P. Bonifacio

The 4MOST instrument concept overview

The 4MOST[1] instrument is a concept for a wide-field, fibre-fed high multiplex spectroscopic instrument facility on the ESO VISTA telescope designed to perform a massive (initially >25x106 spectra in 5 years) combined all-sky public survey. The main science drivers are: Gaia follow up of chemo-dynamical structure of the Milky Way, stellar radial velocities, parameters and abundances, chemical tagging; eROSITA follow up of cosmology with x-ray clusters of galaxies, X-ray AGN/galaxy evolution to z~5, Galactic X-ray sources and resolving the Galactic edge; Euclid/LSST/SKA and other survey follow up of Dark Energy, Galaxy evolution and transients. The surveys will be undertaken simultaneously requiring: highly advanced targeting and scheduling software, also comprehensive data reduction and analysis tools to produce high-level data products. The instrument will allow simultaneous observations of ~1600 targets at R~5,000 from 390-900nm and ~800 targets at R<18,000 in three channels between ~395-675nm (channel bandwidth: 45nm blue, 57nm green and 69nm red) over a hexagonal field of view of ~ 4.1 degrees. The initial 5-year 4MOST survey is currently expect to start in 2020. We provide and overview of the 4MOST systems: optomechanical, control, data management and operations concepts; and initial performance estimates.

The O-Na and Mg-Al anticorrelations in turn-off and early subgiants in globular clusters

High dispersion spectra (R > 40 000) for a quite large number of stars at the main sequence turn-o and at the base of the giant branch in NGC 6397 and NGC 6752 were obtained with the UVES on Kueyen (VLT UT2). The (Fe/H) values we found are 2:03 0:02 0:04 and 1:42 0:02 0:04 for NGC 6397 and NGC 6752 respectively, where the rst error bars refer to internal and the second ones to systematic errors (within the abundance scale dened by our analysis of 25 subdwarfs with good Hipparcos parallaxes). In both clusters the (Fe/H)s obtained for TO-stars agree perfectly (within a few percent) with that obtained for stars at the base of the RGB. The (O=Fe) = 0:21 0:05 value we obtain for NGC 6397 is quite low, but it agrees with previous results obtained for giants in this cluster. Moreover, the star-to-star scatter in both O and Fe is very small, indicating that this small mass cluster is chemically very homogenous. On the other hand, our results show clearly and for the rst time that the O-Na anticorrelation (up to now seen only for stars on the red giant branches of globular clusters) is present among unevolved stars in the globular cluster NGC 6752, a more massive cluster than NGC 6397. A similar anticorrelation is present also for Mg and Al, and C and N. It is very dicult to explain the observed Na-O, and Mg-Al anticorrelation in NGC 6752 stars by a deep mixing scenario; we think it requires some non internal mechanism.

Proceedings of the SPIE

Wide-field multi-object spectroscopy is a high priority for European astronomy over the next decade. Most 8-10m telescopes have a small field of view, making 4-m class telescopes a particularly attractive option for wide-field instruments. We present a science case and design drivers for a wide-field multi-object spectrograph (MOS) with integral field units for the 4.2-m William Herschel Telescope (WHT) on La Palma. The instrument intends to take advantage of a future prime-focus corrector and atmospheric-dispersion corrector (Agocs et al, this conf.) that will deliver a field of view 2 deg in diameter, with good throughput from 370 to 1,000 nm. The science programs cluster into three groups needing three different resolving powers R: (1) high-precision radial-velocities for Gaia-related Milky Way dynamics, cosmological redshift surveys, and galaxy evolution studies (R = 5,000), (2) galaxy disk velocity dispersions (R = 10,000) and (3) high-precision stellar element abundances for Milky Way archaeology (R = 20,000). The multiplex requirements of the different science cases range from a few hundred to a few thousand, and a range of fibre-positioner technologies are considered. Several options for the spectrograph are discussed, building in part on published design studies for E-ELT spectrographs. Indeed, a WHT MOS will not only efficiently deliver data for exploitation of important imaging surveys planned for the coming decade, but will also serve as a test-bed to optimize the design of MOS instruments for the future E-ELT.

