Influence of Inelastic Collisions with Hydrogen Atoms on Non-LTE Oxygen Abundance Determinations

We present the results of our modeling of the O I line formation under non-LTE conditions in the atmospheres of FG stars. The statistical equilibrium of O I has been calculated using Barklem’s quantum-mechanical rates of inelastic collisions with hydrogen atoms. We have determined the non-LTE oxygen abundance from atomic O I lines for the Sun and 46 FG stars in a wide metallicity range, ?2.6 < [Fe/H] < 0.2. The application of accurate atomic data has led to an increase in the departures from LTE and a decrease in the oxygen abundance compared to the use of Drawin’s theoretical approximation. The change in the non-LTE abundance from the infrared O I 7771-5 Å triplet lines is 0.11 dex for solar atmospheric parameters and diminishes in absolute value with decreasing metallicity. We have revised the [O/Fe]–[Fe/H] relationship derived by us previously. The change in [O/Fe] is small in the [Fe/H] range from ?1.5 to 0.2. For stars with [Fe/H] < ?1 the [O/Fe] ratio has increased so that [O/Fe] = 0.60 at [Fe/H] = ?0.8 and rises to [O/Fe] = 0.75 at [Fe/H] = ?2.6.     

New Views on the Early Evolution of Oxygen in the Galaxy

We discuss results on the oxygen abundance in a sample of 23 metal-poor (?3.0≤ [Fe/H] ≤ ?0.3) unevolved stars and one giant. High resolutionspectroscopy of OH lines in the near UV allowed us to trace the early evolution of oxygenversus metallicity. Contrary to previous expectations, we find that oxygen abundances derived from these low excitation lines agree well withthose derived from the high excitation lines of the OI IR triplet and from the [OI] λ 6300 Å line. Our new oxygen abundances show a smooth extension of previouslyknown trends of [O/Fe] versus [Fe/H] in disk stars to much lower metallicities, with a slope of ?0.31± 0.11. The [O/Fe] ratio increasesfrom 0.6 to 1 between [Fe/H] =?1.5 and ?3.0. Comparison with oxygen abundances in giant stars of the same metallicity imply that the lattermay have suffered a process of oxygen depletion. We briefly discussthe impact of these results on the yields of Type II SNe in the early Galaxy and on the age of globular clusters.     

Europium abundances in cool dwarf stars of the galactic thick and thin disks

Abundances of europium for 112 FGK dwarf stars of thick and thin disks have been determined in the metallicity range of ?1.0 < [Fe/H] < +0.3. Spectra of the studied stars have been obtained using the 1.93-m telescope of Haute-Provence Observatory (France) with spectral resolution R = 42000 and signal-to-noise ratio S/N = 100?300. Eu content has been calculated with assumption of LTE using the synthetic spectrum approach with detailed consideration of superfine structure. Analysis of europium abundances as a function of metallicity in kinematically selected stars of the Galactic thick and thin disks revealed different values in the disks. Comparison of europium abundances with magnesium abundances makes it possible to assume that at [Fe/H] < ?0.2 dex the origins of these elements are similar and at [Fe/H] > ?0.2 dex they are, probably, different.     

Spectroscopic analysis of nearby lower-main-sequence stars

Abundances of O, Na, Mg, Al, Si, K, Ca, Sc, Ti, V, Cr, Fe, Ni and Ba are determined for 30 nearby lower-main-sequence stars in the Northern sky using high-resolution, high signal-to-noise ratio spectra. Our results show an equilibrium of  [Fe/H]_I  and  [Fe/H]_II  and a much smaller star-to-star scatter of the abundance ratios as a function of metallicity compared with the results of Kotoneva et al. The non-local thermodynamic equilibrium (non-LTE) corrections for oxygen are considered and found to be small  (∼−0.04 dex)  . A flat trend of [O/Fe] exists over the whole metallicity range. The non-LTE effects for some important elements are discussed, and it is found that the abundance pattern for our programme stars is very similar to that of F and G dwarfs.     

Metallicity measurements using atomic lines in M and K dwarf stars

We report the first survey of chemical abundances in M and K dwarf stars using atomic absorption lines in high-resolution spectra. We have measured Fe and Ti abundances in 35 M and K dwarf stars using equivalent widths measured from  λ/Δλ≈ 33 000  spectra. Our analysis takes advantage of recent improvements in model atmospheres of low-temperature dwarf stars. The stars have temperatures between 3300 and 4700 K, with most cooler than 4100 K. They cover an iron abundance range of  −2.44 < [Fe/H] < +0.16  . Our measurements show [Ti/Fe] decreasing with increasing [Fe/H], a trend similar to that measured for warmer stars, where abundance analysis techniques have been tested more thoroughly. This study is a step towards the observational calibration of procedures to estimate the metallicity of low-mass dwarf stars using photometric and low-resolution spectral indices.     

