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Barklem, Paul
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Publications (10 of 111) Show all publications
Amarsi, A. M., Barklem, P. S., Collet, R., Grevesse, N. & Asplund, M. (2019). 3D non-LTE line formation of neutral carbon in the Sun. Astronomy and Astrophysics, 624, Article ID A111.
Open this publication in new window or tab >>3D non-LTE line formation of neutral carbon in the Sun
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2019 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 624, article id A111Article in journal (Refereed) Published
Abstract [en]

Carbon abundances in late-type stars are important in a variety of astrophysical contexts. However C I lines, one of the main abundance diagnostics, are sensitive to departures from local thermodynamic equilibrium (LTE). We present a model atom for non-LTE analyses of C I lines, that uses a new, physically-motivated recipe for the rates of neutral hydrogen impact excitation. We analyse C I lines in the solar spectrum, employing a three-dimensional (3D) hydrodynamic model solar atmosphere and 3D non-LTE radiative transfer. We find negative non-LTE abundance corrections for C I lines in the solar photosphere, in accordance with previous studies, reaching up to around 0.1 dex in the disk-integrated flux. We also present the first fully consistent 3D non-LTE solar carbon abundance determination: we infer log is an element of(C) = 8.44 ± 0.02, in good agreement with the current standard value. Our models reproduce the observed solar centre-to-limb variations of various C I lines, without any adjustments to the rates of neutral hydrogen impact excitation, suggesting that the proposed recipe may be a solution to the long-standing problem of how to reliably model inelastic collisions with neutral hydrogen in late-type stellar atmospheres.

Keywords
atomic data, radiative transfer, line: formation, Sun: abundances, Sun: photosphere
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-382839 (URN)10.1051/0004-6361/201833603 (DOI)000464916800003 ()
Funder
Swedish Research CouncilKnut and Alice Wallenberg FoundationAustralian Research Council, DP150100250Australian Research Council, FL110100012Swedish National Infrastructure for Computing (SNIC), SNIC2018-3-465Danish National Research Foundation, DNRF106
Available from: 2019-05-20 Created: 2019-05-20 Last updated: 2019-05-20Bibliographically approved
Osorio, Y., Lind, K., Barklem, P. S., Prieto, C. A. & Zatsarinny, O. (2019). Ca line formation in late-type stellar atmospheres: I. The model atom. Astronomy and Astrophysics, 623, Article ID A103.
Open this publication in new window or tab >>Ca line formation in late-type stellar atmospheres: I. The model atom
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2019 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 623, article id A103Article in journal (Refereed) Published
Abstract [en]

Context

Departures from local thermodynamic equilibrium (LTE) distort the calcium abundance derived from stellar spectra in various ways, depending on the lines used and the stellar atmospheric parameters. The collection of atomic data adopted in non-LTE (NLTE) calculations must be sufficiently complete and accurate.

Aims

We derive NLTE abundances from high-quality observations and reliable stellar parameters using a model atom built afresh for this work, and check the consistency of our results over a wide wavelength range with transitions of atomic and singly ionised calcium.

Methods

We built and tested Cat and Can model atoms with state-of-the-art radiative and collisional data, and tested their performance deriving the Ca abundance in three benchmark stars: Procyon, the Sun, and Arcturus. We have excellent-quality observations and accurate stellar parameters for these stars. Two methods to derive the LTE/NLTE abundances were used and compared. The LTE/NLTE centre-to-limb variation (CLV) of Ca lines in the Sun was also investigated.

Results

The two methods used give similar results in all three stars. Several discrepancies found in LTE do not appear in our NLTE results; in particular the agreement between abundances in the visual and infra-red (IR) and the Cat and Can ionisation balance is improved overall, although substantial line-to-line scatter remains. The CLV of the calcium lines around 6165 angstrom can be partially reproduced. We suspect differences between our modelling and CLV results are due to inhomogeneities in the atmosphere that require 3D modelling.

