E. Depagne

First stars VII - Lithium in extremely metal poor dwarfs

Context. The primordial lithium abundance is a key prediction of models of big bang nucleosynthesis, and its abundance in metal-poor dwarfs (the Spite plateau) is an important, independent observational constraint on such models. Aims. This study aims to determine the level and constancy of the Spite plateau as definitively as possible from homogeneous highquality VLT-UVES spectra of 19 of the most metal-poor dwarf stars known. Methods. Our high-resolution (R ∼ 43 000), high S /N spectra are analysed with OSMARCS 1D LTE model atmospheres and turbospectrum synthetic spectra to determine effective temperatures, surface gravities, and metallicities, as well as Li abundances for our stars. Results. Eliminating a cool subgiant and a spectroscopic binary, we find 8 stars to have −3.5 < [Fe/H] < −3. 0a nd 9s tars with −3.0 < [Fe/H] < −2.5. Our best value for the mean level of the plateau is A(Li) = 2.10 ± 0.09. The scatter around the mean is entirely explained by our estimate of the observational error and does not allow for any intrinsic scatter in the Li abundances. In addition, we conclude that a systematic error of the order of 200 K in any of the current temperature scales remains possible. The iron excitation equilibria in our stars support our adopted temperature scale, which is based on a fit to wings of the Hα line, and disfavour hotter scales, which would lead to a higher Li abundance, but fail to achieve excitation equilibrium for iron. Conclusions. We confirm the previously noted discrepancy between the Li abundance measured in extremely metal-poor turnoff stars and the primordial Li abundance predicted by standard Big-Bang nucleosynthesis models adopting the baryonic density inferred from WMAP. We discuss recent work explaining the discrepancy in terms of diffusion and find that uncertain temperature scales remain a major question.

First Stars II. Elemental abundances in the extremely metal-poor star CS 22949-037 ? A diagnostic of early massive supernovae

CS 22949-037 is one of the most metal-poor giants known ((Fe=H) 4:0), and it exhibits large overabundances of carbon and nitrogen (Norris et al.). Using VLT-UVES spectra of unprecedented quality, regarding resolution and S=N ratio, covering a wide wavelength range (from= 350 to 900 nm), we have determined abundances for 21 elements in this star over a wide range of atomic mass. The major new discovery is an exceptionally large oxygen enhancement, (O=Fe)= 1:97 0:1, as measured from the (O I) line at 630.0 nm. We find an enhancement of (N/Fe) of 2:56 0:2, and a milder one of (C=Fe)= 1:17 0:1, similar to those already reported in the literature. This implies Z? = 0:01 Z. We also find carbon isotopic ratios 12 C/ 13 C= 42: 0a nd 13 C/ 14 N= 0:03 +0:035 0:015 , close to the equilibrium value of the CN cycle. Lithium is not detected. Na is strongly enhanced ((Na=Fe)=+2:1 0:2), while S and K are not detected. The silicon-burning elements Cr and Mn are underabundant, while Co and Zn are overabundant ((Zn=Fe)=+0:7). Zn is measured for the first time in such an extremely metal-poor star. The abundances of the neutron-capture elements Sr, Y, and Ba are strongly decreasing with the atomic number of the element: (Sr=Fe) +0:3, (Y=Fe) 0:1, and (Ba=Fe) 0:6. Among possible progenitors of CS 22949-037, we discuss the pair- instability supernovae. Such very massive objects indeed produce large amounts of oxygen, and have been found to be possible sources of primary nitrogen. However, the predicted odd/even eect is too large, and the predicted Zn abundance much too low. Other scenarios are also discussed. In particular, the yields of a recent model (Z35Z) from Heger and Woosley are shown to be in fair agreement with the observations. The only discrepant prediction is the very low abundance of nitrogen, possibly curable by taking into account other eects such as rotationally induced mixing. Alternatively, the absence of lithium in our star, and the values of the isotopic ratios 12 C/ 13 Ca nd 13 C/ 14 N close to the equilibrium value of the CN cycle, suggest that the CNO abundances now observed might have been altered by nuclear processing in the star itself. A 30-40 M supernova, with fallback, seems the most likely progenitor for CS 22949-037.

