NLTE determination of the aluminium abundance in a homogeneous sample of extremely metal-poor stars

Andrievsky, S. M.; Spite, M.; Коротин, С. А.; Spite, F.; Bonifacio, P.; Cayrel, Roger; Hill, V.; François, P.

doi:10.1051/0004-6361:20078837

Cited by 145 publications

(183 citation statements)

References 35 publications

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“…Just as in the analysis of sodium, only the resonance lines of aluminum can be detected at low metallicities, and they are affected by NLTE effects. Andrievsky et al (2008) calculated NLTE corrections for Al in giants, but for temperatures above 4700 K, implying an extrapolation by up to almost 500 K for some of our stars. Furthermore while they mentioned the dependence of the NLTE corrections on line strength, unfortunately they did not give explicit values.…”

Section: Nlte Effectsmentioning

confidence: 82%

“…In metal-rich environments however, with a sufficient amount of neutron rich elements that might act as neutron donors, the yields of Al and Na depend on the neutron excess and their abundances are expected to be metallicity dependent (Woosley & Weaver 1995;Gehren et al 2006). Andrievsky et al (2007Andrievsky et al ( , 2008 found constant [Na/Fe] and [Al/Fe] values of −0.21 ± 0.13 and −0.06 ± 0.10, respectively.…”

Section: Discussionmentioning

confidence: 99%

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Extremely metal-poor stars in classical dwarf spheroidal galaxies: Fornax, Sculptor, and Sextans

Tafelmeyer

Jablonka

Hill

et al. 2010

A&A

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We present the results of a dedicated search for extremely metal-poor stars in the Fornax, Sculptor, and Sextans dSphs. Five stars were selected from two earlier VLT/Giraffe and HET/HRS surveys and subsequently followed up at high spectroscopic resolution with VLT/UVES. All of them turned out to have [Fe/H] < ∼ −3 and three stars are below [Fe/H] ∼ −3.5. This constitutes the first evidence that the classical dSphs Fornax and Sextans join Sculptor in containing extremely metal-poor stars and suggests that all of the classical dSphs contain extremely metal-poor stars. One giant in Sculptor at [Fe/H] = −3.96 ± 0.06 is the most metal-poor star ever observed in an external galaxy. We carried out a detailed analysis of the chemical abundances of the α, iron peak, and the heavy elements, and we performed a comparison with the Milky Way halo and the ultra faint dwarf stellar populations. Carbon, barium, and strontium show distinct features characterized by the early stages of galaxy formation and can constrain the origin of their nucleosynthesis.

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Section: Nlte Effectsmentioning

confidence: 82%

Section: Discussionmentioning

confidence: 99%

Extremely metal-poor stars in classical dwarf spheroidal galaxies: Fornax, Sculptor, and Sextans

Tafelmeyer

Jablonka

Hill

et al. 2010

A&A

Self Cite

151

287

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“…We did not apply NLTE corrections to our abundances, because the used doublet should be relatively free from NLTE effects (Gehren et al 2004;Andrievsky et al 2008). It is interesting to recall here that both Johnson & Pilachowski (2010) and Marino et al (2011) found a tendency, for stars richer than [Fe/H] −1.0 dex, to exhibit no (anti-)correlation among the usual elements (Na, Al, Mg, C, N, and O).…”