Keywords
line: formation, stars: abundances
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-380454 (URN)10.1051/0004-6361/201834680 (DOI)000460891400001 ()
Funder
Swedish Research Council, 2015-00415_3Knut and Alice Wallenberg FoundationSwedish Research Council
Available from: 2019-03-28 Created: 2019-03-28 Last updated: 2019-03-28Bibliographically approved
Amarsi, A. M., Nissen, P. E., Asplund, M., Lind, K. & Barklem, P. (2019). Carbon and oxygen in metal-poor halo stars. Astronomy and Astrophysics, 622, Article ID L4.
Open this publication in new window or tab >>Carbon and oxygen in metal-poor halo stars
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2019 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 622, article id L4Article in journal (Refereed) Published
Abstract [en]

Carbon and oxygen are key tracers of the Galactic chemical evolution; in particular, a reported upturn in [C/O] towards decreasing [O/H] in metal-poor halo stars could be a signature of nucleosynthesis by massive Population III stars. We reanalyse carbon, oxygen, and iron abundances in 39 metal-poor turn-off stars. For the first time, we take into account 3D hydrodynamic effects together with departures from local thermodynamic equilibrium (LTE) when determining both the stellar parameters and the elemental abundances, by deriving effective temperatures from 3D non-LTE H beta profiles, surface gravities from Gaia parallaxes, iron abundances from 3D LTE Fe ii equivalent widths, and carbon and oxygen abundances from 3D non-LTE C-I and O-I equivalent widths. We find that [C/Fe] stays flat with [Fe/H], whereas [O/Fe] increases linearly up to 0.75 dex with decreasing [Fe/H] down to -3.0 dex. Therefore [C/O] monotonically decreases towards decreasing [C/H], in contrast to previous findings, mainly because the non-LTE e ff ects for O i at low [Fe/H] are weaker with our improved calculations.

Place, publisher, year, edition, pages
EDP SCIENCES S A, 2019
Keywords
radiative transfer, stars: abundances, stars: late-type, stars: Population II
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-376813 (URN)10.1051/0004-6361/201834480 (DOI)000456967800001 ()
Funder
Swedish Research Council, 2015-004153Australian Research Council, FL110100012Australian Research Council, DP150100250Swedish Research CouncilKnut and Alice Wallenberg Foundation
Available from: 2019-02-20 Created: 2019-02-20 Last updated: 2019-02-20Bibliographically approved
Amarsi, A. M. & Barklem, P. (2019). Excitation and charge transfer in low-energy hydrogen atom collisions with neutral carbon and nitrogen. Astronomy and Astrophysics, 625, Article ID A78.
Open this publication in new window or tab >>Excitation and charge transfer in low-energy hydrogen atom collisions with neutral carbon and nitrogen
2019 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 625, article id A78Article in journal (Refereed) Published
Abstract [en]

Low-energy inelastic collisions with neutral hydrogen atoms are important processes in stellar atmospheres, and a persistent source of uncertainty in non-LTE modelling of stellar spectra. We have calculated and studied excitation and charge transfer of C I and of N I due to such collisions. We used a previously presented method that is based on an asymptotic two-electron linear combination of atomic orbitals (LCAO) model of ionic-covalent interactions for the adiabatic potential energies, combined with the multichannel Landau-Zener model for the collision dynamics. We find that charge transfer processes typically lead to much larger rate coefficients than excitation processes do, consistent with studies of other atomic species. Two-electron processes were considered and lead to non-zero rate coefficients that can potentially impact statistical equilibrium calculations. However, they were included in the model in an approximate way, via an estimate for the two-electron coupling that was presented earlier in the literature: the validity of these data should be checked in a future work.

Place, publisher, year, edition, pages
EDP SCIENCES S A, 2019
Keywords
atomic data, atomic processes, line: formation, radiative transfer, stars: abundances
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-385004 (URN)10.1051/0004-6361/201935101 (DOI)000468126800001 ()
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Available from: 2019-06-11 Created: 2019-06-11 Last updated: 2019-06-11Bibliographically approved
Eyer, L., Korn, A. J., Barklem, P. S., Edvardsson, B., Eriksson, K., Gavel, A., . . . Zwitter, T. (2019). Gaia Data Release 2 Variable stars in the colour-absolute magnitude diagram. Astronomy and Astrophysics, 623, Article ID A110.
Open this publication in new window or tab >>Gaia Data Release 2 Variable stars in the colour-absolute magnitude diagram
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2019 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 623, article id A110Article in journal (Refereed) Published
Abstract [en]

Context

The ESA Gaia mission provides a unique time-domain survey for more than 1.6 billion sources with G less than or similar to 21 mag.

Aims

We showcase stellar variability in the Galactic colour-absolute magnitude diagram (CaMD). We focus on pulsating, eruptive, and cataclysmic variables, as well as on stars that exhibit variability that is due to rotation and eclipses.