Oxygen Abundances in Solar-Type Stars

The absolute solar oxygen abundance,ε ⊙ = 8.80± 0.06, has been determined from various oxygen abundance indicators in different solar atlases, and a new method is proposed to test collision rate coefficientsfor the NLTE model of OI.Using effective temperatures derived from Balmer lines, oxygen abundances from O triplets in 83 solar-type stars within the solar neighborhood spanning a metallicity range of [Fe/H] = −2.3 ... +0.4 have been determined.NLTE effects are not negligible, especially in warm stars(T_eff ≥ 5800) with [Fe/H] ≥ −0.5. This revised version was published online in July 2006 with corrections to the Cover Date.     

Abundance Ratios in Metal-Rich Halo and Thick-Disk Stars

Recent determinations of precise abundance ratios for nearby halo and thick disk stars in the metallicity range −1.3 < [Fe/H] < −0.5 have revealed a significant cosmic spread in the abundances of oxygen, magnesium, sodium, nickel, s-process and r-process elements relative to iron. Possible explanations of these variations are reviewed. In particular, it is discussed if the differences in abundance ratios are correlated with the kinematics of the stars, and hence can be used to identify stellar populations in the Galaxy. This revised version was published online in July 2006 with corrections to the Cover Date.     

Formation of galactic subsystems in light of the magnesium abundance in field stars: The thin disk

Data from our compiled catalog of spectroscopically determined magnesium abundances in stars with accurate parallaxes are used to select thin-disk dwarfs and subgiants according to kinematic criteria. We analyze the relations between the relative magnesium abundances in stars, [Mg/Fe], and their metallicities, Galactic orbital elements, and ages. The [Mg/Fe] ratios in the thin disk at any metallicity in the range ?1.0 dex <[Fe/H] < ?0.4 dex are shown to be smaller than those in the thick disk, implying that the thin-disk stars are, on average, younger than the thick-disk stars. The relative magnesium abundances in such metal-poor thin-disk stars have been found to systematically decrease with increasing stellar orbital radii in such a way that magnesium overabundances ([Mg/Fe] > 0.2 dex) are essentially observed only in the stars whose orbits lie almost entirely within the solar circle. At the same time, the range of metallicities in magnesium-poor stars is displaced from ?0.5 dex < [Fe/H] < +0.3 dex to ?0.7 dex < [Fe/H] < +0.2 dex as their orbital radii increase. This behavior suggests that, first, the star formation rate decreases with increasing Galactocentric distance and, second, there was no star formation for some time outside the solar circle, while this process was continuous within the solar circle. The decrease in the star formation rate with increasing Galactocentric distance is responsible for the existence of a negative radial metallicity gradient (grad _R[Fe/H] = ?0.05 ± 0.01 kpc^?1) in the disk, which shows a tendency to increase with decreasing age. At the same time, the relative magnesium abundance exhibits no radial gradient. We have confirmed the existence of a steep negative vertical metallicity gradient (grad _Z[Fe/H] = ?0.29 ± 0.06 kpc^?1) and detected a significant positive vertical gradient in relative magnesium abundance (grad _Z[Mg/Fe] = 0.13 ± 0.02 kpc^?1); both gradients increase appreciably in absolute value with decreasing age. We have found that there is not only an age-metallicity relation, but also an age-magnesium abundance relation, in the thin disk. We surmise that the thin disk has a multicomponent structure, but the existence of a negative trend in the star formation rate along the Galactocentric radius does not allow the stars of its various components to be identified in the immediate solar neighborhood.     