Section: Anti-correlationsmentioning

confidence: 87%

The subgiant branch ofω Centauri seen through high-resolution spectroscopy

Pancino

Mucciarelli

Bonifacio

et al. 2011

A&A

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We analyze spectra of 18 stars belonging to the faintest subgiant branch in ω Centauri (the SGB-a), obtained with GIRAFFE@VLT at a resolution of R 17 000 and a S /N ratio between 25 and 50. We measure abundances of Al, Ba, Ca, Fe, Ni, Si, and Ti and we find that these stars have [Fe/H] = −0.73 ± 0.14 dex, similarly to the corresponding red giant branch population (the RGB-a). We also measure [α/Fe] = +0.40 ± 0.16 dex, and [Ba/Fe] = +0.87 ± 0.23 dex, in general agreement with past studies. It is very interesting to note that we found a uniform Al abundance, [Al/Fe] = +0.32 ± 0.14 dex, for all the 18 SGB-a stars analysed here, thus supporting past evidence that the usual (anti-)correlations are not present in this population, and suggesting a non globular cluster-like origin of this particular population. In the dwarf galaxy hypothesis for the formation of ω Cen, this population might be the best candidate for the field population of its putative parent galaxy, although some of its properties appear contradictory. It has also been suggested that the most metal-rich population in ω Cen is significantly enriched in helium. If this is true, the traditional abundance analysis techniques, based on model atmospheres with normal helium content, might lead to errors. We have computed helium enhanced atmospheres for three stars in our sample and verified that the abundance errors due to the use of non-enhanced atmospheres are negligible. Additional, indirect support to the enhanced helium content of the SGB-a population comes from our Li upper limits.

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“…Upper limits are treated as detections for the second generation averages for Al (two stars) and O (one star) when computing the mean and standard deviation. The literature data for halo field stars are assembled as follows; bullets represent Gehren et al (2006), star symbols represent Andrievsky et al (2007Andrievsky et al ( , 2008Andrievsky et al ( , 2010, open triangles Cayrel et al (2004), and open diamonds Nissen et al (2002).…”

Section: O Na Mg and Almentioning

confidence: 99%

Tracing the evolution of NGC 6397 through the chemical composition of its stellar populations

Lind

Charbonnel

Decressin

et al. 2011

A&A

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Context. The chemical compositions of globular clusters provide important information on the star formation that occurred at very early times in the Galaxy. In particular the abundance patterns of elements with atomic number z ≤ 13 may shed light on the properties of stars that early on enriched parts of the star-forming gas with the rest-products of hydrogen-burning at high temperatures. Aims. We analyse and discuss the chemical compositions of a large number of elements in 21 red giant branch stars in the metal-poor globular cluster NGC 6397. We compare the derived abundance patterns with theoretical predictions in the framework of the "wind of fast rotating massive star"-scenario. Methods. High-resolution spectra were obtained with the FLAMES/UVES spectrograph on the VLT. We determined non-LTE abundances of Na, and LTE abundances for the remaining 21 elements, including O (from the [OI] line at 630 nm), Mg, Al, α, iron-peak, and neutron-capture elements, many of which had not been previously analysed for this cluster. We also considered the influence of possible He enrichment in the analysis of stellar spectra. Results. We find that the Na abundances of evolved, as well as unevolved, stars in NGC 6397 show a distinct bimodality, which is indicative of two stellar populations: one primordial stellar generation of composition similar to field stars, and a second generation that is polluted with material processed during hydrogen-burning, i.e., enriched in Na and Al and depleted in O and Mg. The red giant branch exhibits a similar bimodal distribution in the Strömgren colour index c y = c 1 − (b − y), implying that there are also large differences in the N abundance. The two populations have the same composition for all analysed elements heavier than Al, within the measurement uncertainty of the analysis, with the possible exception of [Y/Fe]. Using two stars with almost identical stellar parameters, one from each generation, we estimate the difference in He content, ΔY = 0.01 ± 0.06, given the assumption that the mass fraction of iron is the same for the stars. Conclusions. NGC 6397 hosts two stellar populations that have different chemical compositions of N, O, Na, Mg, and probably Al. The cluster is dominated (75%) by the second generation. We show that massive stars of the first generation can be held responsible for the abundance patterns observed in the second generation long-lived stars of NGC 6397. We estimate that the initial mass of this globular cluster is at least ten times higher than its present-day value.

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NLTE determination of the aluminium abundance in a homogeneous sample of extremely metal-poor stars

Cited by 145 publications

References 35 publications

Extremely metal-poor stars in classical dwarf spheroidal galaxies: Fornax, Sculptor, and Sextans

Extremely metal-poor stars in classical dwarf spheroidal galaxies: Fornax, Sculptor, and Sextans

The subgiant branch ofω Centauri seen through high-resolution spectroscopy

Tracing the evolution of NGC 6397 through the chemical composition of its stellar populations

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