Search

Context. The Gaia Collaboration has recently reported the detection of a 33 M⊙ black hole in a wide binary system located in the solar neighbourhood. Aims. Here we explore the relationship between this black hole, known as Gaia BH3, and the nearby ED-2 halo stellar stream. Methods. We studied the orbital characteristics of the Gaia BH3 binary and present measurements of the chemical abundances of ED-2 member stars derived from high-resolution spectra obtained with the VLT. Results. We find that the Galactic orbit of the Gaia BH3 system and its metallicity are entirely consistent with being part of the ED-2 stream. The characteristics of the stream, particularly its negligible spread in metallicity and in other chemical elements, as well as its single stellar population, suggest that it originated from a disrupted star cluster of low mass. Its age is comparable to that of the globular cluster M92 that has been estimated to be as old as the Universe. Conclusions. This is the first black hole unambiguously associated with a disrupted star cluster. We infer the plausible mass range for the cluster to be relatively narrow, between 2 × 103 M⊙ and 5.2 × 104 M⊙. This implies that the black hole could have formed directly from the collapse of a massive very metal-poor star, but that the alternative scenario of binary interactions inside the cluster environment also deserves to be explored. [ABSTRACT FROM AUTHOR]

Context. Sulfur (S) is one of the lesser-studied α-elements. Published investigations of its behavior have so far focused on local stars, and only a few clusters of the Milky Way have been considered to study this topic. We aim to study the S content of the globular cluster Ruprecht 106 – which has never before been studied for this purpose, but is known to present low levels of the [α/Fe] abundance ratio – and the open cluster Trumpler 5. The only star studied so far in Trumpler 5 shows an unexpectedly low abundance of S. Aims. With this work, we aim to provide the first S abundance in Ruprecht 106 and to investigate the S content of Trumpler 5 with a larger sample of stars. The open cluster Trumpler 20 is considered as a reference object. Methods. We performed a standard abundance analysis based on 1D model atmospheres in local thermodynamical equilibrium (LTE) and on high-resolution and high-signal-to-noise-ratio UVES-slit and UVES/FLAMES spectra. We also applied corrections for nonLTE. The metallicities of the targets were obtained by studying equivalent widths. Sulfur abundances were derived from multiplets 1, 6, and 8 by spectrosynthesis. Results. We find that the metallicities of Ruprecht 106 and Trumpler 5 are [Fe/H] = −1.37±0.11 and [Fe/H] = −0.49±0.14, respectively. Ruprecht 106 is less S-rich than the other Galactic clusters at similar metallicity. The low S content of Ruprecht 106, [S/Fe]NLTE = −0.52±0.13, is consistent with its shortage of α-elements. This supports an extra-galactic origin of this cluster. We obtained a new and more robust S content value of Trumpler 5 of about [S/Fe]NLTE = 0.05±0.20. According to our results, Trumpler 5 follows the trend of the Galactic disk in the [S/Fe]LTE versus [Fe/H] diagram. Our results for Trumpler 20, of namely [Fe/H]= 0.06±0.15 and [S/Fe]NLTE = −0.28±0.21, are in agreement with those in the literature. [ABSTRACT FROM AUTHOR]

The very massive first stars (m > 100  |$\rm M_{\odot }$|⁠) were fundamental to the early phases of reionization, metal enrichment, and supermassive black hole formation. Among them, those with |$140\le \rm m/\rm M_{\odot }\le 260$| are predicted to evolve as Pair Instability Supernovae (PISN) leaving a unique chemical signature in their chemical yields. Still, despite long searches, the stellar descendants of PISN remain elusive. Here we propose a new methodology, the PISN-explorer , to identify candidates for stars with a dominant PISN enrichment. The PISN-explorer is based on a combination of physically driven models, and the FERRE code; and applied to data from large spectroscopic surveys (APOGEE, GALAH, GES, MINCE, and the JINA data base). We looked into more than 1.4 million objects and built a catalogue with 166 candidates of PISN descendants. One of which, 2M13593064+3241036, was observed with UVES at VLT and full chemical signature was derived, including the killing elements, Cu and Zn. We find that our proposed methodology is efficient in selecting PISN candidates from both the Milky Way and dwarf satellite galaxies such as Sextans or Draco. Further high-resolution observations are highly required to confirm our best selected candidates, therefore allowing us to probe the existence and properties of the very massive First Stars. [ABSTRACT FROM AUTHOR]

Metal-poor stars formed from a gas enriched by the ejecta of the explosion of one/few generations of first massive stars. With the Pristine photometry combined with the Gaia data, we selected a sample of bright giants metal-poor candidates to be observed at high resolution. Of the 43 stars observed, 36 were confirmed to be metal-poor, supporting the high success-rate of Pristine in selecting metal-poor stars. We centred the investigation on Cu and Zn, which are elements usually neglected, also because they are the 'killing elements' for identifying the Pair Instability Supernovae (PISN) descendants, the explosion of very massive first stars. We derived detailed chemical abundances of 22 species of 18 elements for 36 giant stars. The study of the kinematics of the stars in the sample revealed the presence of metal-poor stars on disc-like orbits (17 per cent of the sample) and even on thin disc orbits (8 per cent of the sample). Four stars are members of the Gaia–Sausage–Enceladus, one star is also likely a member of it. Three stars in the sample (TYC 1118−595−1, TYC 2207−992−1, and TYC 1194−507−1) show a chemical pattern compatible with the one theoretically derived for the PISN descendants, i.e. stars formed out from a gaseous environment enriched by PISN (>50 per cent level) and subsequent generations of normal stars evolving as core-collapse SNe. [ABSTRACT FROM AUTHOR]

Elemental abundances of the most metal-poor stars reflect the conditions in the early Galaxy and the properties of the first stars. We present a spectroscopic follow-up of two ultra-metal-poor stars ([Fe/H] < −4.0) identified by the survey Pristine : Pristine 221.8781+9.7844 and Pristine 237.8588+12.5660 (hereafter Pr 221 and Pr 237, respectively). Combining data with earlier observations, we find a radial velocity of −149.25 ± 0.27 and −3.18 ± 0.19 km s−1 for Pr 221  and Pr 237, respectively, with no evidence of variability between 2018 and 2020. From a one-dimensional (1D) local thermodynamic equilibrium (LTE) analysis, we measure [Fe/H]LTE = −4.79 ± 0.14 for Pr 221 and −4.22 ± 0.12 for Pr 237, in good agreement with previous studies. Abundances of Li, Na, Mg, Al, Si, Ca, Ti, Fe, and Sr were derived based on the non-LTE (NLTE) line formation calculations. When NLTE effects are included, we measure slightly higher metallicities: [Fe/H]NLTE = −4.40 ± 0.13 and −3.93 ± 0.12, for Pr 221 and Pr 237, respectively. Analysis of the G band yields [C/Fe]1D-LTE ≤ +2.3 and [C/Fe]1D-LTE ≤ +2.0 for Pr 221 and Pr 237. Both stars belong to the low-carbon band. Upper limits on nitrogen abundances are also derived. Abundances for other elements exhibit good agreement with those of stars with similar parameters. Finally, to get insight into the properties of their progenitors, we compare NLTE abundances to theoretical yields of zero-metallicity supernovae (SNe). This suggests that the SNe progenitors had masses ranging from 10.6 to 14.4 M⊙ and low-energy explosions with (0.3–1.2) × 1051 erg. [ABSTRACT FROM AUTHOR]

Context. The general theory of relativity predicts the redshift of spectral lines in the solar photosphere as a consequence of the gravitational potential of the Sun. This effect can be measured from a solar disk-integrated flux spectrum of the Sun's reflected light on Solar System bodies. Aims. The laser frequency comb (LFC) calibration system attached to the HARPS spectrograph offers the possibility of performing an accurate measurement of the solar gravitational redshift (GRS) by observing the Moon or other Solar System bodies. Here, we analyse the line shift observed in Fe absorption lines from five high-quality HARPS-LFC spectra of the Moon. Methods. We selected an initial sample of 326 photospheric Fe lines in the spectral range between 476–585 nm and measured their line positions and equivalent widths (EWs). Accurate line shifts were derived from the wavelength position of the core of the lines compared with the laboratory wavelengths of Fe lines. We also used a CO5BOLD 3D hydrodynamical model atmosphere of the Sun to compute 3D synthetic line profiles of a subsample of about 200 spectral Fe lines centred at their laboratory wavelengths. We fit the observed relatively weak spectral Fe lines (with EW< 180 mÅ) with the 3D synthetic profiles. Results. Convective motions in the solar photosphere do not affect the line cores of Fe lines stronger than about ∼150 mÅ. In our sample, only 15 Fe I lines have EWs in the range 150< EW(mÅ) < 550, providing a measurement of the solar GRS at 639 ± 14 m s−1, which is consistent with the expected theoretical value on Earth of ∼633.1 m s−1. A final sample of about 97 weak Fe lines with EW < 180 mÅ allows us to derive a mean global line shift of 638 ± 6 m s−1, which is in agreement with the theoretical solar GRS. Conclusions. These are the most accurate measurements of the solar GRS obtained thus far. Ultrastable spectrographs calibrated with the LFC over a larger spectral range, such as HARPS or ESPRESSO, together with a further improvement on the laboratory wavelengths, could provide a more robust measurement of the solar GRS and further testing of 3D hydrodynamical models. [ABSTRACT FROM AUTHOR]