Observational restrictions on sodium and aluminium abundance variations in evolution of the galaxy

In this paper we construct and analyze the uniform non-LTE distributions of the aluminium ([Al/Fe]-[Fe/H]) and sodium ([Na/Fe]-[Fe/H]) abundances in the sample of 160 stars of the disk and halo of our Galaxy with metallicities within ?4.07 ≤ [Fe/H] ≤ 0.28. The values of metallicity [Fe/H] and microturbulence velocity ξ _turb indices are determined from the equivalent widths of the Fe II and Fe I lines. We estimated the sodium and aluminium abundances using a 21-level model of the Na I atom and a 39-level model of the Al I atom. The resulting LTE distributions of [Na/Fe]-[Fe/H] and [Al/Fe]-[Fe/H] do not correspond to the theoretical predictions of their evolution, suggesting that a non-LTE approach has to be applied to determine the abundances of these elements. The account of non-LTE corrections reduces by 0.05–0.15 dex the abundances of sodium, determined from the subordinate lines in the stars of the disk with [Fe/H] ≥ ?2.0, and by 0.05–0.70 dex (with a strong dependence on metallicity) the abundances of [Na/Fe], determined by the resonance lines in the stars of the halo with [Fe/H] ≤ ?2.0. The non-LTE corrections of the aluminium abundances are strictly positive and increase from 0.0–0.1 dex for the stars of the thin disk (?0.7 ≤ [Fe/H] ≤ 0.28) to 0.03–0.3 dex for the stars of the thick disk (?1.5 ≤ [Fe/H] ≤ ?0.7) and 0.06–1.2 dex for the stars of the halo ([Fe/H] ≤ ?2.0). The resulting non-LTE abundances of [Na/Fe] reveal a scatter of individual values up to Δ[Na/Fe] = 0.4 dex for the stars of close metallicities. The observed non-LTE distribution of [Na/Fe]-[Fe/H] within 0.15 dex coincides with the theoretical distributions of Samland and Kobayashi et al. The non-LTE aluminium abundances are characterized by a weak scatter of values (up to Δ[Al/Fe] = 0.2 dex) for the stars of all metallicities. The constructed non-LTE distribution of [Al/Fe]-[Fe/H] is in a satisfactory agreement to 0.2 dex with the theoretical data of Kobayashi et al., but strongly differs (up to 0.4 dex) from the predictions of Samland.     

中等贫金属星高分辨率光谱分析Ⅱ.化学元素丰度

在第1篇论文的基础上，确定了样本星的恒星大气参数，得到这些星中9种元素的丰度。讨论了各种元素丰度随[Fe/H]的变化。平均的[Na/Fe]～－0．01dex，接近于太阳丰度。α元素Si和Ca具有几乎相同的丰度模式，而[Ti/Fe]弥散较大，但三者均有随[Fe/H]的减小而增加的趋势。铁峰元素V、Cr、Ni在不同丰度处有较大的弥散，[Cr/Fe]在所有样本星中均表现超丰；而[Mn/Fe]却明显过贫，且随金属丰度的增加而增加。     

Radial mixing and the transition between the thick and thin Galactic discs

The analysis of the kinematics of solar neighbourhood stars shows that the low- and high-metallicity tails of the thin disc are populated by objects which orbital properties suggest an origin in the outer and inner Galactic disc, respectively. Signatures of radial migration are identified in various recent samples, and are shown to be responsible for the high-metallicity dispersion in the age–metallicity distribution. Most importantly, it is shown that the population of low-metallicity wanderers of the thin disc (−0.7 < [Fe/H] < −0.3 dex) is also responsible for the apparent hiatus in metallicity with the thick disc (which terminal metallicity is about −0.2 dex). It implies that the thin disc at the solar circle has started to form stars at about this same metallicity. This is also consistent with the fact that 'transition' objects, which have α-element abundance intermediate between that of the thick and thin discs, are found in the range [−0.4, −0.2] dex. Once the metal-poor thin disc stars are recognized for what they are – wanderers from the outer thin disc – the parenthood between the two discs can be identified on stars genuinely formed at the solar circle through an evolutionary sequence in [α/Fe] and [Fe/H]. Another consequence is that stars that can be considered as truly resulting of the chemical evolution at the solar circle have a metallicity restricted to about [−0.2, +0.2] dex, confirming an old idea that most chemical evolution in the Milky Way have preceded the thin disc formation.     

