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Bladh, S., Liljegren, S., Höfner, S., Aringer, B. & Marigo, P. (2019). An extensive grid of DARWIN models for M-type AGB stars: I. Mass-loss rates and other properties of dust-driven winds. Astronomy and Astrophysics, 626, Article ID A100.
Open this publication in new window or tab >>An extensive grid of DARWIN models for M-type AGB stars: I. Mass-loss rates and other properties of dust-driven winds
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2019 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 626, article id A100Article in journal (Refereed) Published
Abstract [en]

Context: The stellar winds of asymptotic giant branch (AGB) stars are commonly attributed to radiation pressure on dust grains, formed in the wake of shock waves that arise in the stellar atmospheres. The mass loss due to these outflows is substantial, and modelling the dynamical properties of the winds is essential both for studies of individual stars and for understanding the evolution of stellar populations with low to intermediate mass.

Aims: The purpose of this work is to present an extensive grid of dynamical atmosphere and wind models for M-type AGB stars, covering a wide range of relevant stellar parameters.

Methods: We used the DARWIN code, which includes frequency-dependent radiation-hydrodynamics and a time-dependent description of dust condensation and evaporation, to simulate the dynamical atmosphere. The wind-driving mechanism is photon scattering on submicron-sized Mg2SiO4 grains. The grid consists of similar to 4000 models, with luminosities from L-* = 890 L-circle dot to L-* = 40 000 L-circle dot and effective temperatures from 2200 to 3400 K. For the first time different current stellar masses are explored with M-type DARWIN models, ranging from 0.75 M-circle dot to 3 M-circle dot. The modelling results are radial atmospheric structures, dynamical properties such as mass-loss rates and wind velocities, and dust properties (e.g. grain sizes, dust-to-gas ratios, and degree of condensed Si).

Results: We find that the mass-loss rates of the models correlate strongly with luminosity. They also correlate with the ratio L-*/M-* : increasing L-*/M-* by an order of magnitude increases the mass-loss rates by about three orders of magnitude, which may naturally create a superwind regime in evolution models. There is, however, no discernible trend of mass-loss rate with effective temperature, in contrast to what is found for C-type AGB stars. We also find that the mass-loss rates level off at luminosities higher than similar to 14 000 L-circle dot, and consequently at pulsation periods longer than similar to 800 days. The final grain radii range from 0.25 to 0.6 mu m. The amount of condensed Si is typically between 10 and 40%, with gas-to-dust mass ratios between 500 and 4000.

National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-348124 (URN)10.1051/0004-6361/201935366 (DOI)000472465400001 ()
Funder
Swedish Research CouncilEU, European Research Council, 6 15 604
Note

Title in dissertation list of papers: An extensive grid of DARWIN models for M-type AGB stars I. Mass loss and the properties of wind and dust

Available from: 2018-04-10 Created: 2018-04-10 Last updated: 2019-08-05Bibliographically approved
Pastorelli, G., Marigo, P., Girardi, L., Chen, Y., Rubele, S., Trabucchi, M., . . . Cioni, M.-R. L. (2019). Constraining the thermally pulsing asymptotic giant branch phase with resolved stellar populations in the Small Magellanic Cloud. Monthly notices of the Royal Astronomical Society, 485(4), 5666-5692
Open this publication in new window or tab >>Constraining the thermally pulsing asymptotic giant branch phase with resolved stellar populations in the Small Magellanic Cloud
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2019 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 485, no 4, p. 5666-5692Article in journal (Refereed) Published
Abstract [en]

The thermally pulsing asymptotic giant branch (TP-AGB) experienced by low-and intermediate-mass stars is one of the most uncertain phases of stellar evolution and the models need to be calibrated with the aid of observations. To this purpose, we couple high-quality observations of resolved stars in the Small Magellanic Cloud (SMC) with detailed stellar population synthesis simulations computed with the TRILEGAL code. The strength of our approach relies on the detailed spatially resolved star formation history of the SMC, derived from the deep near-infrared photometry of the VISTA survey of the Magellanic Clouds, as well as on the capability to quickly and accurately explore a wide variety of parameters and effects with the COLIBRI code for the TP-AGB evolution. Adopting a well-characterized set of observations - star counts and luminosity functions - we set up a calibration cycle along which we iteratively change a few key parameters of the TP-AGB models until we eventually reach a good fit to the observations. Our work leads to identify two best-fitting models that mainly differ in the efficiencies of the third dredge-up and mass-loss in TP-AGB stars with initial masses larger than about 3 M-circle dot. On the basis of these calibrated models, we provide a full characterization of the TP-AGB stellar population in the SMC in terms of stellar parameters (initial masses, C/O ratios, carbon excess, mass-loss rates). Extensive tables of isochrones including these improved models are publicly available.