A long sought after goal using chemical abundance patterns derived from metal-poor stars is to understand the chemical evolution of the Galaxy and to pin down the nature of the first stars (Pop III). Metal-poor, old, unevolved stars are excellent tracers as they preserve the abundance pattern of the gas from which they were born, and hence they are frequently targeted in chemical tagging studies. Here, we use a sample of 14 metal-poor stars observed with the high-resolution spectrograph called the Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) at the Large Binocular Telescope (LBT) to derive abundances of 32 elements (34 including upper limits). We present well-sampled abundance patterns for all stars obtained using local thermodynamic equilibrium (LTE) radiative transfer codes and one-dimensional (1D) hydrostatic model atmospheres. However, it is currently well-known that the assumptions of 1D and LTE may hide several issues, thereby introducing biases in our interpretation as to the nature of the first stars and the chemical evolution of the Galaxy. Hence, we use non-LTE (NLTE) and correct the abundances using three-dimensional model atmospheres to present a physically more reliable pattern. In order to infer the nature of the first stars, we compare unevolved, cool stars, which have been enriched by a single event ("mono-enriched"), with a set of yield predictions to pin down the mass and energy of the Pop III progenitor. To date, only few bona fide second generation stars that are mono-enriched are known. A simple χ2-fit may bias our inferred mass and energy just as much as the simple 1D LTE abundance pattern, and we therefore carried out our study with an improved fitting technique considering dilution and mixing. Our sample presents Carbon Enhanced Metal-Poor (CEMP) stars, some of which are promising bona fide second generation (mono-enriched) stars. The unevolved, dwarf BD+09_2190 shows a mono-enriched signature which, combined with kinematical data, indicates that it moves in the outer halo and likely has been accreted onto the Milky Way early on. The Pop III progenitor was likely of 25.5 M⊙ and 0.6 foe (0.6 1051 erg) in LTE and 19.2 M⊙ and 1.5 foe in NLTE, respectively. Finally, we explore the predominant donor and formation site of the rapid and slow neutron-capture elements. In BD-10_3742, we find an almost clean r-process trace, as is represented in the star HD20, which is a "metal-poor Sun benchmark" for the r-process, while TYC5481-00786-1 is a promising CEMP-r/-s candidate that may be enriched by an asymptotic giant branch star of an intermediate mass and metallicity. [ABSTRACT FROM AUTHOR]

Context. Unevolved metal-poor stars bore witness to the early evolution of the Galaxy, and the determination of their detailed chemical composition is an important tool to understand its chemical history. The study of their chemical composition can also be used to constrain the nucleosynthesis of the first generation of supernovae that enriched the interstellar medium. Aims. We aim to observe a sample of extremely metal-poor star (EMP stars) candidates selected from the Sloan Digital Sky Survey data release 12 (SDSS DR12) and determine their chemical composition. Methods. We obtained high-resolution spectra of a sample of five stars using HDS on Subaru telescope and used standard 1D models to compute the abundances. The stars we analysed have a metallicity [Fe/H] of between −3.50 and −4.25 dex. Results. We confirm that the five metal-poor candidates selected from low-resolution spectra are very metal poor. We present the discovery of a new ultra metal-poor star (UMP star) with a metallicity of [Fe/H] = −4.25 dex (SDSS J1050032.34−241009.7). We measured in this star an upper limit of lithium (log(Li/H) ≤ 2.0. We found that the four most metal-poor stars of our sample have a lower lithium abundance than the Spite plateau lithium value. We obtain upper limits for carbon in the sample of stars. None of them belong to the high carbon band. We measured abundances of Mg and Ca in most of the stars and found three new α-poor stars. [ABSTRACT FROM AUTHOR]

We present a study of departures from Local Thermodynamic Equilibrium (LTE) in the formation of infrared (IR) lines of Na  i , Mg  i , Al  i , S  i , K  i , and Sr  ii in unevolved stars of spectral types F, G, K and metallicities around the solar metallicity. The purpose of this investigation is to identify lines of these species that can be safely treated with the LTE approximation in the IR spectra of these types of stars. We employ a set of 40 stars observed with the GIANO spectrograph at the 3.5 m Telescopio Nazionale Galileo and previously investigated by Caffau et al. We were able to identify many lines that can be treated in LTE for all the above-mentioned species, except for Sr  ii. The latter species can only be studied using three lines in the J band, but all three of them display significant departures from LTE. With our small-size, but high-quality sample, we can determine robustly the trends of the abundance ratios with metallicity, confirming the trends apparent from a sample that is larger by several orders of magnitude, but of lower quality in terms of resolution and S/N ratio. [ABSTRACT FROM AUTHOR]

Extremely metal-poor (EMP) stars are old objects that mostly formed very early after the big bang. They are rare and, to select them, we have to rely on low-resolution spectroscopic or photometric surveys; specifically the combination of narrow- and broad-band photometry provides a powerful and time efficient way to select MP stars. The Pristine photometric survey is using the Canada–France–Hawaii Telescope MegaCam wide-field imager to obtain narrow-band photometry by utilizing a filter centred at 395.2 nm on the Ca  ii -H and -K lines. Gaia DR 2 is providing us the wide-band photometry as well as parallaxes. Follow-up observations of MP candidates allowed us to improve our photometric calibrations. In this paper of the series we analyse MP stars observed with FORS2 at VLT. We demonstrate the Pristine calibration adopted in this work to be able to provide metallicities accurate to ±0.3 dex for MP giant stars with good parallaxes, while it performs poorly for dwarf and turn-off stars, whatever the accuracy on the parallaxes. We find some MP and very MP stars that are not enhanced in α elements. Such stars have already been found in several other searches, and a higher resolution follow-up of our sample would be useful to put our findings on a firmer ground. This sample of stars analysed has a low fraction of carbon-enhanced MP stars, regardless of the definition adopted. This deficiency could indicate a small sensitivity of the Pristine filter to carbon abundance, issue to be addressed in the future. [ABSTRACT FROM AUTHOR]

We present a novel approach aimed at identifying the key chemical elements to search for the (missing) descendants of very massive first stars exploding as pair instability supernovae (PISN). Our simple and general method consists in a parametric study accounting for the unknowns related to early cosmic star formation and metal-enrichment. Our approach allow us to define the most likely [Fe/H] and abundance ratios of long-lived stars born in interstellar media polluted by the nucleosynthetic products of PISN at a |${\gt } 90{{\ \rm per\ cent}}$|⁠ , |$70{{\ \rm per\ cent}}$|⁠ , and |$50{{\ \rm per\ cent}}$| level. In agreement with previous works, we show that the descendants of very massive first stars can be most likely found at [Fe/H] ≈ −2. Further, we demonstrate that to search for an underabundance of [(N, Cu, Zn)/Fe] < 0 is the key to identify these rare descendants. The 'killing elements' N, Zn, and Cu are not produced by PISN, so that their sub-Solar abundance with respect to iron persists in environments polluted by further generations of normal core-collapse supernovae up to a |$50{{\ \rm per\ cent}}$| level. We show that the star BD+80° 245, which has [Fe/H] = −2.2, [N/Fe] = −0.79, [Cu/Fe] = −0.75, and [Zn/Fe] = −0.12 can be the smoking gun of the chemical imprint from very massive first stars. To this end we acquired new spectra for BD+80° 245 and re-analysed those available from the literature accounting for non-local thermodynamic equilibrium corrections for Cu. We discuss how to find more of these missing descendants in ongoing and future surveys to tightly constrain the mass distribution of the first stars. [ABSTRACT FROM AUTHOR]

With the aim of probing the properties of the bright end of the Pristine survey and its effectiveness in selecting metal-poor stars, we selected a sample of bright candidate metal-poor stars combining Pristine CaHK photometry with APASS gi photometry, before the Gaia second data release became available. These stars were observed with the SOPHIE spectrograph at the 1.93 m telescope of Observatoire de Haute Provence and we used photometry and parallaxes from Gaia DR2 to derive their atmospheric parameters. Chemical abundances were determined from the spectra for 40 stars of the sample. Eight stars were confirmed to be very metal-poor ([Fe/H] < −2.0), as expected from the photometric estimate. No star was found with [Fe/H] < −3.0, although for nine stars the photometric estimate was below this value. Three multiple systems are identified from their multipeaked cross-correlation functions. Two metal-poor stars with [Fe/H] ≈ −1.0 have an age estimate of about 4 Gyr. Accretion from a satellite galaxy is a possible explanation for these 'young metal-poor stars', but they could also be field blue stragglers. Galactic orbits for our sample of stars allowed us to divide them into three classes that we label 'Halo', 'Thick', and 'Thin' and tentatively identify as halo, thick disc, and thin disc. We present a new method for deriving photometric metallicities, effective temperatures, and surface gravities by combining Gaia parallaxes, photometry, and Pristine CaHK photometry. Comparison with spectroscopic metallicities shows a very good agreement and suggests that we can further improve the efficiency of Pristine CaHK in selecting metal-poor stars. [ABSTRACT FROM AUTHOR]

Context. Measurable amounts of Be could have been synthesised primordially if the Universe were non-homogeneous or in the presence of late decaying relic particles. Aims. We investigate the Be abundance in the extremely metal-poor star 2MASS J1808-5104 ([Fe/H] = −3.84) with the aim of constraining inhomogeneities or the presence of late decaying particles. Methods. High resolution, high signal-to-noise ratio (S/N) UV spectra were acquired at ESO with the Kueyen 8.2 m telescope and the UVES spectrograph. Abundances were derived using several model atmospheres and spectral synthesis code. Results. We measured log(Be/H) = −14.3 from a spectrum synthesis of the region of the Be line. Using a conservative approach, however we adopted an upper limit two times higher, i.e. log(Be/H) < −14.0. We measured the O abundance from UV–OH lines and find [O/H] = −3.46 after a 3D correction. Conclusions. Our observation reinforces the existing upper limit on primordial Be. There is no observational indication for a primordial production of 9Be. This places strong constraints on the properties of putative relic particles. This result also supports the hypothesis of a homogeneous Universe, at the time of nucleosynthesis. Surprisingly, our upper limit of the Be abundance is well below the Be measurements in stars of similar [O/H]. This may be evidence that the Be–O relation breaks down in the early Galaxy, perhaps due to the escape of spallation products from the gas clouds in which stars such as 2MASS J1808-5104 have formed. [ABSTRACT FROM AUTHOR]

