| 1. |
- Boyarchuk, A. A., et al.
(författare)
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Scientific problems addressed by the Spektr-UV space project (world space Observatory-Ultraviolet)
- 2016
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Ingår i: Astronomy reports (Print). - 1063-7729 .- 1562-6881. ; 60:1, s. 1-42
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Tidskriftsartikel (refereegranskat)abstract
- The article presents a review of scientific problems and methods of ultraviolet astronomy, focusing on perspective scientific problems (directions) whose solution requires UV space observatories. These include reionization and the history of star formation in the Universe, searches for dark baryonic matter, physical and chemical processes in the interstellar medium and protoplanetary disks, the physics of accretion and outflows in astrophysical objects, from Active Galactic Nuclei to close binary stars, stellar activity (for both low-mass and high-mass stars), and processes occurring in the atmospheres of both planets in the solar system and exoplanets. Technological progress in UV astronomy achieved in recent years is also considered. The well advanced, international, Russian-led Spektr-UV (World Space Observatory-Ultraviolet) project is described in more detail. This project is directed at creating a major space observatory operational in the ultraviolet (115-310 nm). This observatory will provide an effective, and possibly the only, powerful means of observing in this spectral range over the next ten years, and will be an powerful tool for resolving many topical scientific problems.
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| 2. |
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| 3. |
- Martin, N. F., et al.
(författare)
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A stellar stream remnant of a globular cluster below the metallicity floor
- 2022
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 601:7891, s. 45-48
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Tidskriftsartikel (refereegranskat)abstract
- Stellar ejecta gradually enrich the gas out of which subsequent stars form, making the least chemically enriched stellar systems direct fossils of structures formed in the early Universe1. Although a few hundred stars with metal content below 1,000th of the solar iron content are known in the Galaxy2–4, none of them inhabit globular clusters, some of the oldest known stellar structures. These show metal content of at least approximately 0.2% of the solar metallicity ([Fe / H] ≳ − 2.7). This metallicity floor appears universal5,6, and it has been proposed that protogalaxies that merged into the galaxies we observe today were simply not massive enough to form clusters that survived to the present day7. Here we report observations of a stellar stream, C-19, whose metallicity is less than 0.05% of the solar metallicity ([Fe/H]=−3.38±0.06(statistical)±0.20(systematic)). The low metallicity dispersion and the chemical abundances of the C-19 stars show that this stream is the tidal remnant of the most metal-poor globular cluster ever discovered, and is significantly below the purported metallicity floor: clusters with significantly lower metallicities than observed today existed in the past and contributed their stars to the Milky Way halo.
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| 4. |
- Gruyters, Pieter, et al.
(författare)
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Atomic diffusion and mixing in old stars IV. Weak abundance trends in the globular cluster NGC6752
- 2013
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 555, s. A31-
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Tidskriftsartikel (refereegranskat)abstract
- Context. Atomic diffusion in stars can create systematic trends of surface abundances with evolutionary stage. Globular clusters off er useful laboratories to put observational constraints on this theory as one needs to compare abundances in unevolved and evolved stars, all drawn from the same stellar population. Aims. Atomic diffusion and additional mixing has been shown to be at work in the globular cluster NGC6397 at a metallicity of [Fe/H] similar to -2.1. We investigate possible abundance trends in Li, Mg, Ca, Ti, Sc, and Fe with evolutionary stage in another globular cluster NGC6752 at a metallicity of [Fe/H] similar to -1.6. This in order to better constrain stellar structure models including atomic diffusion and additional mixing. Methods. We performed a differential abundance analysis on VLT/FLAMES-UVES data of 16 stars in four groups between the turnoff point and the red giant branch. Continuum normalisation of the stellar spectra was performed in an automated way using DAOSPEC. Differential abundances relative to the sun were derived by fitting synthetic spectra to individual lines in the stellar spectrum. Results. We find weak systematic abundance trends with evolutionary phase for Fe, Sc, Ti, and Ca. The individual trends are weaker than the trends in NGC6397 and only significant at the 1-sigma level. However, the combined trend shows a significance on the 2-sigma level. The trends are best explained by stellar-structure models including atomic diffusion with more efficient additional mixing than needed in NGC6397. The model allows to correct for sub-primordial stellar lithium abundances of the stars on the Spite plateau. Conclusions. Abundance trends for groups of elements, differently affected by atomic diffusion and additional mixing, are identified. Although the significance of the trends is weak, they all seem to indicate that atomic diffusion is operational along the evolutionary sequence of NGC6752. The trends are weaker than those observed in NGC6397, which is perhaps due to more efficient mixing. Using models of atomic diffusion including efficient additional mixing, we find a diffusion-corrected primordial lithium abundance of log epsilon(Li) = 2.58 +/- 0.10, in agreement with WMAP-calibrated Big-Bang nucleosynthesis predictions within the mutual 1-sigma uncertainties.
