii (2.78 and 1.39 R-circle dot) with a 1-3 per cent precision. We also obtained a refined estimate of the effective temperatures (13000 and 9000 K) and studied chemical abundances for both components with the help of disentangled spectra. We demonstrate that the primary component of HD66051 is a typical late-B magnetic chemically peculiar star with a non-uniform surface chemical abundance distribution. It is not an HgMn-type star as suggested by recent studies. The secondary is a metallic-line star showing neither a strong, global magnetic field nor intrinsic spectral variability. Fundamental parameters provided by our work for this interesting system open unique possibilities for probing interior structure, studying atomic diffusion, and constraining binary star evolution.

Place, publisher, year, edition, pages
Oxford University Press, 2018
Keywords
binaries: eclipsing, stars: chemically peculiar, stars: fundamental parameters, stars: individual: HD66051 (V414 Pup), stars: magnetic field
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-362046 (URN)10.1093/mnras/sty1118 (DOI)000439547400022 ()
Funder
Knut and Alice Wallenberg FoundationSwedish Research CouncilEU, European Research Council, 670519: MAMSIE
Available from: 2018-10-11 Created: 2018-10-11 Last updated: 2018-12-11
Rusomarov, N., Kochukhov, O. & Lundin, A. (2018). Magnetic field topology and chemical spot distributions of the Ap star HD 119419. Astronomy and Astrophysics, 609, Article ID A88.
Open this publication in new window or tab >>Magnetic field topology and chemical spot distributions of the Ap star HD 119419
2018 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 609, article id A88Article in journal (Refereed) Published
Abstract [en]

Context. Analysis of high-resolution spectropolarimetric time-series observations of early-type magnetic stars is currently the most advanced method of obtaining detailed information on their surface magnetic field topologies and horizontal spot distributions.

Aims. In this study we analyse a new set of high-quality full Stokes vector observations of the magnetic Ap star HD 119419 – a member of the 14 Myr old Lower Cen-Cru association – for the purpose of studying the surface field topology and mapping the chemical abundance spots.

Methods. We made use of the circular and linear polarisation data collected for HD 119419 with the HARPSpol instrument at the ESO 3.6-m telescope. These observations were analysed with a multi-line magnetic diagnostic technique and modelled in detail with a Magnetic Doppler imaging (MDI) code.

Results. We present a new set of high-precision mean longitudinal magnetic field measurements and derive a revised stellar rotational period by comparing our measurements with the literature data. We also redetermine the basic stellar atmospheric parameters. Our four Stokes parameter magnetic inversions reveal a moderately complex surface field topology with a mean field strength of 18 kG and a maximum local strength of 24 kG. A poloidal dipolar component dominates the magnetic energy spectrum of the surface field in HD 119419. However, significant contributions of the higher-order spherical harmonic components are also present. We show that the dipole plus quadrupole part of the reconstructed field geometry is incapable of reproducing the observed amplitudes and shapes of the Stokes Q and U profiles. The chemical abundance distributions of Fe, Cr, Ti, and Nd, derived self-consistently with the magnetic field geometry, are characterised by large abundance gradients and a lack of clear correlation with the magnetic field structure.

Conclusions. This full Stokes vector analysis of HD 119419 extends the modern hot-star magnetic mapping investigations to an open cluster Ap star with a well-determined age. Further, MDI studies of cluster members will allow us to study the field topologies and chemical abundance spots as a function of stellar age.

Keywords
stars: chemically peculiar, stars: atmospheres, stars: individual: HD 119419, stars: magnetic field
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-343856 (URN)10.1051/0004-6361/201731914 (DOI)000423434200006 ()
Funder
Knut and Alice Wallenberg FoundationSwedish Research CouncilSwedish National Space Board
Available from: 2018-03-02 Created: 2018-03-02 Last updated: 2018-03-02Bibliographically approved
Rosén, L., Kochukhov, O., Alecian, E., Neiner, C., Morin, J. & Wade, G. A. (2018). Magnetic field topology of the cool, active, short-period binary system sigma(2) Coronae Borealis. Astronomy and Astrophysics, 613, Article ID A60.
Open this publication in new window or tab >>Magnetic field topology of the cool, active, short-period binary system sigma(2) Coronae Borealis
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2018 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 613, article id A60Article in journal (Refereed) Published
Abstract [en]

