NLTE determination of the sodium 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:10.1051/0004-6361:20066232

Cited by 138 publications

(182 citation statements)

References 29 publications

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“…This behaviour is observed, for instance, for the sodium D lines (see Fig. 6 of Andrievsky et al 2007). These considerations render NLTE computations for Cu very desirable.…”

Section: Discussionmentioning

confidence: 58%

Cu i resonance lines in turn-off stars of NGC 6752 and NGC 6397

Bonifacio

Caffau

Ludwig

2010

A&A

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Context.Copper is an element whose interesting evolution with metallicity is not fully understood. Observations of copper abundances rely on a very limited number of lines, the strongest are the Cu i lines of Mult. 1 at 324.7 nm and 327.3 nm which can be measured even at extremely low metallicities. Aims. We investigate the quality of these lines as abundance indicators. Methods. We measure these lines in two turn-off (TO) stars in the Globular Cluster NGC 6752 and two TO stars in the Globular Cluster NGC 6397 and derive abundances with 3D hydrodynamical model atmospheres computed with the CO5BOLD code. These abundances are compared to the Cu abundances measured in giant stars of the same clusters, using the lines of Mult. 2 at 510.5 nm and 578.2 nm. Results. The abundances derived from the lines of Mult. 1 in TO stars differ from the abundances of giants of the same clusters. This is true both using CO5BOLD models and using traditional 1D model atmospheres. The LTE 3D corrections for TO stars are large, while they are small for giant stars.Conclusions. The Cu i resonance lines of Mult. 1 are not reliable abundance indicators. It is likely that departures from LTE should be taken into account to properly describe these lines, although it is not clear if these alone can account for the observations. An investigation of these departures is indeed encouraged for both dwarfs and giants. Our recommendation to those interested in the study of the evolution of copper abundances is to rely on the measurements in giants, based on the lines of Mult. 2. We caution, however, that NLTE studies may imply a revision in all the Cu abundances, both in dwarfs and giants.

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“…This behaviour is observed, for instance, for the sodium D lines (see Fig. 6 of Andrievsky et al 2007). These considerations render NLTE computations for Cu very desirable.…”

Section: Discussionmentioning

confidence: 58%

Cu i resonance lines in turn-off stars of NGC 6752 and NGC 6397

Bonifacio

Caffau

Ludwig

2010

A&A

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“…a possible dependence of NLTE corrections on atmospheric parameters. Andrievsky et al (2007) calculated NLTE corrections for a grid of parameters ranging from −2.5 to −4.0 in metallicity, 4500 K to 6350 K in effective temperature and 0.8 to 4.1 in log g (see their Table 2). They applied their results to the samples of giant and turnoff stars of Cayrel et al (2004);and Bonifacio et al (2006).…”

Section: Nlte Effectsmentioning

confidence: 99%

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

Tafelmeyer

Jablonka

Hill

et al. 2010

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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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“…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 sodium abundance in a homogeneous sample of extremely metal-poor stars

Cited by 138 publications

References 29 publications

Cu i resonance lines in turn-off stars of NGC 6752 and NGC 6397

Cu i resonance lines in turn-off stars of NGC 6752 and NGC 6397

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

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

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