First stars IX - Mixing in extremely metal-poor giants. Variation of the 12C/13C, [Na/Mg] and [Al/Mg] ratios

Context: .Extremely metal-poor (EMP) stars preserve a fossil record of the composition of the ISM when the Galaxy formed. It is crucial, however, to verify whether internal mixing has modified their surface composition, especially in the giants where most elements can be studied. Aims: .We aim to understand the CNO abundance variations found in some, but not all EMP field giants analysed earlier. Mixing beyond the first dredge-up of standard models is required, and its origin needs clarification. Methods: .The 12C/^{13C} ratio is the most robust diagnostic of deep mixing, because it is insensitive to the adopted stellar parameters and should be uniformly high in near-primordial gas. We have measured 12C and ^{13C} abundances in 35 EMP giants (including 22 with {[Fe/H] < -3.0}) from high-quality VLT/UVES spectra analysed with LTE model atmospheres. Correlations with other abundance data are used to study the depth of mixing. Results: .The 12C/^{13C} ratio is found to correlate with [C/Fe] (and Li/H), and clearly anti-correlate with [N/Fe], as expected if the surface abundances are modified by CNO processed material from the interior. Evidence for such deep mixing is observed in giants above {log L/L? = 2.6}, brighter than in less metal-poor stars, but matching the bump in the luminosity function in both cases. Three of the mixed stars are also Na- and Al-rich, another signature of deep mixing, but signatures of the ON cycle are not clearly seen in these stars. Conclusions: .Extra mixing processes clearly occur in luminous RGB stars. They cannot be explained by standard convection, nor in a simple way by rotating models. The Na- and Al-rich giants could be AGB stars themselves, but an inhomogeneous early ISM or pollution from a binary companion remain possible alternatives.

First stars X. The nature of three unevolved carbon-enhanced metal-poor stars

Context. On the order of 20% of the very metal-poor stars in the Galaxy exhibit large carbon enhancements. It is important to establish which astrophysical sites and processes are responsible for the elemental abundance patterns of this early Galactic population. Aims. We seek to understand the nature of the progenitors of three main-sequence turnoff Carbon-Enhanced Metal-Poor (CEMP) stars, CS 31080-095, CS 22958-042, and CS 29528-041, based on a detailed abundance analysis. Methods. From high-resolution VLT/UVES spectra (R similar to 43 000), we determine abundances or upper limits for Li, C, N, O, and other important elements, as well as C-12/C-13 isotopic ratios. Results. All three stars have -3.30 <= [Fe/H]<= -2.85 and moderate to high CNO abundances. CS 22958-042 is one of the most carbon-rich CEMP stars known ([C/Fe] = +3.2), while CS 29528-041 (one of the few N-enhanced metal-poor stars known) is one of the most nitrogen rich ([N/Fe] = +3.0). Oxygen is very high in CS 31080-095 ([O/Fe] = +2.35) and in CS 22958-042 ([O/Fe] = +1.35). All three stars exhibit [Sr/Fe] < 0; Ba is not detected in CS 22958-042 ([Ba/Fe] < -0.53),but it is moderately enhanced ([Ba/Fe] similar to 1) in the other two stars. CS 22958-042 displays one of the largest sodium overabundances yet found in CEMP stars ([Na/Fe] = +2.8). CS 22958-042 has C-12/C-13 = 9, similar to most other CEMP stars without enhanced neutron-capture elements, while C-12/C-13 = 40 in CS 31080-095. CS 31080-095 and CS 29528-041 have A(Li) similar to 1.7, below the Spite Plateau, while Li is not detected in CS 22958-042. Conclusions. CS 22958-042 is a CEMP-no star, but the other two stars are in no known class of CEMP star and thus either constitute a new class or are a link between the CEMP-no and CEMP-s classes, adding complexity to the abundance patterns for CEMP stars. We interpret the abundance patterns in our stars to imply that current models for the presumed AGB binary progenitors lack an extra-mixing process, similar to those apparently operating in RGB stars. (Less)

First stars IV. CS 29497-030: Evidence for operation of the s-process at very low metallicity