Context. Space observations by the CoRoT and Kepler missions have provided a wealth of high-quality seismic data for a large number of stars from the main sequence to the red giant phases. One main goal of these missions is to take advantage of the rich spectra of solar-like oscillations to perform precise determinations of stellar characteristic parameters. To make the best of such data, we need theoretical stellar models with a precise near-surface structure since a near-surface structure of a solar-like star has significant influence on solar-like oscillation frequencies. The mixing-length parameter is a key factor to determine the near-surface structure of stellar models. In current versions of the convection formulations used in stellar evolution codes, the mixing-length parameter is a free parameter that needs to be properly specified. Aims. We aim at determining appropriate values of the mixing-length parameter, α, to be used consistently with the adopted convection formulation when computing stellar evolution models across the Hertzsprung–Russell diagram. This determination is based on 3D hydrodynamical simulation models. Methods. We calibrated α values by matching entropy profiles of 1D envelope models with those of hydrodynamical 3D models of solar-like stars produced by the CO5BOLD code. For such calibration, previous works concentrated on the classical mixing-length theory (MLT). We also analyzed full spectrum turbulence (FST) models. To construct the atmosphere in the 1D models, we used the Eddington gray T(τ) relation and that with the solar-calibrated Hopf-like function. Results. For both MLT and FST models with a mixing length l = αHp, calibrated α values increase with increasing surface gravity or decreasing effective temperature. For the FST models, we carried out an additional calibration using an α* value defined as l = rtop − r + α*Hp, top, where α* is found to increase with surface gravity and effective temperature. We provide tables of the calibrated α values across the Teff–log g plane for solar metallicity. By computing stellar evolution with varying α based on our 3D α calibration, we find that the change from solar α to varying α shifts evolutionary tracks particularly for the FST model. As for the correspondence to the 3D models, the solar Hopf-like function generally gives a photospheric-minimum entropy closer to a 3D model than the Eddington T(τ). The structure below the photosphere depends on the adopted convection model. However, we cannot obtain a definitive conclusion about which convection model gives the best correspondence to the 3D models. This is because each 1D physical quantity is related via an equation of state (EoS), but it is not the case for the averaged 3D quantities. Although the FST models with l = rtop − r + α*Hp, top are found to give the oscillation frequencies closest to the solar observed frequencies, their acoustic cavities are formed with compensatory effects between deviating density and temperature profiles near the top of the convective envelope. In future work, an appropriate treatment of the top part of the 1D convective envelope is necessary, for example, by considering turbulent pressure and overshooting. [ABSTRACT FROM AUTHOR]

Context. Extremely metal-poor stars are keys to understand the early evolution of our Galaxy. The ESO large programme TOPoS has been tailored to analyse a new set of metal-poor turn-off stars, whereas most of the previously known extremely metal-poor stars are giant stars. Aims. Sixty five turn-off stars (preselected from SDSS spectra) have been observed with the X-shooter spectrograph at the ESO VLT Unit Telescope 2, to derive accurate and detailed abundances of magnesium, silicon, calcium, iron, strontium and barium. Methods. We analysed medium-resolution spectra (R ≃ 10 000) obtained with the ESO X-shooter spectrograph and computed the abundances of several α and neutron-capture elements using standard one-dimensional local thermodynamic equilibrium (1D LTE) model atmospheres. Results. Our results confirms the super-solar [Mg/Fe] and [Ca/Fe] ratios in metal-poor turn-off stars as observed in metal-poor giant stars. We found a significant spread of the [α/Fe] ratios with several stars showing subsolar [Ca/Fe] ratios. We could measure the abundance of strontium in 12 stars of the sample, leading to abundance ratios [Sr/Fe] around the Solar value. We detected barium in two stars of the sample. One of the stars (SDSS J114424−004658) shows both very high [Ba/Fe] and [Sr/Fe] abundance ratios (>1 dex). [ABSTRACT FROM AUTHOR]

Context. The most metal-poor stars are the relics of the early chemical evolution of the Galaxy. Their chemical composition is an important tool to constrain the nucleosynthesis in the first generation of stars. The aim is to observe a sample of extremely metal-poor star (EMP stars) candidates selected from the Sloan Digital Sky Survey Data Release 12 (SDSS DR12) and determine their chemical composition. Aims. We obtain medium resolution spectra of a sample of six stars using the X-shooter spectrograph at the Very Large Telescope (VLT) and we used ATLAS models to compute the abundances. Methods. Five stars of the sample have a metallicity [Fe/H] between −2.5 dex and −3.2 dex. We confirm the recent discovery of SDSS J002314.00+030758.0 as a star with an extremely low [Fe/H] ratio. Assuming the α-enhancement [Ca/Fe] = +0.4 dex, we obtain [Fe/H] = −6.1 dex. Results. We could also determine its magnesium abundance and found that this star exhibits a very high ratio [Mg/Fe]≤ +3.60 dex assuming [Fe/H] = −6.13 dex. We determined the carbon abundance and found A(C) = 6.4 dex. From this carbon abundance, this stars belongs to the lower band of the A(C)–[Fe/H] diagram. [ABSTRACT FROM AUTHOR]

Aims. A probable carbon enhanced metal-poor (CEMP) star, Pisces II 10694, was discovered recently in the ultra-faint (UFD) galaxy Pisces II. This galaxy is supposed to be very old, suspected to include dark matter, and likely formed the bulk of its stars before the reionisation of the Universe. Methods. New abundances have been obtained from observations of Pisces II 10694 at the Kueyen ESO VLT telescope, using the high-efficiency spectrograph: X-shooter. Results. We found that Pisces II 10694 is a CEMP-no star with [Fe/H] = −2.60 dex. Careful measurements of the CH and C2  bands confirm the enhancement of the C abundance ([C/Fe] = +1.23). This cool giant has very probably undergone extra mixing and thus its original C abundance could be even higher. Nitrogen, O, Na, and Mg are also strongly enhanced, but from Ca to Ni the ratios [X/Fe] are similar to those observed in classical very metal-poor stars. With its low Ba abundance ([Ba/Fe] = −1.10 dex) Pisces II 10694 is a CEMP-no star. No variation in the radial velocity could be detected between 2015 and 2017. The pattern of the elements has a shape similar to the pattern found in galactic CEMP-no stars like CS 22949-037 ([Fe/H] = −4.0) or SDSS J1349+1407 ([Fe/H] = −3.6). Conclusions. The existence of a CEMP-no star in the UFD galaxy Pisc II suggests that this small galaxy likely hosted zero-metallicity stars. This is consistent with theoretical predictions of cosmological models supporting the idea that UFD galaxies are the living fossils of the first star-forming systems. [ABSTRACT FROM AUTHOR]

We present the status of an ongoing project to map the detailed chemical abundances of stars across the main body of the Sagittarius dwarf Spheroidal galaxy (Sgr dSph). The Sgr dSph is the closest known dwarf galaxy, and is being tidally destroyed by its interaction with the Milky Way (MW), leaving behind a massive stellar stream. Sgr dSph is a chemically outstanding object, with peculiar abundance ratios, clear center-outskirts abundance gradients, and spanning more than 3 orders of magnitude in metallicity. We present here detailed abundances from UVES@VLT spectra for more than 50 giants across 8 fields along the major and minor axes of Sgr dSph, and 5 more outside the galaxy main body, but possibly associated to its stellar stream. [ABSTRACT FROM AUTHOR]

Aims. We determined abundances of Mg and K in the atmospheres of 53 (Mg) and 75 (K) turn-off (TO) stars of the Galactic globular cluster 47 Tuc. The obtained abundances, together with those of Li, O, and Na that we had earlier determined for the same sample of stars, were used to search for possible relations between the abundances of K and other light elements, Li, O, Na, and Mg, as well as the connections between the chemical composition of TO stars and their kinematical properties. Methods. Abundances of Mg and K were determined using archival high resolution VLT FLAMES/GIRAFFE spectra, in combination with the one-dimensional (1D) non-local thermodynamic equilibrium (NLTE) spectral synthesis methodology. Spectral line profiles were computed with the MULTI code, using 1D hydrostatic ATLAS9 stellar model atmospheres. We also utilized three-dimensional (3D) hydrodynamical CO5 BOLD and 1D hydrostatic LHD model atmospheres for computing 3D–1D LTE abundance corrections for the spectral lines of Mg and K, in order to assess the influence of convection on their formation in the atmospheres of TO stars. Results. The determined average abundance-to-iron ratios and their root mean square variations due to star-to-star abundance spreads were 〈[Mg/Fe]〉1D NLTE = 0.47 ± 0.12, and 〈[K/Fe]〉1D NLTE = 0.39 ± 0.09. Although the data suggest the possible existence of a weak correlation in the [K/Fe]–[Na/Fe] plane, its statistical significance is low. No statistically significant relations between the abundance of K and other light elements were detected. Also, we did not find any significant correlations or anti-correlations between the [Mg/Fe] and [K/Fe] ratios and projected distance from the cluster center. Similarly, no relations between the absolute radial velocities of individual stars and abundances of Mg and K in their atmospheres were detected. The 3D–1D abundance corrections were found to be small (≤0.1 dex) for the lines of Mg and K used in this study, thus indicating that the influence of convection on their formation is small. [ABSTRACT FROM AUTHOR]

Aims. We studied the influence of convection on the spectral energy distributions (SEDs), photometric magnitudes, and colour indices of different types of stars across the H-R diagram. Methods. The 3D hydrodynamical CO5BOLD, averaged <3D>, and 1D hydrostatic LHD model atmospheres were used to compute SEDs of stars on the main sequence (MS), main sequence turn-off (TO), subgiant branch (SGB), and red giant branch (RGB), in each case at two different effective temperatures and two metallicities, [M/H] = 0.0 and -2.0. Using the obtained SEDs, we calculated photometric magnitudes and colour indices in the broad-band Johnson-Cousins UBVRI and 2MASS JHKs, and the medium-band Strömgren uvby photometric systems. Results. The 3D-1D differences in photometric magnitudes and colour indices are small in both photometric systems and typically do not exceed ±0.03 mag. Only in the case of the coolest giants located on the upper RGB are the differences in the U and u bands able reach ≈ -0.2 mag at [M/H] = 0.0 and ≈ -0.1 mag at [M/H] = -2.0. Generally, the 3D-1D differences are largest in the blue-UV part of the spectrum and decrease towards longer wavelengths. They are also sensitive to the effective temperature and are significantly smaller in hotter stars. Metallicity also plays a role and leads to slightly larger 3D-1D differences at [M/H] = 0.0. All these patterns are caused by a complex interplay between the radiation field, opacities, and horizontal temperature fluctuations that occur due to convective motions in stellar atmospheres. Although small, the 3D-1D differences in the magnitudes and colour indices are nevertheless comparable to or larger than typical photometric uncertainties and may therefore cause non-negligible systematic differences in the estimated effective temperatures. [ABSTRACT FROM AUTHOR]