Chemical abundances in the young galaxy at z=2.309 towards PHL 957

We present high-resolution Utrecht Echelle Spectrograph spectra of the quasar PHL 957, obtained in order to study the foreground damped Lyα (DLA) galaxy at z =2.309. Measurements of absorption lines lead to accurate abundance determinations of Fe, S and N which complement measurements of Zn, Cr and Ni already available for this system. We find [Fe/H]=−2.0±0.1, [S/H]=−1.54±0.06 and [N/H]=−2.76±0.07. The ratio [Fe/Zn]=−0.44 provides evidence that ≈74 per cent of iron and ≈28 per cent of zinc are locked into dust grains with a dust-to-gas ratio of ≈3 per cent of the Galactic one. The total iron content in both gas and dust in the DLA system is [Fe/H]=−1.4. This confirms a rather low metallicity in the galaxy, which is in the early stages of its chemical evolution. The detection of S ii allows us to measure the S ii /Zn ii ratio, which is a unique diagnostic tool for tracing back its chemical history, since it is not affected by the presence of dust. Surprisingly, the resulting relative abundance is [S/Zn]=0.0±0.1, at variance with the overabundance found in the Galactic halo stars with similar metallicity. We emphasize that the [S/Zn] ratio is solar in all the three DLA absorbers with extant data. Upper limits are also found for Mn, Mg, O and P and, once the dust depletion is accounted for, we obtain [Mg/Fe]_c<+0.2, [O/Fe]_c<+0.4, [Mn/Fe]_c<+0.0 and [P/Fe]_c<−0.7. The [α/Fe] values do not support Galactic halo-like abundances, implying that the chemical evolution of this young galaxy is not reproducing the evolution of our own Galaxy.     

Intermediate-metallicity, high-velocity stars and Galactic chemical evolution

High signal-to-noise ratio spectra were obtained of 10 high-proper-motion stars having  −1 ≲[Fe/H] < 0  , and a comparable number of disc stars. All but two of the high-proper-motion stars were confirmed to have  [Fe/H] > −1.0  , some approaching solar metallicity, but, even so, earlier measurements overestimated the metallicities and velocities of some of these stars. Models of stellar populations were used to assign membership probabilities to the Galactic components to which the high-velocity stars might belong. Many were found to be more probably thick-disc than halo objects, despite their large space motions, and two might be associated with the inner Galaxy. It may be necessary to reassess contamination of previous halo samples, such as those used to define the metallicity distribution, to account for contamination by high-velocity thick-disc stars, and to consider possible subcomponents of the halo.
The change in [α/Fe] ratios at  [Fe/H]≃−1.0  is often used to constrain the degree and timing of Type Ia supernova nucleosynthesis in Galactic chemical-evolution models. [Ti/Fe] values were measured for eight of the high-velocity stars. Both high- and low-[Ti/Fe] halo stars exist; likewise high- and low-[Ti/Fe] thick-disc stars exist. We conclude that the [Ti/Fe]'break' is not well defined for a given population; nor is there a simple, continuous evolutionary sequence through the break. Implications for the interpretation of the [α/Fe] break in terms of SN Ia time-scales and progenitors are discussed. The range of [Ti/Fe] found for high -velocity (low rotation) thick-disc stars contrasts with that for the low -velocity (high rotation) thick-disc sample studied by Prochaska et al.     

Abundance Ratios in Extreme Metal-Poor Stars

In extremely metal-poor stars ([Fe/H]≤ − 2.5) the neutron capture elementsare characterized by a 300-fold dispersion in M/Fe ratios which decreases with increasing metallicity, the median M/Fe ratio increases with increasing [Fe/H], but the averageM/Fe number ratio is approximately constant. These observations are consistent witha highly dispersed intrinsic yield of neutron-capture elements in supernova (SN) events,and a progression to increasing metallicity by stochastic chemical evolution.The abundance trends indicate that the synthesis of elements heavier thanbarium was dominated by the r-process. The Sr/Ba ratio shows a dispersionwhich suggests a stochastic source of Sr in excess of the r-process value;possibly due to the alpha-rich freeze out.The iron-peak elements Cr, Mn, and Co show non-solar abundance ratios forextreme metal-poor stars, and no measurableintrinsic dispersion relative to iron. We discuss chemical evolution models which explain these observations. This revised version was published online in July 2006 with corrections to the Cover Date.     