Place, publisher, year, edition, pages
OXFORD UNIV PRESS, 2019
Keywords
stars: AGB and post-AGB, stars: evolution, stars: carbon, stars: mass-loss, Magellanic Clouds
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-390640 (URN)10.1093/mnras/stz725 (DOI)000474880400090 ()
Funder
EU, European Research Council, 615604EU, Horizon 2020, 682115
Available from: 2019-08-21 Created: 2019-08-21 Last updated: 2019-08-21Bibliographically approved
Liljegren, S., Höfner, S., Freytag, B. & Bladh, S. (2018). Atmospheres and wind properties of non-spherical AGB stars. Astronomy and Astrophysics, 619, Article ID A47.
Open this publication in new window or tab >>Atmospheres and wind properties of non-spherical AGB stars
2018 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 619, article id A47Article in journal (Refereed) Published
Abstract [en]

Context. The wind-driving mechanism of asymptotic giant branch (AGB) stars is commonly attributed to a two-step process: first, gas in the stellar atmosphere is levitated by shockwaves caused by stellar pulsation, then accelerated outwards by radiative pressure on newly formed dust, inducing a wind. Dynamical modelling of such winds usually assumes a spherically symmetric star.

Aims. We explore the potential consequences of complex stellar surface structures, as predicted by three-dimensional (3D) star-in-a-box modelling of M-type AGB stars, on the resulting wind properties with the aim to improve the current wind models.

Methods. Two different modelling approaches are used; the CO5BOLD 3D star-in-a-box code to simulate the convective, pulsating interior and lower atmosphere of the star, and the DARWIN one-dimensional (1D) code to describe the dynamical atmosphere where the wind is accelerated. The gas dynamics of the inner atmosphere region at distances of R ∼ 1−2 R, which both modelling approaches simulate, are compared. Dynamical properties and luminosity variations derived from CO5BOLD interior models are used as input for the inner boundary in DARWIN wind models in order to emulate the effects of giant convection cells and pulsation, and explore their influence on the dynamical properties.

Results. The CO5BOLD models are inherently anisotropic, with non-uniform shock fronts and varying luminosity amplitudes, in contrast to the spherically symmetrical DARWIN wind models. DARWIN wind models with CO5BOLD-derived inner boundary conditions produced wind velocities and mass-loss rates comparable to the standard DARWIN models, however the winds show large density variations on time-scales of 10–20 yr.

Conclusions. The method outlined in this paper derives pulsation properties from the 3D star-in-a-box CO5BOLD models, to be used in the DARWIN models. If the current grid of CO5BOLD models is extended, it will be possible to construct extensive DARWIN grids with inner boundary conditions derived from 3D interior modelling of convection and pulsation, and avoid the free parameters of the current approach.