Context. The atmospheres of cool stars are temporally and spatially inhomogeneous due to the effects of convection. The influence of this inhomogeneity, referred to as granulation, on colours has never been investigated over a large range of effective temperatures and gravities. Aims. We aim to study, in a quantitative way, the impact of granulation on colours. Methods. We use the CIFIST (Cosmological Impact of the FIrst Stars) grid of CO5BOLD (COnservative COde for the COmputation of COmpressible COnvection in a BOx of L Dimensions, L = 2, 3) hydrodynamical models to compute emerging fluxes. These in turn are used to compute theoretical colours in the UBVRI, 2MASS, HIPPARCOS, Gaia and SDSS systems. Every CO5BOLD model has a corresponding one dimensional (1D) plane-parallel LHD (Lagrangian HydroDynamics) model computed for the same atmospheric parameters, which we used to define a "3D correction" that can be applied to colours computed from fluxes computed from any 1D model atmosphere code. As an example, we illustrate these corrections applied to colours computed from ATLAS models. Results. The 3D corrections on colours are generally small, of the order of a few hundredths of a magnitude, yet they are far from negligible. We find that ignoring granulation effects can lead to underestimation of Teff by up to 200 K and overestimation of gravity by up to 0.5 dex, when using colours as diagnostics. We have identified a major shortcoming in how scattering is treated in the current version of the CIFIST grid, which could lead to offsets of the order 0.01 mag, especially for colours involving blue and UV bands. We have investigated the Gaia and HIPPARCOS photometric systems and found that the (G - Hp), (BP - RP) diagram is immune to the effects of granulation. In addition, we point to the potential of the RVS photometry as a metallicity diagnostic. Conclusions. Our investigation shows that the effects of granulation should not be neglected if one wants to use colours as diagnostics of the stellar parameters of F, G, K stars. A limitation is that scattering is treated as true absorption in our current computations, thus our 3D corrections are likely an upper limit to the true effect. We are already computing the next generation of the CIFIST grid, using an approximate treatment of scattering. [ABSTRACT FROM AUTHOR]

Aims. The abundance patterns of the neutron-capture elements in metal-poor stars provide a unique record of the nucleosynthesis products of the earlier massive primitive objects. Methods. We measured new abundances of so-called light neutron-capture of first peak elements using local thermodynamic equilibrium (LTE) 1D analysis; this analysis resulted in a sample of 11 very metal-poor stars, from [Fe/H] = -2.5 to [Fe/H] = -3.4 and one carbon-rich star, CS 22949-037 with [Fe/H] = -4.0. The abundances were compared to those observed in two classical metal-poor stars: the typical r-rich star CS 31082-001 ([Eu/Fe] > +1.0) and the r-poor star HD122563 ([Eu/Fe] < 0.0), which are known to present a strong enrichment of the first peak neutron-capture elements relative to the second peak. Results. Within the first peak, the abundances are well correlated in analogy to the well-known correlation inside the abundances of the second-peak elements. In contrast, there is no correlation between any first peak element with any second peak element. We show that the scatter of the ratio of the first peak abundance over second peak abundance increases when the mean abundance of the second peak elements decreases from r-rich to r-poor stars. We found two new r-poor stars that are very similar to HD122563. A third r-poor star, CS 22897-008, is even more extreme; this star shows the most extreme example of first peak elements enrichment to date. On the contrary, another r-poor star (BD-18 5550) has a pattern of first peak elements that is similar to the typical r-rich stars CS 31082-001, however this star has some Mo enrichment. Conclusions. The distribution of the neutron-capture elements in our very metal-poor stars can be understood as the combination of at least two mechanisms: one that enriches the forming stars cloud homogeneously through the main r-process and leads to an element pattern similar to the r-rich stars, such as CS 31082-001; and another that forms mainly lighter, first peak elements. [ABSTRACT FROM AUTHOR]

We have developed a model atom for Cu with which we perform statistical equilibrium computations that allow us to compute the line formation of Cu I lines in stellar atmospheres without assuming local thermodynamic equilibrium (LTE). We validate this model atom by reproducing the observed line profiles of the Sun, Procyon and 11 metal-poor stars. Our sample of stars includes both dwarfs and giants. Over a wide range of stellar parameters, we obtain excellent agreement among different Cu I lines. The 11 metal-poor stars have iron abundances in the range -4.2 ≤ [Fe/H] ≤ -1.4, the weighted mean of the [Cu/Fe] ratios is -0.22 dex, with a scatter of -0.15 dex. This is very different from the results from LTE analysis (the difference between NLTE and LTE abundances reaches 1 dex) and in spite of the small size of our sample, it prompts for a revision of the Galactic evolution of Cu. [ABSTRACT FROM AUTHOR]

Context. The first generation of stars formed in the Galaxy left behind the chemical signatures of their nucleosynthesis in the interstellar medium, visible today in the atmospheres of low-mass stars that formed afterwards. Sampling the chemistry of those low-mass provides insight into the first stars. Aims. We aim to increase the samples of stars with extremely low metal abundances, identifying ultra metal-poor stars from spectra with modest spectral resolution and signal-to-noise ratio (S/N). Achieving this goal involves deriving reliable metallicities and carbon abundances from such spectra. Methods. We carry out follow-up observations of faint, V > 19, metal-poor candidates selected from SDSS spectroscopy and observed with the Optical System for Imaging and low-Intermediate-Resolution Integrated Spectroscopy (OSIRIS) at GTC. The SDSS and follow-up OSIRIS spectra were analyzed using the FERRE code to derive effective temperatures, surface gravities, metallicities and carbon abundances. In addition, a well-known extremely metal-poor star has been included in our sample to calibrate the analysis methodology. Results. We observed and analyzed five metal-poor candidates from modest-quality SDSS spectra. All stars in our sample have been confirmed as extremely metal-poor stars, in the [Fe/H] < -3.3 regime. We report the recognition of J173403+644632, a carbon- enhanced ultra metal-poor dwarf star with [Fe/H] = -4.3 and [C/Fe] = +3.1. [ABSTRACT FROM AUTHOR]

Extremely metal-poor (EMP) stars are old objects formed in the first Gyr of the Universe. They are rare and, to select them the most successful strategy has been to build on large and low-resolution spectroscopic surveys. The combination of narrow- and broad-band photometry provides a powerful and cheaper alternative to select metal-poor stars. The ongoing Pristine Survey is adopting this strategy, conducting photometry with the Canada France Hawaii Telescope MegaCam wide-field imager and a narrow-band filter centered at 395.2 nm on the Ca II-H and -K lines. In this paper, we present the results of the spectroscopic follow-up conducted on a sample of 26 stars at the bright end of the magnitude range of the Survey ( g⩽15), using FEROS at the MPG/ESO 2.2-m telescope (manufactured by Zeiss, Oberkochen, Germany). From our chemical investigation on the sample, we conclude that this magnitude range is too bright to use the Sloan Digital Sky Survey (SDSS) g r i bands, which are typically saturated. Instead, the Pristine photometry can be usefully combined with the AAVSO Photometric All Sky Survey (APASS) g r iphotometry to provide reliable metallicity estimates. [ABSTRACT FROM AUTHOR]

Context. The space-borne missions CoRoT and Kepler have provided a wealth of highly accurate data. However, our inability to properly model the upper-most region of solar-like stars prevents us from making the best of these observations. This problem is called "surface effect" and a key ingredient to solve it is turbulent pressure for the computation of both the equilibrium models and the oscillations. While 3D hydrodynamic simulations help to include properly the turbulent pressure in the equilibrium models, the way this surface effect is included in the computation of stellar oscillations is still subject to uncertainties. Aims. We aim at determining how to properly include the effect of turbulent pressure and its Lagrangian perturbation in the adiabatic computation of the oscillations. We also discuss the validity of the gas-gamma model and reduced gamma model approximations, which have been used to compute adiabatic oscillations of equilibrium models including turbulent pressure. Methods. We use a patched model of the Sun with an inner part constructed by a 1D stellar evolution code (CESTAM) and an outer part by the 3D hydrodynamical code (CO5BOLD). Then, the adiabatic oscillations are computed using the ADIPLS code for the gas-gamma and reduced gamma model approximations and with the MAD code imposing the adiabatic condition on an existing time-dependent convection formalism. Finally, all those results are compared to the observed solar frequencies. Results. We show that the computation of the oscillations using the time-dependent convection formalism in the adiabatic limit improves significantly the agreement with the observed frequencies compared to the gas-gamma and reduced gamma model approximations. Of the components of the perturbation of the turbulent pressure, the perturbation of the density and advection term is found to contribute most to the frequency shift. Conclusions. The turbulent pressure is certainly the dominant factor responsible for the surface effects. Its inclusion into the equilibrium models is thus necessary but not sufficient. Indeed, the perturbation of the turbulent pressure must be properly taken into account for computing adiabatic oscillation frequencies. We propose a formalism to evaluate the frequency shift due to the inclusion of the term with the turbulent pressure perturbation in the variational principle in order to extrapolate our result to other stars at various evolutionary stages. Although this work is limited to adiabatic oscillations and the inclusion of the turbulent pressure, future works will have to account for the nonadiabatic effect and convective backwarming. [ABSTRACT FROM AUTHOR]

The past decade and a half has seen the design and execution of several ground-based spectroscopic surveys, both Galactic and Extragalactic. Additionally, new surveys are being designed that extend the boundaries of current surveys. In this context, many important considerations must be done when designing a spectrograph for the future. Among these is the determination of the optimum wavelength coverage. In this work, we present a new code for determining the wavelength ranges that provide the optimal amount of information to achieve the required science goals for a given survey. In its first mode, it utilizes a user-defined list of spectral features to compute a figure-of-merit for different spectral configurations. The second mode utilizes a set of flux-calibrated spectra, determining the spectral regions that show the largest differences among the spectra. Our algorithm is easily adaptable for any set of science requirements and any spectrograph design. We apply the algorithm to several examples, including 4MOST, showing the method yields important design constraints to the wavelength regions. [ABSTRACT FROM AUTHOR]