The first generations of stars

Summary. Up to a decade ago, searches for population III stars (i.e. with strictly the chemical composition left by the Big Bang) had led to the results that (1) no such star had been found, (2) stars with metallicities significantly below [Fe/H] = were exceedingly rare. Thanks to a major survey, undertaken by Beers, Preston and Shectman 18 years ago, covering about 7500 square degrees in the sky, and down to magnitude =16.0, the situation has drastically changed. The observational limit towards the lowest metallicities is now about [Fe/H] = , i.e. 4 dex below the solar metallicity , (a level of pollution by supernova ejecta of only a few ppm), and over 100 stars are known with metallicities [Fe/H] in the range to . The study of this sample, and of a few stars found more serendipitously, has allowed a number of new conclusions: (i) The cosmological element Li stays constant (prolongation of the Spite's plateau) down to the lowest metallicities, a great observational gift to the hot Big Bang cosmology (ii) All heavier elements show a roughly linear increase with the abundance of O (or even Fe if the metallicity is below [Fe/H] = ), including the other light elements, Be and B. This last point has led to a reappraisal of the current view that they were produced by spallation of interstellar nuclei by galactic cosmic rays, because the rise of those elements with metallicity should then have been more quadratic than linear. An alternative new perspective is that these elements are produced by spallation of the primary nuclei ejected by SNe ii against protons of the interstellar medium. (iii) The ratio of the alpha elements (O, Si, Mg,...) to iron also stays constant down to the lowest metallicities, at about 3 times the solar value. (iv) Significant deviations to a lockstep variation of the various elements within the iron-peak start to appear below [Fe/H] = . The strongest are a decrease of [Cr/Fe] and an increase of [Co/Fe] when [Fe/H] decreases from to . These trends are not explained by the current status of explosive nucleosynthesis. (v) A great scatter of the abundances of the neutron capture elements relative to iron appears at very low metallicities. Similar scatter is seen for [Al/Fe]. A remarkable star with [Fe/H] = , CS 22892-052, has been found, with a superb spectrum of the -elements, involving over-abundances of those with respect to iron by factors ranging between 10 and 50. (vi) The kinematics of the very metal-poor stars is similar to that of other halo stars, with a complete lack of systemic rotation in an inertial frame, if not a small amount of counter-rotation in the Galaxy. Evidence exists that the velocity ellipsoid is radially elongated for stars within 10 kpc from the galactic center, whereas it is more spherical or even radially contracted at 20 kpc from the galactic center. (vii) The low metallicity stars were likely formed at an early cosmological epoch ( if H km/s), before the Galaxy had developed a disk. The new views concerning the sizes of the Ly clouds open the possibility that the low-metallicity Ly systems are large halos having the right metallicity for being protogalaxies, just forming early stellar generations. (viii) One may wonder why, if more than 100 stars are known with metallicities between [Fe/H] = to no pop. III has been found, or even not one star near [Fe/H] = . Different kinds of explanations have been proposed, with none conclusive at present. Either we have already observed a pop. III star, but its pristine Big Bang composition has been corrupted by a small amount of interstellar matter accreted during its 10 Gyr of orbiting in an already-enriched gas, or the collective process of star formation has polluted the medium before it has produced the low-mass stars we can still observe now, or, simpler, pop. III stars exist, but are sufficiently rare that we have not yet observed a volume large enough to have found one. Received: April 3, 1996     

Formation of galactic subsystems in light of the magnesium abundance in field stars: The thick disk

The space velocities and Galactic orbital elements of stars calculated from the currently available high-accuracy observations in our compiled catalog of spectroscopic magnesium abundances in dwarfs and subgiants in the solar neighborhood are used to identify thick-disk objects. We analyze the relations between chemical, spatial, and kinematic parameters of F–G stars in the identified subsystem. The relative magnesium abundances in thick-disk stars are shown to lie within the range 0.0 < [Mg/Fe] < 0.5 and to decrease with increasingmetallicity starting from [Fe/H] ≈ ?1.0. This is interpreted as evidence for a longer duration of the star formation process in the thick disk. We have found vertical gradients in metallicity (grad_Z[Fe/H] = ?0.13 ± 0.04 kpc^?1) and relative magnesium abundance (grad_Z[Mg/Fe] = 0.06 ± 0.02 kpc^?1), which can be present in the subsystem only in the case of its formation in a slowly collapsing protogalaxy. However, the gradients in the thick disk disappear if the stars whose orbits lie in the Galactic plane, but have high eccentricities and low azimuthal space velocities atypical of the thin-disk stars are excluded from the sample. The large spread in relative magnesium abundance (?0.3 < [Mg/Fe] < 0.5) in the stars of the metal-poor “tail” of the thick disk, which constitute ≈8% of the subsystem, can be explained in terms of their formation inside isolated interstellar clouds that interacted weakly with the matter of a single protogalactic cloud. We have found a statistically significant negative radial gradient in relative magnesium abundance in the thick disk (grad_R[Mg/Fe] = ?0.03 ± 0.01 kpc^{? 1}) instead of the expected positive gradient. The smaller perigalactic orbital radii and the higher eccentricities for magnesium-richer stars, which, among other stars, are currently located in a small volume of the Galactic space near the Sun, are assumed to be responsible for the gradient inversion. A similar, but statistically less significant inversion is also observed in the subsystem for the radial metallicity gradient.     