Aims. The goal of this work is to study the cool, active binary star sigma(2) CrB, focussing on its magnetic field. The two F9-G0 components of this system are tidally locked and in a close orbit, increasing the chance of interaction between their magnetospheres. Methods. We used Stokes IV data from the twin spectropolarimeters Narval at the TBL and ESPaDOnS at the CFHT. The leastsquares deconvolution multi-line technique was used to increase the signal-to-noise ratio of the data. We then applied a new binary Zeeman-Doppler imaging code to reconstruct simultaneously the magnetic topology and brightness distribution of both components of sigma(2) CrB. This analysis was carried out for two observational epochs in 2014 and 2017. Results. A previously unconfirmed magnetic field of the primary star has been securely detected. At the same time, the polarisation signatures of the secondary appear to have a systematically larger amplitude than that of the primary. This corresponds to a stronger magnetic field, for which the magnetic energy of the secondary exceeds that of the primary by a factor of 3.3-5.7. While the magnetic energy is similar for the secondary star in the two epochs, the magnetic energy is about twice as high in 2017 for the primary. The magnetic field topology of the two stars in the earlier epoch (2014) is very different. The fractions of energy in the dipole and quadrupole components of the secondary are similar and thereafter decrease with increasing harmonic angular degree l. At the same time, for the primary the fraction of energy in the dipole component is low and the maximum energy contribution comes from l = 4. However, in the 2017 epoch both stars have similar field topologies and a systematically decreasing energy with increasing l. In the earlier epoch, the magnetic field at the visible pole appears to be of opposite polarity for the primary and secondary, suggesting linked magnetospheres. The apparent rotational periods of both sigma(2) CrB components are longer than the orbital period, which we interpret as an evidence of a solar-like differential rotation. Conclusions. Despite their nearly identical fundamental parameters, the components of sigma(2) CrB system exhibit different magnetic field properties. This indicates that the magnetic dynamo process is a very sensitive function of stellar parameters.

Place, publisher, year, edition, pages
EDP Sciences, 2018
Keywords
polarization, stars: activity, stars: magnetic field, stars: late-type, stars: individual: sigma(2) CrB
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-357716 (URN)10.1051/0004-6361/201731706 (DOI)000433880200006 ()
Funder
Swedish Research CouncilKnut and Alice Wallenberg FoundationEU, Horizon 2020, ANR10 LABX56
Available from: 2018-08-31 Created: 2018-08-31 Last updated: 2018-12-11
Farihi, J., Fossati, L., Wheatley, P. J., Metzger, B. D., Mauerhan, J., Bachman, S., . . . Stone, N. (2018). Magnetism, X-rays and accretion rates in WD 1145+017 and other polluted white dwarf systems. Monthly notices of the Royal Astronomical Society, 474(1), 947-960
Open this publication in new window or tab >>Magnetism, X-rays and accretion rates in WD 1145+017 and other polluted white dwarf systems
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2018 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 474, no 1, p. 947-960Article in journal (Refereed) Published
Abstract [en]

This paper reports circular spectropolarimetry and X-ray observations of several polluted white dwarfs including WD 1145+017, with the aim to constrain the behaviour of disc material and instantaneous accretion rates in these evolved planetary systems. Two stars with previously observed Zeeman splitting, WD 0322-019 and WD 2105-820, are detected above 5 sigma and < B-z > > 1 kG, while WD 1145+017, WD 1929+011, and WD 2326+049 yield (null) detections below this minimum level of confidence. For these latter three stars, high-resolution spectra and atmospheric modelling are used to obtain limits on magnetic field strengths via the absence of Zeeman splitting, finding B-* < 20 kG based on data with resolving power R approximate to 40 000. An analytical framework is presented for bulk Earth composition material falling on to the magnetic polar regions of white dwarfs, where X-rays and cyclotron radiation may contribute to accretion luminosity. This analysis is applied to X-ray data for WD 1145+017, WD 1729+371, and WD 2326+049, and the upper bound count rates are modelled with spectra for a range of plasma kT = 1-10 keV in both the magnetic and non-magnetic accretion regimes. The results for all three stars are consistent with a typical dusty white dwarf in a steady state at 108-109 g s(-1). In particular, the non-magnetic limits for WD 1145+017 are found to be well below previous estimates of up to 10(12) g s(-1), and likely below 1010 g s(-1), thus suggesting the star-disc system may be average in its evolutionary state, and only special in viewing geometry.

Place, publisher, year, edition, pages
OXFORD UNIV PRESS, 2018
Keywords
circumstellar matter, stars: magnetic field, planetary systems, white dwarfs, X-rays: stars
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-348400 (URN)10.1093/mnras/stx2664 (DOI)000424339500070 ()
Available from: 2018-04-12 Created: 2018-04-12 Last updated: 2018-04-12Bibliographically approved
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Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0003-3061-4591

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