Keywords
stars: AGB and post-AGB, stars: atmospheres, stars: winds outflows, stars: oscillations, shock waves
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy with specialization in Astrophysics
Identifiers
urn:nbn:se:uu:diva-348123 (URN)10.1051/0004-6361/201833203 (DOI)000449278400001 ()
Funder
Swedish Research CouncilSwedish National Infrastructure for Computing (SNIC)
Available from: 2018-04-10 Created: 2018-04-10 Last updated: 2019-06-26Bibliographically approved
Bladh, S., Susanne, H., Aringer, B. & Eriksson, K. (2015). Exploring wind-driving dust species in cool luminous giants III: Wind models for M-type AGB stars. Astronomy and Astrophysics, 575, Article ID A105.
Open this publication in new window or tab >>Exploring wind-driving dust species in cool luminous giants III: Wind models for M-type AGB stars
2015 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 575, article id A105Article in journal (Refereed) Published
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:uu:diva-230644 (URN)10.1051/0004-6361/201424917 (DOI)000360710400006 ()
Funder
Swedish Research Council
Available from: 2014-08-27 Created: 2014-08-27 Last updated: 2017-12-05Bibliographically approved
Bladh, S. (2014). Dynamical atmospheres and winds of M-type AGB stars. (Doctoral dissertation). Uppsala: Acta Universitatis Upsaliensis
Open this publication in new window or tab >>Dynamical atmospheres and winds of M-type AGB stars
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Mass loss, in the form of slow stellar winds, is a decisive factor for the evolution of cool luminous giants, eventually turning them into white dwarfs. These dense outflows are also a key factor in the enrichment of the interstellar medium with newly produced elements from the interior of these stars. There are strong indications that these winds are accelerated by radiation pressure on dust grains, but the actual grain species responsible for driving the outflows in M-type Asymptotic Giant Branch stars are still a matter of debate. Observations of dust features in the circumstellar environment of these stars suggest that magnesium-iron silicates are possible wind-drivers. However, the optical properties of these silicate grains are strongly influenced by the Fe-content. Fe-bearing condensates heat up strongly when interacting with the radiation field and therefore cannot form close enough to the star to trigger outflows. Fe-free condensates, on the other hand, have a low absorption cross-section at near-IR wavelengths where AGB stars emit most of their flux.  To solve this conundrum, it has been suggested that winds of M-type AGB stars may be driven by photon scattering on Fe-free silicate grains with sizes comparable to the wavelength of the flux maximum, rather than by true absorption. In this thesis we investigate dynamical models of M-type AGB stars, using Fe-free silicates as the wind-driving dust species. According to our findings these models produce both dynamic and photometric properties consistent with observations. Especially noteworthy are the large photometric variations in the visual band during a pulsation cycle, seen both in the observed and synthetic fluxes. A closer examination of the models reveals that these variations are caused by changes in the molecular layers, and not by changes in the dust. This is a strong indication that stellar winds of M-type AGB stars are driven by dust materials that are very transparent in the visual and near-infrared wavelength regions, otherwise these molecular effects would not be visible.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. p. 54
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1170
Keywords
Late-type stars, AGB stars, stellar winds, atmospheres, mass-loss, outflows, circumstellar matter, dust, hydroynamics, radiative transfer
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy with specialization in Astrophysics
Identifiers
urn:nbn:se:uu:diva-230645 (URN)978-91-554-9015-7 (ISBN)
Public defence
2014-10-10, Polhemssalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:00 (English)
Opponent
Supervisors
Available from: 2014-09-18 Created: 2014-08-27 Last updated: 2015-01-22
Bladh, S., Höfner, S., Nowotny, W., Aringer, B. & Eriksson, K. (2013). Exploring wind-driving dust species in cool luminous giants II. Constraints from photometry of M-type AGB stars. Astronomy and Astrophysics, 553, A20
Open this publication in new window or tab >>Exploring wind-driving dust species in cool luminous giants II. Constraints from photometry of M-type AGB stars
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2013 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 553, p. A20-Article in journal (Refereed) Published
Abstract [en]

Context. The heavy mass loss observed in evolved asymptotic giant branch (AGB) stars is usually attributed to a two-stage process: atmospheric levitation by pulsation-induced shock waves, followed by radiative acceleration of newly formed dust grains. The dust transfers momentum to the surrounding gas through collisions and thereby triggers a general outflow. Radiation-hydrodynamical models of M-type AGB stars suggest that these winds can be driven by photon scattering - in contrast to absorption - on Fe-free silicate grains of sizes 0.1-1 mu m. Aims. In this paper we study photometric constraints for wind-driving dust species in M-type AGB stars, as part of an ongoing effort to identify likely candidates among the grain materials observed in circumstellar envelopes. Methods. To investigate the scenario of stellar winds driven by photon scattering on dust, and to explore how different optical and chemical properties of wind-driving dust species affect photometry we focus on two sets of dynamical models atmospheres: (i) models using a detailed description for the growth of Mg2SiO4 grains, taking into account both scattering and absorption cross-sections when calculating the radiative acceleration; and (ii) models using a parameterized dust description, constructed to represent different chemical and optical dust properties. By comparing synthetic photometry from these two sets of models to observations of M-type AGB stars we can provide constraints on the properties of wind-driving dust species. Results. Photometry from wind models with a detailed description for the growth of Mg2SiO4 grains reproduces well both the values and the time-dependent behavior of observations of M-type AGB stars, providing further support for the scenario of winds driven by photon scattering on dust. The photometry from the models with a parameterized dust description suggests that wind-drivers need to have a low absorption cross-section in the visual and near-IR to reproduce the time-dependent behavior, i. e. small variations in (J-K) and spanning a larger range in (V-K). This places constraints on the optical and chemical properties of the wind-driving dust species. Conclusions. To reproduce the observed photometric variations in (V-K) and (J-K) both detailed and parameterized models suggest that the wind-driving dust materials have to be quite transparent in the visual and near-IR. Consequently, strong candidates for outflows driven by photon scattering on dust grains are Mg2SiO4, MgSiO3, and potentially SiO2.