We present a detailed analysis of seven young stars observed with the spectrograph SOPHIE at the Observatoire de Haute-Provence for which the chemical composition was incomplete or absent in the literature. For five stars, we derived the stellar parameters and chemical compositions using our automatic pipeline optimized for F, G, and K stars, while for the other two stars with high rotational velocity, we derived the stellar parameters by using other information (parallax), and performed a line-by-line analysis. Chromospheric emission-line fluxes from Caii are obtained for all targets. The stellar parameters we derive are generally in good agreement with what is available in the literature. We provide a chemical analysis of two of the stars for the first time. The star HIP 80124 shows a strong Li feature at 670.8 nm implying a high lithium abundance. Its chemical pattern is not consistent with it being a solar sibling, as has been suggested. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Context. The solar photospheric oxygen abundance is still widely debated. Adopting the solar chemical composition based on the "low" oxygen abundance, as determined with the use of three-dimensional (3D) hydrodynamical model atmospheres, results in a well-known mismatch between theoretical solar models and helioseismic measurements that is so far unresolved. Aims. We carry out an independent redetermination of the solar oxygen abundance by investigating the center-to-limb variation of the Oi IR triplet lines at 777 nm in different sets of spectra. Methods. The high-resolution and high signal-to-noise solar center-to-limb spectra are analyzed with the help of detailed synthetic line profiles based on 3D hydrodynamical CO5BOLD model atmospheres and 3D non-LTE line formation calculations with NLTE3D. The idea is to exploit the information contained in the observations at different limb angles to simultaneously derive the oxygen abundance, A(O), and the scaling factor SH that describes the cross-sections for inelastic collisions with neutral hydrogen relative to the classical Drawin formula. Using the same codes and methods, we compare our 3D results with those obtained from the semi-empirical Holweger-Müller model atmosphere as well as from different one-dimensional (1D) reference models. Results. With the CO5BOLD 3D solar model, the best fit of the center-to-limb variation of the triplet lines is obtained when the collisions by neutral hydrogen atoms are assumed to be efficient, i.e., when the scaling factor SH is between 1.2 and 1.8, depending on the choice of the observed spectrum and the triplet component used in the analysis. The line profile fits achieved with standard 1D model atmospheres (with fixed microturbulence, independent of disk position μ) are clearly of inferior quality compared to the 3D case, and give the best match to the observations when ignoring collisions with neutral hydrogen (SH = 0). The results derived with the Holweger-Müller model are intermediate between 3D and standard 1D. Conclusions. The analysis of various observations of the triplet lines with different methods yields oxygen abundance values (on a logarithmic scale where A(H) = 12) that fall in the range 8.74

Keywords: stars: Population II; stars: abundances; Galaxy: abundances; Galaxy: halo; errata, addenda EN stars: Population II stars: abundances Galaxy: abundances Galaxy: halo errata, addenda 1 1 1 10/03/22 20220929 NES 220929 We realised that there was a mistake in the code that computed parameters for the photometric data sample. Furthermore, for 301 763 stars, the interpolation arrived at the edge in metallicity of the synthetic grid (+0.5) and the program was not extrapolating. Stars: Population II, stars: abundances, Galaxy: abundances, Galaxy: halo, errata, addenda. [Extracted from the article]

From stellar spectra, a variety of physical properties of stars can be derived. In particular, the chemical composition of stellar atmospheres can be inferred from absorption line analyses. These provide key information on large scales, such as the formation of our Galaxy, down to the small-scale nucleosynthesis processes that take place in stars and supernovae. By extending the observed wavelength range toward bluer wavelengths, we optimize such studies to also include critical absorption lines in metal-poor stars, and allow for studies of heavy elements ( Z ≥ 38) whose formation processes remain poorly constrained. In this context, spectrographs optimized for observing blue wavelength ranges are essential, since many absorption lines at redder wavelengths are too weak to be detected in metal-poor stars. This means that some elements cannot be studied in the visual-redder regions, and important scientific tracers and science cases are lost. The present era of large public surveys will target millions of stars. It is therefore important that the next generation of spectrographs are designed such that they cover a wide wavelength range and can observe a large number of stars simultaneously. Only then, we can gain the full information from stellar spectra, from both metal-poor to metal-rich ones, that will allow us to understand the aforementioned formation scenarios in greater detail. Here we describe the requirements driving the design of the forthcoming survey instrument 4MOST, a multi-object spectrograph commissioned for the ESO VISTA 4 m-telescope. While 4MOST is also intended for studies of active galactic nuclei, baryonic acoustic oscillations, weak lensing, cosmological constants, supernovae and other transients, we focus here on high-density, wide-area survey of stars and the science that can be achieved with high-resolution stellar spectroscopy. Scientific and technical requirements that governed the design are described along with a thorough line blending analysis. For the high-resolution spectrograph, we find that a sampling of ≥2.5 (pixels per resolving element), spectral resolution of 18000 or higher, and a wavelength range covering 393-436 nm, is the most well-balanced solution for the instrument. A spectrograph with these characteristics will enable accurate abundance analysis (±0.1 dex) in the blue and allow us to confront the outlined scientific questions. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Context. We present a grid of computed non-local thermodynamic equilibrium (NLTE) equivalent widths (EW) and NLTE abundance corrections for four Ba II lines: 4554, 5853, 6141, and 6496 Å. Aims. The grid can be useful in deriving the NLTE barium abundance in stars having parameters in the following ranges: effective temperature from 4000 K to 6500 K, surface gravity log g from 0 to 5, microturbulent velocity 0 km s-1 to 3 km s-1, metallicity [Fe/H] from -2 to +0.5, and [Ba/Fe] from -0.4 to +0.6. The NLTE abundance can be either derived by EW interpolation (using the observed Ba II line EW) or by using the NLTE correction applied to a previously determined LTE abundance. Methods. Ba II line equivalent widths and the NLTE corrections were calculated using the updated MULTI code and the Ba II atomic model that was previously applied to determine the NLTE barium abundance in different types of stars. Results. The grid is available on-line through the web, and we find that the grid Ba NLTE corrections are almost as accurate as direct NLTE profile fitting (to within 0.05-0.08 dex). For the weakest Ba II line (5853 Å) the LTE abundances almost agree with the NLTE abundances, whereas the other three Ba II lines, 4554, 6141, and 6496 Å, need NLTE corrections even at the highest metallicities tested here. The 4554 Å line is extremely strong and should not be used for abundance analysis above [Fe/H] = -1. Furthermore, we tested the impact of different model atmospheres and spectrum synthesis codes and found average differences of 0.06 dex and 0.09 dex, respectively, for all four lines. At these metallicities we find an average ΔNLTE of ±0.1 dex for the three useful Ba lines for subsolar cool dwarfs. [ABSTRACT FROM AUTHOR]

Aims. We obtained spectra for two giants of Crater (Crater J113613-105227 and Crater J113615-105244) using X-Shooter at the VLT, with the purpose of determining their radial velocities and metallicities. Methods. Radial velocities were determined by cross-correlating the spectra with that of a standard star. The spectra were analysed with the MyGIsFOS code using a grid of synthetic spectra computed from one-dimensional, local thermodynamic equilibrium (LTE) model atmospheres. Effective temperature and surface gravity were derived from photometry measured from images obtained by the Dark Energy Survey. Results. The radial velocities are 144.3±4.0 kms-1 for Crater J113613-105227 and and 134.1±4.0 kms-1 for Crater J113615-105244. The metallicities are [Fe/H] = -1.73 and [Fe/H] = -1.67, respectively. In addition to the iron abundance, we were able to determine abundances for nine elements: Na, Mg, Ca, Ti, V, Cr, Mn, Ni, and Ba. For Na and Ba we took into account deviations from LTE because the corrections are significant. The abundance ratios are similar in the two stars and resemble those of Galactic stars of the same metallicity. In the deep photometric images we detected several stars that lie to the blue of the turn-off. Conclusions. The radial velocities imply that both stars are members of the Crater stellar system. The difference in velocity between the two taken at face value implies a velocity dispersion >3.7 kms-1 at a 95% confidence level. Our spectroscopic metallicities agree excellently well with those determined by previous investigations using photometry. Our deep photometry and the spectroscopic metallicity imply an age of 7 Gyr for the main population of the system. The stars to the blue of the turn-off can be interpreted as a younger population that is of the same metallicity and an age of 2.2Gyr. Finally, spatial and kinematical parameters support the idea that this system is associated with the galaxies Leo IV and Leo V. All the observations favour the interpretation of Crater as a dwarf galaxy. [ABSTRACT FROM AUTHOR]

Context. The solar photospheric abundance of oxygen is still a matter of debate. For about ten years some determinations have favoured a low oxygen abundance which is at variance with the value inferred by helioseismology. Among the oxygen abundance indicators, the forbidden line at 630 nm has often been considered the most reliable even though it is blended with a Ni I line. In Papers I and II of this series we reported a discrepancy in the oxygen abundance derived from the 630 nm and the subordinate [OI] line at 636 nm in dwarf stars, including the Sun. Aims. Here we analyse several, in part new, solar observations of the centre-to-limb variation of the spectral region including the blend at 630 nm in order to separate the individual contributions of oxygen and nickel. Methods. We analyse intensity spectra observed at different limb angles in comparison with line formation computations performed on a CO5BOLD 3D hydrodynamical simulation of the solar atmosphere. Results. The oxygen abundances obtained from the forbidden line at different limb angles are inconsistent if the commonly adopted nickel abundance of 6.25 is assumed in our local thermodynamic equilibrium computations. With a slightly lower nickel abundance, A(Ni) ≈ 6.1, we obtain consistent fits indicating an oxygen abundance of A(O) = 8.73 ± 0.05. At this value the discrepancy with the subordinate oxygen line remains. Conclusions. The derived value of the oxygen abundance supports the notion of a rather low oxygen abundance in the solar photosphere. However, it is disconcerting that the forbidden oxygen lines at 630 and 636 nm give noticeably different results, and that the nickel abundance derived here from the 630 nm blend is lower than expected from other nickel lines. [ABSTRACT FROM AUTHOR]