Methods

We describe the locations of variable star classes, variable object fractions, and typical variability amplitudes throughout the CaMD and show how variability-related changes in colour and brightness induce "motions". To do this, we use 22 months of calibrated photometric, spectro-photometric, and astrometric Gaia data of stars with a significant parallax. To ensure that a large variety of variable star classes populate the CaMD, we crossmatched Gaia sources with known variable stars. We also used the statistics and variability detection modules of the Gaia variability pipeline. Corrections for interstellar extinction are not implemented in this article.

Results

Gaia enables the first investigation of Galactic variable star populations in the CaMD on a similar, if not larger, scale as was previously done in the Magellanic Clouds. Although the observed colours are not corrected for reddening, distinct regions are visible in which variable stars occur. We determine variable star fractions to within the current detection thresholds of Gaia. Finally, we report the most complete description of variability-induced motion within the CaMD to date.

Conclusions

Gaia enables novel insights into variability phenomena for an unprecedented number of stars, which will benefit the understanding of stellar astrophysics. The CaMD of Galactic variable stars provides crucial information on physical origins of variability in a way that has previously only been accessible for Galactic star clusters or external galaxies. Future Gaia data releases will enable significant improvements over this preview by providing longer time series, more accurate astrometry, and additional data types (time series BP and RP spectra, RVS spectra, and radial velocities), all for much larger samples of stars.

Keywords
stars: general, stars: variables: general, stars: oscillations, binaries: eclipsing, surveys, methods: data analysis
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-380450 (URN)10.1051/0004-6361/201833304 (DOI)000461008700002 ()
Funder
EU, Horizon 2020, 670519
Note

For complete list of authors see http://dx.doi.org/10.1051/0004-6361/201833304

Available from: 2019-03-29 Created: 2019-03-29 Last updated: 2019-03-29Bibliographically approved
Allende Prieto, C., Koesterke, L., Hubeny, I., Bautista, M. A., Barklem, P. & Nahar, S. N. (2018). A collection of model stellar spectra for spectral types B to early-M. Astronomy and Astrophysics, 618, Article ID A25.
Open this publication in new window or tab >>A collection of model stellar spectra for spectral types B to early-M
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2018 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 618, article id A25Article in journal (Refereed) Published
Abstract [en]

Context: Models of stellar spectra are necessary for interpreting light from individual stars, planets, integrated stellar populations, nebulae, and the interstellar medium.

Aims: We provide a comprehensive and homogeneous collection of synthetic spectra for a wide range of atmospheric parameters and chemical compositions.

Methods: We compile atomic and molecular data from the literature. We adopt the largest and most recent set of ATLAS9 model atmospheres, and use the radiative code ASS epsilon T.

Results: The resulting collection of spectra is made publicly available at medium and high-resolution (R lambda/delta lambda = 10 000, 100 000 and 300 000) spectral grids, which include variations in effective temperature between 3500K and 30 000 K, surface gravity (0 <= log g <= 5), and metallicity (-5 <= [Fe/H] <= +0 : 5), spanning the wavelength interval 120-6500 nm. A second set of denser grids with additional dimensions, [alpha/ Fe] and micro-turbulence, are also provided (covering 200-2500 nm). We compare models with observations for a few representative cases.

Place, publisher, year, edition, pages
EDP SCIENCES S A, 2018
Keywords
radiative transfer, atlases, stars: atmospheres
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-367029 (URN)10.1051/0004-6361/201732484 (DOI)000446750800003 ()
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Available from: 2018-11-28 Created: 2018-11-28 Last updated: 2018-11-28Bibliographically approved
Roederer, I. U. & Barklem, P. (2018). A New Test of Copper and Zinc Abundances in Late-type Stars Using Ultraviolet Cu II and Zn II Lines. Astrophysical Journal, 857(1), Article ID 2.
Open this publication in new window or tab >>A New Test of Copper and Zinc Abundances in Late-type Stars Using Ultraviolet Cu II and Zn II Lines
2018 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 857, no 1, article id 2Article in journal (Refereed) Published
Abstract [en]