Keywords
stars: AGB and post-AGB, stars: late-type, stars: mass-loss, stars: winds, outflows, stars: atmospheres, circumstellar matter
National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-203560 (URN)10.1051/0004-6361/201220590 (DOI)000319858700020 ()
Available from: 2013-07-15 Created: 2013-07-15 Last updated: 2017-12-06Bibliographically approved
Sacuto, S., Ramstedt, S., Höfner, S., Olofsson, H., Bladh, S., Eriksson, K., . . . Maercker, M. (2013). The wind of the M-type AGB star RT Virginis probed by VLTI/MIDI. Astronomy and Astrophysics, 551, A72
Open this publication in new window or tab >>The wind of the M-type AGB star RT Virginis probed by VLTI/MIDI
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2013 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 551, p. A72-Article in journal (Refereed) Published
Abstract [en]

Aims. We study the circumstellar environment of the M-type AGB star RT Vir using mid-infrared high spatial resolution observations from the ESO-VLTI focal instrument MIDI. The aim of this study is to provide observational constraints on theoretical prediction that the winds of M-type AGB objects can be driven by photon scattering on iron-free silicate grains located in the close environment (about 2 to 3 stellar radii) of the star. Methods. We interpreted spectro-interferometric data, first using wavelength-dependent geometric models. We then used a self-consistent dynamic model atmosphere containing a time-dependent description of grain growth for pure forsterite dust particles to reproduce the photometric, spectrometric, and interferometric measurements of RT Vir. Since the hydrodynamic computation needs stellar parameters as input, a considerable effort was first made to determine these parameters. Results. MIDI differential phases reveal the presence of an asymmetry in the stellar vicinity. Results from the geometrical modeling give us clues to the presence of aluminum and silicate dust in the close circumstellar environment (<5 stellar radii). Comparison between spectro-interferometric data and a self-consistent dust-driven wind model reveals that silicate dust has to be present in the region between 2 to 3 stellar radii to reproduce the 59 and 63 m baseline visibility measurements around 9.8 mu m. This gives additional observational evidence in favor of winds driven by photon scattering on iron-free silicate grains located in the close vicinity of an M-type star. However, other sources of opacity are clearly missing to reproduce the 10-13 mu m visibility measurements for all baselines. Conclusions. This study is a first attempt to understand the wind mechanism of M-type AGB stars by comparing photometric, spectrometric, and interferometric measurements with state-of-the-art, self-consistent dust-driven wind models. The agreement of the dynamic model atmosphere with interferometric measurements in the 8-10 mu m spectral region gives additional observational evidence that the winds of M-type stars can be driven by photon scattering on iron-free silicate grains. Finally, a larger statistical study and progress in advanced self-consistent 3D modeling are still required to solve the remaining problems.

Keywords
techniques: interferometric, instrumentation: high angular resolution, stars: AGB and post-AGB, stars: atmospheres, circumstellar matter, stars: mass-loss
National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-199474 (URN)10.1051/0004-6361/201220524 (DOI)000316460600072 ()
Available from: 2013-05-07 Created: 2013-05-06 Last updated: 2017-12-06Bibliographically approved
Lebzelter, T., Heiter, U., Abia, C., Eriksson, K., Ireland, M., Neilson, H., . . . Wing, R. F. (2012). Comparative modelling of the spectra of cool giants. Astronomy and Astrophysics, 547, A108
Open this publication in new window or tab >>Comparative modelling of the spectra of cool giants
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2012 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 547, p. A108-Article in journal (Refereed) Published
Abstract [en]

Context. Our ability to extract information from the spectra of stars depends on reliable models of stellar atmospheres and appropriate techniques for spectral synthesis. Various model codes and strategies for the analysis of stellar spectra are available today. Aims. We aim to compare the results of deriving stellar parameters using different atmosphere models and different analysis strategies. The focus is set on high-resolution spectroscopy of cool giant stars. Methods. Spectra representing four cool giant stars were made available to various groups and individuals working in the area of spectral synthesis, asking them to derive stellar parameters from the data provided. The results were discussed at a workshop in Vienna in 2010. Most of the major codes currently used in the astronomical community for analyses of stellar spectra were included in this experiment. Results. We present the results from the different groups, as well as an additional experiment comparing the synthetic spectra produced by various codes for a given set of stellar parameters. Similarities and differences of the results are discussed. Conclusions. Several valid approaches to analyze a given spectrum of a star result in quite a wide range of solutions. The main causes for the differences in parameters derived by different groups seem to lie in the physical input data and in the details of the analysis method. This clearly shows how far from a definitive abundance analysis we still are.