We present an abundance analysis of the very metal-poor, carbon-enhanced star CS 29497-030. Our results indicate that this unusually hot turno star (Te = 6650 K, logg= 3.5) has a metallicity (Fe/H)= 2:8, and exhibits large overabun- dances of carbon ((C/Fe)=+2.38), nitrogen ((N/Fe)=+1.88), and oxygen ((O/Fe)=+1.67). This star also exhibits a large enhancement in its neutron-capture elements; the pattern follows that expected to arise from the s-process. In particular, the Pb abundance is found to be very high with respect to iron ((Pb/Fe)=+3.5), and also with respect to the second peak s-process elements (e.g., Ba, La, Ce, Nd), which fits into the newly introduced classification of lead (Pb) stars. The known spectroscopic binary status of this star, along with the observed s-process abundance pattern, suggest that it has accreted matter from a com- panion, which formerly was an Asymptotic Giant-Branch (AGB) star. In a preliminary analysis, we have also identified broad absorption lines of metallic species that suggest a large axial rotational velocity for this star, which may be the result of spin-up associated with the accretion of material from its previous AGB companion. In addition, this star is clearly depleted in the light element Li. When considered along with its rather high inferred temperature, these observations are consistent with the expected properties of a very low metallicity halo blue straggler.

First Stars. III. A detailed elemental abundance study of four extremely metal-poor giant stars ?;??

This paper reports detailed abundance analyses for four extremely metal-poor (XMP) giant stars with (Fe=H)< 3:8, based on high-resolution, high-S=N spectra from the ESO VLT (Kueyen/UVES) and LTE model atmosphere calculations. The derived (/Fe) ratios in our sample exhibit a small dispersion, confirming previous findings in the literature, i.e. a constant overabundance of the-elements with a very small (if any) dependence on (Fe/H). In particular, the very small scatter we determine for (Si/Fe) suggests that this element shows a constant overabundance at very low metallicity, a conclusion which could not have been derived from the widely scattered (Si/Fe) values reported in the literature for less metal-poor stars. For the iron-peak elements, our precise abundances for the four XMP stars in our sample confirm the decreasing trend of Cr and Mn with decreasing (Fe/H), as well as the increasing trend for Co and the absence of any trend for Sc and Ni. In contrast to the significant spread of the ratios (Sr/Fe) and (Ba/Fe), we find (Sr/Ba) in our sample to be roughly solar, with a much lower dispersion than previously found for stars in the range 3:5< (Fe=H)< 2:5.

Oxygen Abundance in the Template Halo Giant HD 122563

HD 122563 is a well-known bright (V ¼ 6:2) halo giant of low metallicity ((Fe/H ��� 2:7). We have observed HD 122563 for infrared OH lines at 1.5-1.7 lm in the H band with the NIRSPEC high-resolution spectrograph at the 10 m Keck Telescope. Optical spectra were obtained with the UVES spectrograph at the 8 m VLT UT2 telescope at ESO (Paranal) and the FEROS spectrograph at ESO (La Silla). Based on the opti- cal high-resolution data, a detailed analysis has been carried out, and data on the forbidden (O i) 6300 Aline, unblended by telluric or sky lines, was obtained with the FEROS spectrograph. Signal-to-noise ratios of 200- 400 were obtained at resolutions of 37,000 in the H band and 45,000 in the optical. For the analysis we have adopted a photometric effective temperature Teff ¼ 4600 K. Two values for the gravity were adopted: a value deduced from ionization equilibrium, log g ¼ 1:1, with corresponding metallicity (Fe/H �¼� 2:8 and micro- turbulence velocity vt ¼ 2: 0k m s � 1 ; and log g ¼ 1:5, derived from the Hipparcos parallax, implying (Fe/H �¼� 2:71 and vt ¼ 2: 0k m s � 1 . The forbidden (O i) 6300 Aand the permitted O i 7771 Alines give O/Fe ratios essentially insensitive to model parameter variations, whereas the oxygen abundances from OH lines are sensitive to gravity, giving (O/Fe �¼þ 0:9 and +0.7, respectively, for log g ¼ 1:1 and 1.5. We derive the following oxygen abundances: for model 1, (O/Fe �¼þ 0:6, +1.1, and +0.9; and for model 2, (O/Fe �¼þ 0:6, +1.1, and +0.7, based on the (O i) 6300 A ˚ ,O i 7771 A ˚ , and IR OH 1.6 lm lines, respectively. The different oxygen abundance indicators give different oxygen abundances, illustrating the problem of oxy- gen abundance derivation in metal-poor giants. This is important because the age of globular clusters and the production of Li, Be, and B from spallation of C, N, and O atoms in the early Galaxy depend on the oxygen abundance adopted for the metal-poor stars. Subject headings: stars: abundances — stars: individual (HD 122563) — stars: Population II On-line material: machine-readable table

High-resolution abundance analysis of red giants in the globular cluster NGC 6522