Measurement of stellar age from uranium decay

The ages of the oldest stars in the Galaxy indicate when star formation began, and provide a minimum age for the Universe. Radioactive dating of meteoritic material and stars relies on comparing the present abundance ratios of radioactive and stable nuclear species to the theoretically predicted ratios of their production. The radioisotope 232Th (half-life 14 Gyr) has been used to date Galactic stars, but it decays by only a factor of two over the lifetime of the Universe. 238U (half-life 4.5 Gyr) is in principle a more precise age indicator, but even its strongest spectral line, from singly ionized uranium at a wavelength of 385.957 nm, has previously not been detected in stars. Here we report a measurement of this line in the very metal-poor star CS31082-0018, a star which is strongly overabundant in its heavy elements. The derived uranium abundance, log(U/H) = -13.7 ± 0.14 ± 0.12 yields an age of 12.5 ± 3 Gyr, though this is still model dependent. The observation of this cosmochronometer gives the most direct age determination of the Galaxy. Also, with improved theoretical and laboratory data, it will provide a highly precise lower limit to the age of the Universe.

The Primordial lithium abundance

Lithium abundances in a selected sample of halo stars have been revised by using the new accurate IRFM effective temperatures by Alonso, Arribas & Mart´onez-Roger (1996a). From 41 plateau stars (Teff > 5700 and [Fe/H] � -1.5) we found no evidence for intrinsic dispersion, a tiny trend with Teff and no trend with [Fe/H]. The trend with the Teff is fully consistent with the standard Li isochrones of Deliyannis, Demarque & Kawaler (1990) implying a primordial value for Li of A(Li)= 2.238 ± 0.0121� ± 0.05sys . The present results argue against any kind of depletion predicted by diffusion, rotational mixing or stellar winds. Therefore the Li observed in Pop II stars provides a direct and reliable estimate of the baryonic density that can rival other baryonic indicators such as the deuterium in high redshift systems. The present upwards revision of primordial Li in the framework of SBBN gives at 1� two solutions for the baryonic density: Bh 2 = 0.0062 +0.0018

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.

The lithium content of the globular cluster NGC 6397

We make use of high resolution, high signal-to-noise ratio spectra of 12 turn-o stars in the metal-poor globular cluster NGC 6397 to measure its lithium content. We conclude that they all have the same lithium abundance A(Li)= 2:34 with a standard deviation of 0.056 dex. We use this result, together with Monte Carlo simulations, to estimate that the maximum allowed intrinsic scatter is of the order of 0.035 dex. This is a new stringent constraint to be fulfilled by stellar models which predict Li depletion. We argue that although a mild depletion of 0.1-0.2 dex, as predicted by recent models, cannot be ruled out, there is no compelling reason for it. This fact, together with the good agreement with the Li abundance observed in field stars, supports the primordial origin of lithium in metal-poor stars. Taking the above value as the primordial lithium abundance implies a cosmic baryonic density which is eitherbh 2 = 0:016 0:004 orbh 2 = 0:005 +0:0026 0:0006 , from the predictions of standard big bang nucleosynthesis. The high baryonic density solution is in agreement with recent results on the primordial abundance of deuterium and 3 He and on the estimates derived from the fluctuations of the cosmic microwave background.

Ionization Properties and Elemental Abundances in Damped Lyα Systems

We analyze extant data of Al+2, Al+, and other low ions with the aim of studying the ionization properties of damped Lyα systems (DLAs) from the analysis of the ratio R(Al+2/Al+) ≡ N(Al+2)/N(Al+). We find the good correlations log N(Al+)-log N(Si+) and log N(Al+)-log N(Fe+) that we use to indirectly estimate N(Al+) from N(Si+) and/or N(Fe+) measurements. In this way, we determine the ratio R(Al+2/Al+) for a sample of 20 DLAs. Contrary to common belief, the ratio can attain relatively high values (up to 0.6), suggesting that the gas of the intermediate ionization state plays an important role in DLAs. On the other hand, the lack of any trend between abundance ratios, such as Si/H and Si/Fe and R(Al+2/Al+) indicates that abundances are not severely influenced by ionization effects. We find a log R(Al+2/Al+)-log N(H0) anticorrelation that we use in conjunction with idealized photoionization equilibrium calculations to constrain the ionization properties and to predict ionization corrections in DLAs. We consider two possible origins for the species of the low- and intermediate-ionization state: (1) neutral regions devoid of Al+2 and/or (2) partially ionized, Al+2-bearing regions. The log R(Al+2/Al+)-log N(H0) anticorrelation can be naturally explained in terms of a two-region model with a soft, stellar-type ionizing radiation field. We present abundance ionization corrections for 14 elements of astrophysical interest derived with different types of ionizing spectra. For most of these elements, the corrections are generally below measurements errors, which is contrary to the predictions of recent models presented in the literature. We briefly discuss the potential effects of inaccuracies in the Al recombination rates used in the photoionization calculations.

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.