Keywords
Stars: abundances, Stars: atmospheres, Stars: fundamental parameters, Stars: late-type
National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-186191 (URN)10.1051/0004-6361/201219142 (DOI)000311031700108 ()
Available from: 2012-11-29 Created: 2012-11-28 Last updated: 2017-12-07Bibliographically approved
Bladh, S. & Höfner, S. (2012). Exploring wind-driving dust species in cool luminous giants: I. Basic criteria and dynamical models of M-type AGB stars. Astronomy and Astrophysics, 546, A76
Open this publication in new window or tab >>Exploring wind-driving dust species in cool luminous giants: I. Basic criteria and dynamical models of M-type AGB stars
2012 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 546, p. A76-Article in journal (Refereed) Published
Abstract [en]

Context. The heavy mass loss observed in evolved asymptotic giant branch stars is usually attributed to a two-stage process: atmospheric levitation by pulsation-induced shock waves followed by radiative acceleration of dust grains, which transfer momentum to the surrounding gas through collisions. In order for an outflow to occur the two stages of the mass-loss scheme have to connect, i.e., the radiative acceleration can only be initiated if the levitated gas reaches a distance from the stellar photosphere where dust particles can condense. This levitation distance is limited by the kinetic energy transferred to the gas by the shock waves, which imposes strict constraints on potential wind-driving dust species. Aims. This work is part of an ongoing effort aiming at identifying the actual wind-drivers among the dust species observed in circumstellar envelopes. In particular, we focus on the interplay between a strong stellar radiation field and the dust formation process. Methods. To identify critical properties of potential wind-driving dust species we use detailed radiation-hydrodynamical models which include a parameterized dust description, complemented by simple analytical estimates to help with the physical interpretation of the numerical results. The adopted dust description is constructed to mimic different chemical and optical dust properties in order to systematically study the effects of a realistic radiation field on the second stage of the mass loss mechanism. Results. We see distinct trends in which combinations of optical and chemical dust properties are needed to trigger an outflow. Dust species with a low condensation temperature and a near-infrared absorption coefficient that decreases strongly with wavelength will not condense close enough to the stellar surface to be considered as potential wind-drivers. Conclusions. Our models confirm that metallic iron and Fe-bearing silicates are not viable as wind-drivers due to their near-infrared optical properties and resulting large condensation distances. TiO2 is also excluded as a wind-driver due to the low abundance of Ti. Other species, such a SiO2 and Al2O3, are less clear-cut cases due to uncertainties in the optical and chemical data and further work is needed. A strong candidate is Mg2SiO4 with grain sizes of 0.1-1 mu m, where scattering contributes significantly to the radiative acceleration, as suggested by earlier theoretical work and supported by recent observations.

Keywords
stars: AGB and post-AGB, stars: mass-loss, stars: winds, outflows, circumstellar matter, radiative transfer, hydrodynamics
National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-187096 (URN)10.1051/0004-6361/201219138 (DOI)000310349100076 ()
Available from: 2012-12-04 Created: 2012-12-03 Last updated: 2017-12-07Bibliographically approved
Bladh, S., Höfner, S. & Aringer, B. (2011). Dust in AGB Stars: Transparent or Opaque?. In: Why Galaxies Care about AGB Stars II: Shining Examples and Common Inhabitants. Paper presented at Astronomical Society of the Pacific Conference Series, 445. , 445
Open this publication in new window or tab >>Dust in AGB Stars: Transparent or Opaque?
2011 (English)In: Why Galaxies Care about AGB Stars II: Shining Examples and Common Inhabitants, 2011, Vol. 445Conference paper, Published paper (Refereed)
Abstract [en]

The optical properties of the dust particles that drive the winds of cool giant stars affect the stellar spectra in two ways: (1) indirectly, through their influence on the dynamical structure of the atmosphere/envelope and the resulting molecular features, and (2) directly, by changes of the spectral energy distribution due to absorption and scattering on dust grains. The qualitative differences in the energy distributions of C-type and M-type AGB stars in the visual and near-infrared regions suggest that the dust particles in oxygen rich atmospheres are relatively transparent to radiation. By using detailed dynamical models of gas and radiation combined with a simple description for the dust opacity (which can be adjusted to mimic different wavelength dependences and condensation temperatures) and also by adjusting the fraction of the opacity that is treated as true absorption, we investigate which dust properties produce synthetic photometry consistent with observations. The goal of this study is to narrow down the possible dust species that may be driving the winds in M-type AGB stars.

Series
Astronomical Society of the Pacific Conference Series
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-170170 (URN)
Conference
Astronomical Society of the Pacific Conference Series, 445
Available from: 2012-03-09 Created: 2012-03-09 Last updated: 2012-10-11
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0003-2964-7943

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