Chemical evolution of the galaxy: Abundances of the light elements (sodium to calcium)

The abundances of the light (Na to Ca) elements in disc and halo stars are reviewed. New analyses are emphasized. Elements considered are the α-nuclei (Mg, Si, and Ca), and the odd-even nuclei (Na and Al, also²⁵Mg and²⁶Mg). The α-nuclei are overabundant (relative to Fe) in the old disc and halo stars. Halo stars ([Fe/H] < —1.2) have [α/Fe] ∼0.3 with extreme halo ([Fe/H] ≲ −2.0) stars showing possibly higher overabundances. The scatter in [α/Fe] at a given [Fe/H] is small. To within the observational errors, the abundance patterns for Mg, Si, and Ca are identical. For disc stars, the Na and Al abundances relative to Mg are almost independent of the [Fe/H]. Halo stars ([Fe/H] < −1) show [Na/Mg] < 0 and [AI/Mg] < 0, but the form of the mean relation and the scatter about the relation between [odd-even/Mg] and [Fe/H] remains uncertain.     

A Statistical Model for Predicting the Average Abundance Patterns of Heavier Elements in Metal-Poor Stars

We have collected nearly all the available observed data of the elements from Ba to Dy in halo and disk stars in the metallicity range -4.0 <[Fe/H]< 0.5. Based on the observed data of Ba and Eu, we evaluated the least-squares regressions of [Ba/Fe] on [Fe/H], and [Eu/H] on [Ba/H]. Assuming that the heavy elements (heavier than Ba) are produced by a combination of the main components of s- and r-processes in metal-poor stars, and choosing Ba and Eu as respective representative elements of the main s- and the main r-processes, a statistical model for predicting the Galactic chemical evolution of the heavy elements is presented. With this model, we calculate the mean abundance trends of the heavy elements La, Ce, Pr, Nd, Sm, and Dy with the metallicity. We compare our results with the observed data at various metallicities, showing that the predicted trends are in good agreement with the observed trends, at least for the metallicity range [Fe/H]> -2.5. Finally, we discuss our results and deduce some importa     

Inhomogeneous Chemical Evolution of the Galaxy in the Solar Neighbourhood

N-body numerical simulations of an inhomogeneous Galactic Chemical Evolution (GCE) of the solar neighbourhood with a high temporal resolution are presented. The solar annular ring is divided into distinct spatial grids of area ~1–2 kpc². Each grid evolves distinctly in terms of star formation and nucleosynthetic yields from numerous generations of stars. The evolution of the galaxy is simulated by considering discrete episodes of star formation. Subsequent to the evolution of the simulated stars within each grid the stellar nucleosynthetic yields are homogenized within the grid rather than the traditionally adopted criteria of homogenizing over the entire solar annular ring. This provides a natural mechanism of generating heterogeneities in the elemental abundance distribution of stars. A complex chemical evolutionary history is inferred that registers episodes of time-dependent contributions from SN II+Ib/c with respect to SN Ia. It was observed that heterogeneities can remerge even after episodes of large scale homogenizations on scales larger than the grid size. However, a comparison of the deduced heterogeneities with the observed scatter in the elemental abundances of the dwarf stars suggest only a partial match, specifically, for [Fe/H] > ?0.5. The deduced heterogeneities in the case of carbon, oxygen, magnesium, silicon, sulphur, calcium and titanium can explain the observed heterogeneities for [Fe/H] < ?0.5. It may not be possible to explain the entire observed spread exclusively on the basis of the inhomogeneous GCE.     

The chemical composition of the Small Magellanic Cloud

The Small Magellanic Cloud is a close, irregular galaxy that has experienced a complex star formation history due to the strong interactions occurred both with the Large Magellanic Cloud and the Galaxy. Despite its importance, the chemical composition of its stellar populations older than ∼ 1–2 Gyr is still poorly investigated. I present the first results of a spectroscopic survey of ∼ 200 Small Magellanic Cloud giant stars performed with FLAMES@VLT. The derived metallicity distribution peaks at [Fe/H] ∼ –0.9/–1.0 dex, with a secondary peak at [Fe/H] ∼ –0.6 dex. All these stars show [α /Fe] abundance ratios that are solar or mildly enhanced (∼+0.1 dex). Also, three metal‐poor stars (with [Fe/H] ∼ –2.5 dex and enhanced [α /Fe] ratios compatible with those of the Galactic Halo) have been detected in the outskirts of the SMC: These giants are the most metal‐poor stars discovered so far in the Magellanic Clouds. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)