Context. The tidal disruption of the Sagittarius dwarf spheroidal galaxy (Sgr dSph) is producing the most prominent substructure in the Milky Way (MW) halo, the Sagittarius Stream. Aside from field stars, it is suspected that the Sgr dSph has lost a number of globular clusters (GC). Many Galactic GC are thought to have originated in the Sgr dSph. While for some candidates an origin in the Sgr dSph has been confirmed owing to chemical similarities, others exist whose chemical composition has never been investigated. Aims. NGC 5053 and NGC 5634 are two of these scarcely studied Sgr dSph candidate-member clusters. To characterize their composition we analyzed one giant star in NGC 5053, and two in NGC 5634. Methods. We analyze high-resolution and signal-to-noise spectra by means of the MyGIsFOS code, determining atmospheric parameters and abundances for up to 21 species between O and Eu. The abundances are compared with those of MW halo field stars, of unassociated MW halo globulars, and of the metal-poor Sgr dSph main body population. Results. We derive a metallicity of [Fe II/H] = -2:26 ± 0:10 for NGC 5053, and of [Fe I/H] = 1:99 ± 0:075 and -1:97 ± 0:076 for the two stars in NGC 5634. This makes NGC 5053 one of the most metal-poor globular clusters in the MW. Both clusters display an α enhancement similar to the one of the halo at comparable metallicity. The two stars in NGC 5634 clearly display the Na-O anticorrelation widespread among MW globulars. Most other abundances are in good agreement with standard MW halo trends. Conclusions. The chemistry of the Sgr dSph main body populations is similar to that of the halo at low metallicity. It is thus difficult to discriminate between an origin of NGC 5053 and NGC 5634 in the Sgr dSph, and one in the MW. However, the abundances of these clusters do appear closer to that of Sgr dSph than of the halo, favoring an origin in the Sgr dSph system. [ABSTRACT FROM AUTHOR]

Context. In the course of the Turn Off Primordial Stars (TOPoS) survey, aimed at discovering the lowest metallicity stars, we have found several carbon-enhanced metal-poor (CEMP) stars. These stars are very common among the stars of extremely low metallicity and provide important clues to the star formation processes. We here present our analysis of six CEMP stars. Aims. We want to provide the most complete chemical inventory for these six stars in order to constrain the nucleosynthesis processes responsible for the abundance patterns. Methods. We analyse both X-Shooter and UVES spectra acquired at the VLT. We used a traditional abundance analysis based on OSMARCS 1D local thermodynamic equilibrium (LTE) model atmospheres and the turbospectrum line formation code. Results. Calcium and carbon are the only elements that can be measured in all six stars. The range is -5.0 ≤ [Ca/H] < -2.1 and 7.12 ≤ A(C) ≤ 8.65. For star SDSS J1742+2531 we were able to detect three Fe i lines from which we deduced [Fe/H] = -4.80, from four Ca II lines we derived [Ca/H] = -4.56, and from synthesis of the G-band we derived A(C) = 7.26. For SDSS J1035+0641 we were not able to detect any iron lines, yet we could place a robust (3σ) upper limit of [Fe/H] < -5.0 and measure the Ca abundance, with [Ca/H] = -5.0, and carbon, A(C) = 6.90, suggesting that this star could be even more metal-poor than SDSS J1742+2531. This makes these two stars the seventh and eighth stars known so far with [Fe/H] < -4.5, usually termed ultra-iron-poor (UIP) stars. No lithium is detected in the spectrum of SDSS J1742+2531 or SDSS J1035+0641, which implies a robust upper limit of A(Li) < 1.8 for both stars. Conclusions. Our measured carbon abundances confirm the bimodal distribution of carbon in CEMP stars, identifying a high-carbon band and a low-carbon band. We propose an interpretation of this bimodality according to which the stars on the high-carbon band are the result of mass transfer from an AGB companion, while the stars on the low-carbon band are genuine fossil records of a gas cloud that has also been enriched by a faint supernova (SN) providing carbon and the lighter elements. The abundance pattern of the UIP stars shows a large star-to-star scatter in the [X/Ca] ratios for all elements up to aluminium (up to 1 dex), but this scatter drops for heavier elements and is at most of the order of a factor of two. We propose that this can be explained if these stars are formed from gas that has been chemically enriched by several SNe, that produce the roughly constant [X/Ca] ratios for the heavier elements, and in some cases the gas has also been polluted by the ejecta of a faint SN that contributes the lighter elements in variable amounts. The absence of lithium in four of the five known unevolved UIP stars can be explained by a dominant role of fragmentation in the formation of these stars. This would result either in a destruction of lithium in the pre-main-sequence phase, through rotational mixing or to a lack of late accretion from a reservoir of fresh gas. The phenomenon should have varying degrees of efficiency. [ABSTRACT FROM AUTHOR]

Context. Sulphur is an important volatile α element, but its role in the Galactic chemical evolution is still uncertain, and more observations constraining the sulphur abundance in stellar photospheres are required. Aims. We derive the sulphur abundances in red giant branch (RGB) stars in three Galactic halo globular clusters (GC) that cover a wide metallicity range (-2.3 < [Fe/H] < -1.2): M4 (NGC6121), M22 (NGC6656), and M30 (NGC7099). The halo field stars show a large scatter in the [S/Fe] ratio in this metallicity span, which is inconsistent with canonical chemical evolution models. To date, very few measurements of [S/Fe] exist for stars in GCs, which are good tracers of the chemical enrichment of their environment. However, some light and α elements show star-to-star variations within individual GCs, and it is as yet unclear whether the α element sulphur also varies between GC stars. Methods. We used the infrared spectrograph CRIRES to obtain high-resolution (R ~ 50 000), high signal-to-noise (S/N ~ 200 per px) spectra in the region of the S I multiplet 3 at 1045 nm for 15 GC stars selected from the literature (six stars in M4, six stars in M22, and three stars in M30). Multiplet 3 is better suited for S abundance derivation than the more commonly used lines of multiplet 1 at 920 nm, since its lines are not blended by telluric absorption or other stellar features at low metallicity. Results. We used spectral synthesis to derive the [S/Fe] ratio of the stars assuming local thermodynamic equilibrium (LTE). We find mean [S/Fe]LTE = 0.58 ± 0.01 ± 0.20 dex (statistical and systematic error) for M4, [S/Fe]LTE = 0.57 ± 0.01 ± 0.19 dex for M22, and [S/Fe]LTE = 0.55 ± 0.02 ± 0.16 dex for M30. The negative NLTE corrections are estimated to be in the order of the systematic uncertainties. We do not detect star-to-star variations of the S abundance in any of the observed GCs, with the possible exception of two individual stars, one in M22 and one in M30, which appear to be highly enriched in S. Conclusions. With the tentative exception of two stars with measured high S abundances, we conclude that sulphur behaves like a typical α element in the studied Galactic GCs, showing enhanced abundances with respect to the solar value at metallicities below [Fe/H] - 1.0 dex without a considerable spread. [ABSTRACT FROM AUTHOR]

Aims. We study how diffuse interstellar bands (DIBs) measured toward distance-distributed target stars can be used to locate dense interstellar (IS) clouds in the Galaxy and probe a line-of-sight (LOS) kinematical structure, a potentially useful tool when gaseous absorption lines are saturated or not available in the spectral range. Cool target stars are numerous enough for this purpose. Methods. We devised automated DIB-fitting methods appropriate for cool star spectra and multiple IS components. The data were fitted with a combination of a synthetic stellar spectrum, a synthetic telluric transmission, and empirical DIB profiles. The initial number of DIB components and their radial velocity were guided by HI 21 cm emission spectra, or, when available in the spectral range, IS neutral sodium absorption lines. For NaI, radial velocities of NaI lines and DIBs were maintained linked during a global simultaneous fit. In parallel, stellar distances and extinctions were estimated self-consistently by means of a 2D Bayesian method from spectroscopically-derived stellar parameters and photometric data. Results. We have analyzed Gaia-ESO Survey (GES) spectra of 225 stars that probe between ~2 and 10 kpc long LOS in five different regions of the Milky Way. The targets are the two CoRoT fields, two open clusters (NGC 4815 and λ Vel), and the Galactic bulge. Two OGLE fields toward the bulge observed before the GES are also included (205 target stars). Depending on the observed spectral intervals, we extracted one or more of the following DIBs: λλ 6283.8, 6613.6, and 8620.4. For each field, we compared the DIB strengths with the Bayesian distances and extinctions, and the DIB Doppler velocities with the HI emission spectra. Conclusions. For all fields, the DIB strength and the target extinction are well correlated. For targets that are widely distributed in distance, marked steps in DIBs and extinction radial distance profiles match each other and broadly correspond to the expected locations of spiral arms. For all fields, the DIB velocity structure agrees with HI emission spectra, and all detected DIBs correspond to strong NaI lines. This illustrates how DIBs can be used to locate the Galactic interstellar gas and to study its kinematics at the kpc scale, as illustrated by Local and Perseus Arm DIBs that differ by ≳~30 km s-1, in agreement with HI emission spectra. On the other hand, if most targets are located beyond the main absorber, DIBs can trace the differential reddening within the field. [ABSTRACT FROM AUTHOR]

We report on a chemical-abundance analysis of two very metal-poor horizontal-branch stars in the Milky Way halo: CS 22186-005 ([Fe/H] = -2.70) and CS 30344-033 ([Fe/H] = -2.90). The analysis is based on high-resolution spectra obtained at ESO, with the spectrographs HARPS at the 3.6m telescope, and UVES at the VLT.We adopted one-dimensional, plane-parallel model atmospheres assuming local thermodynamic equilibrium. We derived elemental abundances for 13 elements for CS 22186-005 and 14 elements for CS 30344-033. This study is the first abundance analysis of CS 30344-033. CS 22186-005 has been analyzed previously, but we report here the first measurement of nickel (Ni; Z = 28) for this star, based on twenty-two Ni i lines ([Ni/Fe] = -0.21 ± 0.02); the measurement is significantly below the mean found for most metal-poor stars. Differences of up to 0.5 dex in [Ni/Fe] ratios were determined by different authors for the same type of stars in the literature, which means that it is not yet possible to conclude that there is a real intrinsic scatter in the [Ni/Fe] ratios. For the other elements for which we obtained estimates, the abundance patterns in these two stars match the Galactic trends defined by giant and turnoff stars well. This confirms the value of horizontal-branch stars as tracers of the chemical properties of stellar populations in the Galaxy. Our radial velocities measurements for CS 22186-005 differ from previously published measurements by more than the expected statistical errors. More measurements of the radial velocity of this star are encouraged to confirm or refute its radial velocity variability. [ABSTRACT FROM AUTHOR]