We present new abundances derived from Cu I, Cu II, Zn I, and Zn II lines in six warm (5766 <= T-eff <= 6427 K), metal-poor (-2.50 <= [Fe/H] <= -0.95) dwarf and subgiant (3.64 <= log g <= 4.44) stars. These abundances are derived from archival high-resolution ultraviolet spectra from the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope and ground-based optical spectra from several observatories. Ionized Cu and Zn are the majority species, and abundances derived from Cu II and Zn II lines should be largely insensitive to departures from local thermodynamic equilibrium (LTE). We find good agreement between the [Zn/H] ratios derived separately from Zn I and Zn II lines, suggesting that departures from LTE are, at most, minimal (less than or similar to 0.1 dex). We find that the [Cu/H] ratios derived from Cu II lines are 0.36 +/- 0.06 dex larger than those derived from Cu I lines in the most metal-poor stars ([Fe/H] < 1.8), suggesting that LTE underestimates the Cu abundance derived from Cu I lines. The deviations decrease in more metal-rich stars. Our results validate previous theoretical non-LTE calculations for both Cu and Zn, supporting earlier conclusions that the enhancement of [Zn/Fe] in metal-poor stars is legitimate, and the deficiency of [Cu/Fe] in metal-poor stars may not be as large as previously thought.

Place, publisher, year, edition, pages
IOP PUBLISHING LTD, 2018
Keywords
nuclear reactions, nucleosynthesis, abundances, stars: abundances, stars: atmospheres, stars: individual, stars: population II, ultraviolet: stars
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-352482 (URN)10.3847/1538-4357/aab71f (DOI)000429348600002 ()
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Available from: 2018-06-08 Created: 2018-06-08 Last updated: 2018-12-11
Nandakumar, G., Ryde, N., Schultheis, M., Thorsbro, B., Jönsson, H., Barklem, P., . . . Fragkoudi, F. (2018). Chemical characterization of the inner Galactic bulge: North-South symmetry. Monthly notices of the Royal Astronomical Society, 478(4), 4374-4389
Open this publication in new window or tab >>Chemical characterization of the inner Galactic bulge: North-South symmetry
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2018 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 478, no 4, p. 4374-4389Article in journal (Refereed) Published
Abstract [en]

While the number of stars in the Galactic bulge with detailed chemical abundance measurements is increasing rapidly, the inner Galactic bulge (vertical bar b vertical bar < 2 degrees) remains poorly studied, due to heavy interstellar absorption and photometric crowding. We have carried out a high-resolution IR spectroscopic study of 72 M giants in the inner bulge using the CRIRES (ESO/VLT) facility. Our spectra cover the wavelength range of 2.0818-2.1444 mu m with the resolution of R similar to 50 000 and have signal-to-noise ratio of 50: 100. Our stars are located along the bulge minor axis at l = 0 degrees, b= +/- 0 degrees, +/- 1 degrees, +/- 2 degrees, and + 3 degrees. Our sample was analysed in a homogeneous way using the most current K-band line list. We clearly detect a bimodal metallicity distribution function with a metal-rich peak at similar to + 0.3 dex and a metal-poor peak at similar to -0.5 dex and no stars with [Fe/H] > + 0.6 dex. The Galactic Centre field reveals in contrast a mainly metal-rich population with a mean metallicity of + 0.3 dex. We derived [Mg/Fe] and [Si/Fe] abundances that are consistent with trends from the outer bulge. We confirm for the supersolar metallicity stars the decreasing trend in [Mg/Fe] and [Si/Fe] as expected from chemical evolution models. With the caveat of a relatively small sample, we do not find significant differences in the chemical abundances between the Northern and the Southern fields; hence, the evidence is consistent with symmetry in chemistry between North and South.

Place, publisher, year, edition, pages
OXFORD UNIV PRESS, 2018
Keywords
stars: abundances, stars: late-type, Galaxy: bulge
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-362839 (URN)10.1093/mnras/sty1255 (DOI)000441288300008 ()
Funder
Swedish Research Council, 621-2014-5640Knut and Alice Wallenberg FoundationThe Crafoord FoundationStiftelsen Olle Engkvist Byggmästare
Available from: 2018-10-12 Created: 2018-10-12 Last updated: 2018-10-12Bibliographically approved
Yakovleva, S. A., Barklem, P. S. & Belyaev, A. K. (2018). Data on inelastic processes in low-energy potassium-hydrogen and rubidium-hydrogen collisions. Monthly notices of the Royal Astronomical Society, 473(3), 3810-3817
Open this publication in new window or tab >>Data on inelastic processes in low-energy potassium-hydrogen and rubidium-hydrogen collisions
2018 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 473, no 3, p. 3810-3817Article in journal (Refereed) Published
Abstract [en]

