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| 2. |
- 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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| 3. |
- Lucchesi, R., et al.
(författare)
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The Pristine survey – XV. A CFHT ESPaDOnS view on the Milky Way halo and disc populations
- 2022
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 511:1, s. 1004-1021
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Tidskriftsartikel (refereegranskat)abstract
- We present a one-dimensional, local thermodynamic equilibrium homogeneous analysis of 132 stars observed at high resolutionwith ESPaDOnS. This represents the largest sample observed at high resolution (R ∼ 40 000) from the Pristine survey. Thissample is based on the first version of the Pristine catalogue and covers the full range of metallicities from [Fe/H] ∼−3 to∼+0.25, with nearly half of our sample (58 stars) composed of very metal-poor (VMP) stars ([Fe/H] ≤ −2). This wide rangeof metallicities provides the opportunity of a new detailed study of the Milky Way stellar population. Because it includes bothdwarf and giant stars, it also enables the analysis of any potential bias induced by the Pristine selection process. Based on GaiaEDR3, the orbital analysis of this Pristine-ESPaDOnS sample shows that it is composed of 65 halo stars and 67 disc stars. Aftera general assessment of the sample chemical properties with the α-elements Mg and Ca, we focus on the abundance of carbonand the neutron capture elements Ba and Sr. While most of our VMP subsample is carbon normal, we also find that 14 starsout of the 38 stars with [Fe/H] ≤ −2 and measured carbon abundances turn out to be carbon-enhanced metal-poor (CEMP)stars. We show that these CEMP stars are nearly exclusively (i.e. 12 stars out of 14) in the regime of low luminosity, unevolved,dwarf stars, which we interpret as the consequence of bias of the Pristine filter against C-rich giants. Among the VMP stars,we identify two CEMP stars with no enhancement in neutron-capture process elements and another one enriched in s-processelement. Finally, one VMP star is found with a very low [Sr/Fe] abundance ratio for its metallicity, as expected if it had beenaccreted from an ultra-faint dwarf galaxy.
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| 4. |
- Tautvaišiene, G., et al.
(författare)
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Gaia -ESO Survey : Detailed elemental abundances in red giants of the peculiar globular cluster NGC 1851
- 2022
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 658
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Tidskriftsartikel (refereegranskat)abstract
- Context. NGC 1851 is one of several globular clusters for which multiple stellar populations of the subgiant branch have been clearly identified and a difference in metallicity detected. A crucial piece of information on the formation history of this cluster can be provided by the sum of A(C+N+O) abundances. However, these values have lacked a general consensus thus far. The separation of the subgiant branch can be based on age and/or A(C+N+O) abundance differences. Aims. Our main aim was to determine carbon, nitrogen, and oxygen abundances for evolved giants in the globular cluster NGC 1851 in order to check whether or not the double populations of stars are coeval. Methods. High-resolution spectra, observed with the FLAMES-UVES spectrograph on the ESO VLT telescope, were analysed using a differential model atmosphere method. Abundances of carbon were derived using spectral synthesis of the C2 band heads at 5135 and 5635.5 Å. The wavelength interval 6470-6490 Å, with CN features, was analysed to determine nitrogen abundances. Oxygen abundances were determined from the [O I] line at 6300 Å. Abundances of other chemical elements were determined from equivalent widths or spectral syntheses of unblended spectral lines. Results. We provide abundances of up to 29 chemical elements for a sample of 45 giants in NGC 1851. The investigated stars can be separated into two populations with a difference of 0.07 dex in the mean metallicity, 0.3 dex in the mean C/N, and 0.35 dex in the mean s-process dominated element-to-iron abundance ratios [s/Fe]. No significant difference was determined in the mean values of A(C+N+O) as well as in abundance to iron ratios of carbon, α- and iron-peak-elements, and of europium. Conclusions. As the averaged A(C+N+O) values between the two populations do not differ, additional evidence is given that NGC 1851 is composed of two clusters, the metal-rich cluster being by about 0.6 Gyr older than the metal-poor one. A global overview of NGC 1851 properties and the detailed abundances of chemical elements favour its formation in a dwarf spheroidal galaxy that was accreted by the Milky Way.
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