Context. It has been noted that, in classical extremely metal-poor (EMP) stars, the abundance ratio of two well-observed neutron-capture elements, Sr and Ba, is always higher than [Sr/Ba] = -0.5, which is the value of the solar r-only process; however, a handful of EMP stars have recently been found with a very low Sr/Ba ratio. Aims. We try to understand the origin of this anomaly by comparing the abundance pattern of the elements in these stars and in the classical EMP stars. Methods. For a rigorous comparison with previous data, four stars with very low Sr/Ba ratios were observed and analyzed in the same way as in the First Stars program: analysis within LTE approximation through 1D (hydrostatic) model atmosphere, providing homogeneous abundances of nine neutron-capture elements. Results. In CS 22950-173, the only turnoff star of the sample, the Sr/Ba ratio is, in fact, found to be higher than the r-only solar ratio, so the star is discarded. The remaining stars (CS 29493-090, CS 30322-023, HE 305-4520) are cool evolved giants. They do not present a clear carbon enrichment, but in evolved giants C is partly burned into N, and owing to their high N abundance, they could still have initially been carbon-rich EMP stars (CEMP). The abundances of Na to Mg present similar anomalies to those in CEMP stars. The abundance patterns of the neutron-capture elements in the three stars are strikingly similar to a theoretical s-process pattern. This pattern could at first be attributed to pollution by a nearby AGB, but none of the stars presents a clear variation in the radial velocity indicating the presence of a companion. The stellar parameters seem to exclude any internal pollution in a TP-AGB phase for at least two of these stars. The possibility that the stars are early-AGB stars polluted during the core He flash does not seem compatible with the theory. [ABSTRACT FROM AUTHOR]

Context. The abundances of α-elements are a powerful diagnostic of the star formation history and chemical evolution of a galaxy. Sulphur, being moderately volatile, can be reliably measured in the interstellar medium (ISM) of damped Ly-α galaxies and extragalactic Hii regions. Measurements in stars of different metallicity in our Galaxy can then be readily compared to the abundances in external galaxies. Such a comparison is not possible for Si or Ca that suffer depletion onto dust in the ISM. Furthermore, studying sulphur is interesting because it probes nucleosynthetic conditions that are very different from those of O or Mg. In this context measurements in star clusters are a reliable tracers of the Galactic evolution of sulphur. Aims. The aim of this paper is to determine sulphur abundances in several Galactic clusters that span a metallicity range −1.5 < [Fe/H] < 0.0. Methods. We use a standard abundance analysis, based on 1D model atmospheres in local thermodynamical equilibrium (LTE) and literature corrections for non-LTE (NLTE), as well as 3D corrections based on hydrodynamical model atmospheres, to derive sulphur abundances in a sample of stars in the globular cluster M4, and the open clusters Trumpler 5, NGC2477, and NGC 5822. Results. We find A (S)NLTE = 6.11 ± 0.04 for M4, A (S)NLTE = 7.17 ± 0.02 for NGC2477, and A (S)NLTE = 7.13 ± 0.06 for NGC5822. For the only star studied in Trumpler 5 we find A(S)NLTE = 6.43 ± 0.03 and A(S)LTE = 6.94 ± 0.05. Conclusions. Our measurements show that, by and large, the S abundances in Galactic clusters trace reliably those in field stars. The only possible exception is Trumpler 5, for which the NLTE sulphur abundance implies an [S/Fe] ratio lower by roughly 0.4 dex than observed in field stars of comparable metallicity, even though its LTE sulphur abundance is in line with abundances of field stars. Moreover the LTE sulphur abundance is consistent only with the abundance of another α-element, Mg, in the same star, while the low NLTE value is consistent with Si and Ca. We believe that further investigation of departures from LTE is necessary, as well as observation of other S i lines in this star and in other stars of the same cluster, before one can conclude that the sulphur abundance in Trumpler 5 is indeed 0.4 dex lower than in field stars of comparable metallicity. The S abundances in our sample of stars in clusters imply that the clusters are chemically homogeneous for S within 0.05 dex. [ABSTRACT FROM AUTHOR]

Context. Moderately r-process-enriched stars (r-I; +0.3 ≤ [Eu/Fe] ≤ +1.0) are at least four times as common as those that are greatly enriched in r-process elements (r-II; [Eu/Fe] > +1.0), and the abundances in their atmospheres are important tools for obtaining a better understanding of the nucleosynthesis processes responsible for the origin of the elements beyond the iron peak. Aims. The main aim of this work is to derive abundances for a sample of seven metal-poor stars with -3.4 ≤ [Fe/H] ≤ -2.4 classified as r-I stars, to understand the role of these stars for constraining the astrophysical nucleosynthesis event(s) that is (are) responsible for the production of the r-process, and to investigate whether they differ, in any significant way, from the r-II stars. Methods. We carried out a detailed abundance analysis based on high-resolution spectra obtained with the VLT/UVES spectrograph, using spectra in the wavelength ranges 3400–4500 Å, 6800–8200 Å, and 8700–10 000 Å, with resolving power R ∼ 40 000 (blue arm) and R ∼ 55 000 (red arm). The OSMARCS LTE 1D model atmosphere grid was employed, along with the spectrum synthesis code Turbospectrum. Results. We have derived abundances of the light elements Li, C, and N, the α-elements Mg, Si, S, Ca, and Ti, the odd-Z elements Al, K, and Sc, the iron-peak elements V, Cr,Mn, Fe, Co, and Ni, and the trans-iron elements from the first peak (Sr, Y, Zr,Mo, Ru, and Pd), the second peak (Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb), the third peak (Os and Ir, as upper limits), and the actinides (Th) regions. The results are compared with values for these elements for r-II and "normal" very and extremely metal-poor stars reported in the literature, ages based on radioactive chronometry are explored using different models, and a number of conclusions about the r-process and the r-I stars are presented. Hydrodynamical models were used for some elements, and general behaviors for the 3D corrections were presented. Although the abundance ratios of the second r-process peak elements (usually associated with the main r-process) are nearly identical for r-I and r-II stars, the first r-process peak abundance ratios (probably associated with the weak r-process) are more enhanced in r-I stars than in r-II stars, suggesting that differing nucleosynthesis pathways were followed by stars belonging to these two different classifications. [ABSTRACT FROM AUTHOR]

Context. The cluster 47 Tuc is among the most metal-rich Galactic globular clusters and its metallicity is similar to that of metal-poor disc stars and open clusters. Like other globular clusters, it displays variations in the abundances of elements lighter than Si, which is generally interpreted as evidence of the presence of multiple stellar populations. Aims. We aim to determine abundances of Li, O, and Na in a sample of of 110 turn-off (TO) stars, in order to study the evolution of light elements in this cluster and to put our results in perspective with observations of other globular and open clusters, as well as with field stars. Methods. We use medium resolution spectra obtained with the GIRAFFE spectrograph at the ESO 8.2m Kueyen VLT telescope and use state of the art 1D model atmospheres and NLTE line transfer to determine the abundances. We also employ CO5BOLD hydrodynamical simulations to assess the impact of stellar granulation on the line formation and inferred abundances. Results. Our results confirm the existence of Na-O abundance anti-correlation and hint towards a possible Li-O anti-correlation in the TO stars of 47 Tuc. At the same time, we find no convincing evidence supporting the existence of Li-Na correlation. The obtained 3D NLTE mean lithium abundance in a sample of 94 TO stars where Li lines were detected reliably, 〈A(Li)3D NLTE〉 = 1.78±0.18 dex, appears to be significantly lower than what is observed in other globular clusters. At the same time, star-to-star spread in Li abundance is also larger than seen in other clusters. The highest Li abundance observed in 47 Tuc is about 0.1 dex lower than the lowest Li abundance observed among the un-depleted stars of the metal-poor open cluster NGC2243. Conclusions. The correlations/anti-correlations among light element abundances confirm that chemical enrichment history of 47 Tuc was similar to that of other globular clusters, despite the higher metallicity of 47 Tuc. The lithium abundances in 47 Tuc, when put into context with observations in other clusters and field stars, suggest that stars that are more metal-rich than [Fe/H] ∼ −1.0 experience significant lithium depletion during their lifetime on the main sequence, while the more metal-poor stars do not. Rather strikingly, our results suggest that initial lithium abundance with which the star was created may only depend on its age (the younger the star, the higher its Li content) and not on its metallicity. [ABSTRACT FROM AUTHOR]

Context. The existence of lithium-rich low-mass red giant stars still represents a challenge for stellar evolution models. Stellar clusters are privileged environments for this kind of investigation. Aims. To investigate the chemical abundance pattern of the old open cluster Trumpler 5, we observed a sample of four red-clump stars with high-resolution optical spectrographs. One of them (#3416) reveals extremely strong lithium lines in its spectrum. Methods. One-dimensional, local thermodynamic equilibrium analysis was performed on the spectra of the observed stars. A 3DNLTE analysis was performed to derive the lithium abundance of star #3416. Results. Star #3416 is super Li-rich with A(Li) = 3.75 dex. The lack of 6Li enrichment (6Li/7Li < 2%), the low carbon isotopic ratio (12C/13C = 14 ± 3), and the lack of evidence for radial velocity variation or enhanced rotational velocity (v sin i = 2.8 km s-1) all suggest that lithium production has occurred in this star through the Cameron & Fowler mechanism. Conclusions. We identified a super Li-rich core helium-burning, red-clump star in an open cluster. Internal production is the most likely cause of the observed enrichment. Given the expected short duration of a star's Li-rich phase, enrichment is likely to have occurred at the red clump or in the immediately preceding phases, namely during the He-flash at the tip of the red giant branch (RGB) or while ascending the brightest portion of the RGB. [ABSTRACT FROM AUTHOR]