Two sets of rate coefficients for low-energy inelastic potassium-hydrogen and rubidium-hydrogen collisions were computed for each collisional system based on two model electronic structure calculations, performed by the quantum asymptotic semi-empirical and the quantum asymptotic linear combinations of atomic orbitals (LCAO) approaches, followed by quantum multichannel calculations for the non-adiabatic nuclear dynamics. The rate coefficients for the charge transfer (mutual neutralization, ion-pair formation), excitation and de-excitation processes are calculated for all transitions between the five lowest lying covalent states and the ionic states for each collisional system for the temperature range 1000–10 000 K. The processes involving higher lying states have extremely low rate coefficients and, hence, are neglected. The two model calculations both single out the same partial processes as having large and moderate rate coefficients. The largest rate coefficients correspond to the mutual neutralization processes into the K(5s 2S) and Rb(4d 2D) final states and at temperature 6000 K have values exceeding 3 × 10−8 cm3 s−1 and 4 × 10−8 cm3 s−1, respectively. It is shown that both the semi-empirical and the LCAO approaches perform equally well on average and that both sets of atomic data have roughly the same accuracy. The processes with large and moderate rate coefficients are likely to be important for non-LTE modelling in atmospheres of F, G and K-stars, especially metal-poor stars.

Keywords
atomic data, atomic processes, stars: atmospheres
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-346361 (URN)10.1093/mnras/stx2580 (DOI)000423809400074 ()
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Available from: 2018-03-16 Created: 2018-03-16 Last updated: 2018-03-16Bibliographically approved
Amarsi, A. M., Nordlander, T., Barklem, P., Asplund, M., Collet, R. & Lind, K. (2018). Effective temperature determinations of late-type stars based on 3D non-LTE Balmer line formation. Astronomy and Astrophysics, 615, Article ID A139.
Open this publication in new window or tab >>Effective temperature determinations of late-type stars based on 3D non-LTE Balmer line formation
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2018 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 615, article id A139Article in journal (Refereed) Published
Abstract [en]

Hydrogen Balmer lines are commonly used as spectroscopic effective temperature diagnostics of late-type stars. However, reliable inferences require accurate model spectra, and the absolute accuracy of classical methods that are based on one-dimensional (1D) hydrostatic model atmospheres and local thermodynamic equilibrium (LTE) is still unclear. To investigate this, we carry out 3D non-LTE calculations for the Balmer lines, performed, for the first time, over an extensive grid of 3D hydrodynamic STAGGER model atmospheres. For H alpha, H beta, and H gamma we find significant 1D non-LTE versus 3D non-LTE differences (3D effects): the outer wings tend to be stronger in 3D models, particularly for H gamma, while the inner wings can be weaker in 3D models, particularly for H alpha. For H alpha, we also find significant 3D LTE versus 3D non-LTE differences (non-LTE effects): in warmer stars (T-eff approximate to 6500 K) the inner wings tend to be weaker in non-LTE models, while at lower effective temperatures (T-eff approximate to 4500 K) the inner wings can be stronger in non-LTE models; the non-LTE effects are more severe at lower metallicities. We test our 3D non-LTE models against observations of well-studied benchmark stars. For the Sun, we infer concordant effective temperatures from H alpha, H beta, and H gamma; however the value is too low by around 50 K which could signal residual modelling shortcomings. For other benchmark stars, our 3D non-LTE models generally reproduce the effective temperatures to within 1 sigma uncertainties. For H alpha, the absolute 3D effects and non-LTE effects can separately reach around 100 K, in terms of inferred effective temperatures. For metal-poor turn-off stars, 1D LTE models of H alpha can underestimate effective temperatures by around 150 K. Our 3D non-LTE model spectra are publicly available, and can be used for more reliable spectroscopic effective temperature determinations.

Keywords
radiative transfer, line: formation, line: profiles, stars: atmospheres, stars: late-type
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-361994 (URN)10.1051/0004-6361/201732546 (DOI)000440220000006 ()
Funder
Swedish Research Council, 2015004153Australian Research Council, DP150100250Swedish Research CouncilKnut and Alice Wallenberg Foundation
Available from: 2018-10-03 Created: 2018-10-03 Last updated: 2018-10-03Bibliographically approved
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