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| 5. |
- Kielty, Collin L., et al.
(författare)
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The Pristine survey - XII. Gemini-GRACES chemo-dynamical study of newly discovered extremely metal-poor stars in the Galaxy
- 2021
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 506:1, s. 1438-1461
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Tidskriftsartikel (refereegranskat)abstract
- High-resolution optical spectra of 30 metal-poor stars selected from the Pristine survey are presented, based on observations taken with the Gemini Observatory GRACES spectrograph. Stellar parameters Teff and log g are determined using a Gaia DR2 colour–temperature calibration and surface gravity from the Stefan–Boltzmann equation. GRACES spectra are used to determine chemical abundances (or upper limits) for 20 elements (Li, O, Na, Mg, K, Ca, Ti, Sc, Cr, Mn, Fe, Ni, Cu, Zn, Y, Zr, Ba, La, Nd, Eu). These stars are confirmed to be metal-poor ([Fe/H] < −2.5), with higher precision than from earlier medium-resolution analyses. The chemistry for most targets is similar to other extremely metal-poor stars in the Galactic halo. Three stars near [Fe/H] = −3.0 have unusually low Ca and high Mg, suggestive of contributions from few SN II where alpha-element formation through hydrostatic nucleosynthesis was more efficient. Three new carbon-enhanced metal-poor (CEMP) stars are also identified (two CEMP-s and one potential CEMP-no star) when our chemical abundances are combined with carbon from previous medium-resolution analyses. The GRACES spectra also provide precision radial velocities (σRV ≤ 0.2 km s−1) for dynamical orbit calculations with the Gaia DR2 proper motions. Most of our targets are dynamically associated with the Galactic halo; however, five stars with [Fe/H] < −3 have planar-like orbits, including one retrograde star. Another five stars are dynamically consistent with the Gaia-Sequoia accretion event; three have typical halo [α/Fe] ratios for their metallicities, whereas two are [Mg/Fe]-deficient, and one is a new CEMP-s candidate. These results are discussed in terms of the formation and early chemical evolution of the Galaxy.
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| 7. |
- Velichko, A. B., et al.
(författare)
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Formation of Zr I and II lines under non-LTE conditions of stellar atmospheres
- 2010
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Ingår i: Astronomy letters. - : Pleiades Publishing Ltd. - 1063-7737 .- 1562-6873. ; 36:9, s. 664-679
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Tidskriftsartikel (refereegranskat)abstract
- The formation of Zr I and Zr II lines in stellar atmospheres under non-LTE conditions has been considered for the first time. A model zirconium atom has been composed using 148 Zr I levels, 772 Zr II levels, and the ground Zr III state. Non-LTE calculations have been performed for model atmospheres with T eff = 5500 and 6000 K, log g = 2.0 and 4.0, [M/H] = −3, −2, −1, 0. In the entire investigated range of parameters, the Zr I levels are shown to be underpopulated relative to their LTE populations in the line formation region. In contrast, the excited Zr II levels are overpopulated, while the ground state and lower excited levels of Zr II retain their LTE populations. Since the non-LTE effects cause the Zr I and Zr II spectral lines being investigated to weaken, the non-LTE corrections to the abundance derived from Zr I and Zr II lines are positive. For Zr II lines, they increase with decreasing metallicity and surface gravity up to 0.34 dex for the model with T eff = 5500, log g = 2.0, and [M/H] = −2. The non-LTE effects depend weakly on temperature. The non-LTE corrections for Zr I lines reach 0.33 dex for solar-metallicity models. Zr I and Zr II lines in the solar spectrum have been analyzed. The non-LTE zirconium abundances derived from lines in the two ionization stages are shown to agree between themselves within the error limits, while the LTE abundance difference is 0.28 dex. The zirconium abundance in the solar atmosphere (averaged over Zr I and Zr II lines) is log ɛZr,⊙ = 2.63 ± 0.07.
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