Context. The current and planned high-resolution, high-multiplexity stellar spectroscopic surveys, as well as the swelling amount of underutilized data present in public archives, have led to an increasing number of efforts to automate the crucial but slow process of retrieving stellar parameters and chemical abundances from spectra. Aims. We present MyGIsFOS1, a code designed to derive atmospheric parameters and detailed stellar abundances from medium- to high-resolution spectra of cool (FGK) stars. We describe the general structure and workings of the code, present analyses of a number of well-studied stars representative of the parameter space MyGIsFOS is designed to cover, and give examples of the exploitation of MyGIsFOS very fast analysis to assess uncertainties through Monte Carlo tests. Methods. MyGIsFOS aims to reproduce a "traditional" manual analysis by fitting spectral features for different elements against a precomputed grid of synthetic spectra. The lines of Fe I and Fe II can be employed to determine temperature, gravity, microturbulence, and metallicity by iteratively minimizing the dependence of Fe i abundance from line lower energy and equivalent width, and imposing Fe I-Fe II ionization equilibrium. Once parameters are retrieved, detailed chemical abundances are measured from lines of other elements. Results. MyGIsFOS replicates closely the results obtained in similar analyses on a set of well-known stars. It is also quite fast, performing a full parameter determination and detailed abundance analysis in about two minutes per star on a mainstream desktop computer. Currently, its preferred field of application are high-resolution and/or large spectral coverage data (e.g., UVES, X-shooter, HARPS, Sophie). [ABSTRACT FROM AUTHOR]

The measurement of isotopic ratios provides a privileged insight both into nucleosynthesis and into the mechanisms operating in stellar envelopes, such as gravitational settling. In this article, we give a few examples of how isotopic ratios can be determined from high-resolution, high-quality stellar spectra. We consider examples of the lightest elements, H and He, for which the isotopic shifts are very large and easily measurable, and examples of heavier elements for which the determination of isotopic ratios is more difficult. The presence of 6Li in the stellar atmospheres causes a subtle extra depression in the red wing of the 7Li 670.7 nm doublet which can only be detected in spectra of the highest quality. But even with the best spectra, the derived 6Li abundance can only be as good as the synthetic spectra used for their interpretation. It is now known that 3D non-LTE modelling of the lithium spectral line profiles is necessary to account properly for the intrinsic line asymmetry, which is produced by convective flows in the atmospheres of cool stars, and can mimic the presence of 6Li. We also discuss briefly the case of the carbon isotopic ratio in metal-poor stars, and provide a new determination of the nickel isotopic ratios in the solar atmosphere. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Context. The metal-weak tail of the metallicity distribution function (MDF) of the Galactic Halo stars contains crucial information on the formation mode of the first generation of stars. To determine this observationally, it is necessary to observe large numbers of extremely metal-poor stars. Aims. We present here the Turn-Off Primordial Stars survey (TOPoS) that is conducted as an ESO Large Programme at the VLT. This project has four main goals: (i) to understand the formation of low-mass stars in a low-metallicity gas: determine the metal-weak tail of the halo MDF below [M/H] = -3.5; in particular, we aim at determining the critical metallicity, that is the lowest metallicity sufficient for the formation of low-mass stars; (ii) to determine in extremely metal-poor stars the relative abundances of the elements that are the signature of the massive first stars; (iii) to determine the trend of the lithium abundance at the time when the Galaxy formed; and (iv) to derive the fraction of C-enhanced extremely metal-poor stars with respect to normal extremely metal-poor stars. The large number of stars observed in the SDSS provides a good sample of candidate stars at extremely low metallicity. Methods. Candidates with turn-off colours down to magnitude g = 20 were selected from the low-resolution spectra of SDSS by means of an automated procedure. X-Shooter has the potential of performing the necessary follow-up spectroscopy, providing accurate metallicities and abundance ratios for several key elements for these stars. Results. We present here the stellar parameters of the first set of stars. The nineteen stars range in iron abundance between -4.1 and -2.9 dex relative to the Sun. Two stars have a high radial velocity and, according to our estimate of their kinematics, appear to be marginally bound to the Galaxy and are possibly accreted from another galaxy. [ABSTRACT FROM AUTHOR]

Context. The extremely metal-poor stars are the direct descendants of the first generation stars. They carry the chemical signature of the pristine Universe at the time they formed, shortly after the Big Bang. Aims. We aim to derive information about extremely metal-poor stars from their observed spectra. Methods. Four extremely metal-poor stars were selected from the Sloan Digital Sky Survey (SDSS) and observed during the guaranteed observing time of X-shooter. The X-shooter spectra were analysed using an automatic code, MyGIsFOS, which is based on a traditional analysis method. It makes use of a synthetic grid computed from one-dimensional, plane-parallel, hydrostatic model atmospheres. Results. The low metallicity derived from the SDSS spectra is confirmed here. Two kinds of stars are found. Two stars are confirmed to be extremely metal-poor, with no evidence of any enhancement in carbon. The two other stars are strongly enhanced in carbon. We could not derive iron abundance for one of them, while [Ca/H] is below -4.5. Two of the stars are members of the rare population of extremely metal-poor stars low in alpha elements. [ABSTRACT FROM AUTHOR]

Aims. We utilize state-of-the-art three-dimensional (3D) hydrodynamical and classical 1D stellar model atmospheres to study the influence of convection on the formation properties of various atomic and molecular spectral lines in the atmospheres of four red giant stars, located close to the base of the red giant branch, RGB (Teff ≈ 5000 K, log g = 2.5), and characterized by four different metallicities, [M/H] = 0.0, -1.0, -2.0, -3.0. Methods. The role of convection in the spectral line formation is assessed with the aid of abundance corrections, i.e., the differences in abundances predicted for a given equivalent width of a particular spectral line with the 3D and 1D model atmospheres. The 3D hydrodynamical and classical 1D model atmospheres used in this study were calculated with the CO5BOLD and 1D LHD codes, respectively. Identical atmospheric parameters, chemical composition, equation of state, and opacities were used with both codes, therefore allowing a strictly differential analysis of the line formation properties in the 3D and 1D models. Results. We find that for lines of certain neutral atoms, such as Mg i, Ti i, Fei, and Ni i, the abundance corrections strongly depend both on the metallicity of a given model atmosphere and the line excitation potential, ?. While abundance corrections for all lines of both neutral and ionized elements tend to be small at solar metallicity (⩽±0.1 dex), for lines of neutral elements with low ionization potential and low-to-intermediate ? they quickly increase with decreasing metallicity, reaching in their extremes -0.6 to -0.8dex. In all such cases the large abundance corrections are due to horizontal temperature fluctuations in the 3D hydrodynamical models. Lines of neutral elements with higher ionization potentials (Eion ≳ 10 eV) generally behave very similarly to lines of ionized elements characterized by low ionization potentials (Eion ≲ 6 eV). In the latter case, the abundance corrections are small (generally, ⩽±0.1dex) and are caused by approximately equal contributions from the horizontal temperature fluctuations and differences between the temperature profiles in the 3D and 1D model atmospheres. Abundance corrections of molecular lines are very sensitive to the metallicity of the underlying model atmosphere and may be larger (in absolute value) than ∼-0.5dex at [M/H] = -3.0 (∼-1.5 dex in the case of CO). At fixed metallicity and excitation potential, the abundance corrections show little variation within the wavelength range studied here, 400-1600 nm.We also find that an approximate treatment of scattering in the 3D model calculations (i.e., ignoring the scattering opacity in the outer, optically thin, atmosphere) leads to abundance corrections that are altered by less than ∼0.1 dex, both for atomic and molecular (CO) lines, with respect to the model where scattering is treated as true absorption throughout the entire atmosphere, with the largest differences for the resonance and low-excitation lines Aims. We utilize state-of-the-art three-dimensional (3D) hydrodynamical and classical 1D stellar model atmospheres to study the influence of convection on the formation properties of various atomic and molecular spectral lines in the atmospheres of four red giant stars, located close to the base of the red giant branch, RGB (Teff ≈ 5000 K, log g = 2.5), and characterized by four different metallicities, [M/H] = 0.0, -1.0, -2.0, -3.0. Methods. The role of convection in the spectral line formation is assessed with the aid of abundance corrections, i.e., the differences in abundances predicted for a given equivalent width of a particular spectral line with the 3D and 1D model atmospheres. The 3D hydrodynamical and classical 1D model atmospheres used in this study were calculated with the CO5BOLD and 1D LHD codes, respectively. Identical atmospheric parameters, chemical composition, equation of state, and opacities were used with both codes, therefore allowing a strictly differential analysis of the line formation properties in the 3D and 1D models. Results. We find that for lines of certain neutral atoms, such as Mg i, Ti i, Fei, and Ni i, the abundance corrections strongly depend both on the metallicity of a given model atmosphere and the line excitation potential, ?. While abundance corrections for all lines of both neutral and ionized elements tend to be small at solar metallicity (⩽±0.1 dex), for lines of neutral elements with low ionization potential and low-to-intermediate ? they quickly increase with decreasing metallicity, reaching in their extremes -0.6 to -0.8dex. In all such cases the large abundance corrections are due to horizontal temperature fluctuations in the 3D hydrodynamical models. Lines of neutral elements with higher ionization potentials (Eion ≳ 10 eV) generally behave very similarly to lines of ionized elements characterized by low ionization potentials (Eion ≲ 6 eV). In the latter case, the abundance corrections are small (generally, ⩽±0.1dex) and are caused by approximately equal contributions from the horizontal temperature fluctuations and differences between the temperature profiles in the 3D and 1D model atmospheres. Abundance corrections of molecular lines are very sensitive to the metallicity of the underlying model atmosphere and may be larger (in absolute value) than ∼-0.5dex at [M/H] = -3.0 (∼-1.5 dex in the case of CO). At fixed metallicity and excitation potential, the abundance corrections show little variation within the wavelength range studied here, 400-1600 nm.We also find that an approximate treatment of scattering in the 3D model calculations (i.e., ignoring the scattering opacity in the outer, optically thin, atmosphere) leads to abundance corrections that are altered by less than ∼0.1 dex, both for atomic and molecular (CO) lines, with respect to the model where scattering is treated as true absorption throughout the entire atmosphere, with the largest differences for the resonance and low-excitation lines. [ABSTRACT FROM AUTHOR]