Context. The [Sr/Ba] and [Y/Ba] scatter observed in some galactic halo stars that are very metal-poor and in a few individual stars of the oldest known Milky Way globular cluster NGC 6522 have been interpreted as evidence of early enrichment by massive fastrotating stars (spinstars). Because NGC 6522 is a bulge globular cluster, the suggestion was that not only the very-metal poor halo stars, but also bulge stars at [Fe/H] ∼−1 could be used as probes of the stellar nucleosynthesis signatures from the earlier generations of massive stars, but at much higher metallicity. For the bulge the suggestions were based on early spectra available for stars in NGC 6522, with a medium resolution of R ∼ 22 000 and a moderate signal-to-noise ratio. Aims. The main purpose of this study is to re-analyse the NGC 6522 stars reported previously by using new high-resolution (R ∼ 45 000) and high signal-to-noise spectra (S /N > 100). We aim at re-deriving their stellar parameters and elemental ratios, in particular the abundances of the neutron-capture s-process-dominated elements such as Sr, Y, Zr, La, and Ba, and of the r-element Eu. Methods. High-resolution spectra of four giants belonging to the bulge globular cluster NGC 6522 were obtained at the 8m VLT UT2-Kueyen telescope with the UVES spectrograph in FLAMES-UVES configuration. The spectroscopic parameters were derived based on the excitation and ionization equilibrium of Fe i and Fe ii. Results. Our analysis confirms a metallicity [Fe/H] = −0.95 ± 0.15 for NGC 6522 and the overabundance of the studied stars in Eu (with +0.2 < [Eu/Fe] < +0.4) and alpha-elements O and Mg. The neutron-capture s-element-dominated Sr, Y, Zr, Ba, and La now show less pronounced variations from star to star. Enhancements are in the range 0.0 < [Sr/Fe] < +0.4, +0.23 < [Y/Fe] < +0.43, 0.0 < [Zr/Fe] < +0.4, 0.0 < [La/Fe] < +0.35, and 0.05 < [Ba/Fe] < +0.55. Conclusions. The very high overabundances of [Y/Fe] previously reported for the four studied stars is not confirmed with the new high-quality spectra. The moderate enhancement in [Sr/Fe] previously reported for one of the re-studied stars is confirmed, but the strong enhancements of this ratio for the other two stars are not confirmed. Despite the moderate enhancements found for the neutroncapture s-element-dominated species, none of the four stars studied here show positive values for all [Sr/Ba], [Y/Ba] and [Zr/Ba] ratios. The re-studied stars are now compatible not only with the interpretation that the s-process enhancements in these very old stars are due to spinstars, but also with alternative models such as mass transfer from s-process-rich AGB stars. Note, however, that when our results are interpreted in the context of more extended datasets from the literature, the spinstar scenario still seems to be favoured.

Looking for imprints of the first stellar generations in metal-poor bulge field stars

Context. Efforts to look for signatures of the first stars have concentrated on metal-poor halo objects. However, the low end of the bulge metallicity distribution has been shown to host some of the oldest objects in the Milky Way and hence this Galactic component potentially offers interesting targets to look at imprints of the first stellar generations. As a pilot project, we selected bulge field stars already identified in the ARGOS survey as having [Fe/H] ~ -1 and oversolar [alpha/Fe] ratios, and we used FLAMES-UVES to obtain detailed abundances of key elements that are believed to reveal imprints of the first stellar generations. Aims. The main purpose of this study is to analyse selected ARGOS stars using new high-resolution (R~45,000) and high-signal-to-noise (S/N >100) spectra. We aim to derive their stellar parameters and elemental ratios, in particular the abundances of C, N, the alpha-elements O, Mg, Si, Ca, and Ti, the odd-Z elements Na and Al, the neutron-capture s-process dominated elements Y, Zr, La, and Ba, and the r-element Eu. Methods. High-resolution spectra of five field giant stars were obtained at the 8m VLT UT2-Kueyen telescope with the UVES spectrograph in FLAMES-UVES configuration. Spectroscopic parameters were derived based on the excitation and ionization equilibrium of Fe I and Fe II. The abundance analysis was performed with a MARCS LTE spherical model atmosphere grid and the Turbospectrum spectrum synthesis code.

Abundance of Heavy Elements in Extremely Metal-Poor Stars

This paper reports on the abundance determination of neutron‐capture elements in 32 extremely metal‐poor stars. The study is based on the analysis of high quality spectra obtained with UVES+Kueyen. The results are compared with the most recent analyses of spectra mostly taken with other 10m class telescopes.