Context. To alleviate the discrepancy between the prediction of the primordial lithium abundance in the universe and the abundances observed in Pop II dwarfs and subgiant stars, it has been suggested ...that the stars observable today have undergone photospheric depletion of lithium. Aims. To identify the cause of this depletion, it is important to accurately establish the behaviour of lithium abundance with effective temperature and evolutionary phase. Stars in globular clusters are ideal objects for such an abundance analysis, because relative stellar parameters can be determined precisely. Methods. We conducted a homogeneous analysis of a very large sample of stars in the metal-poor globular cluster NGC 6397, covering all evolutionary phases from below the main sequence turn-off to high up on the red giant branch. Non-LTE Li abundances or abundance upper limits were obtained for all stars, and for a sizeable subset of the targets sodium abundances were also obtained. The Na abundances were used to distinguish stars formed out of pristine material from stars formed out of material affected by pollution from a previous generation of more massive stars. Results. The dwarf, turn-off, and early subgiant stars in our sample form a thin abundance plateau, disrupted in the middle of the subgiant branch by the Li dilution caused by the first dredge-up. A second steep abundance drop is seen at the luminosity of the red giant branch bump. The turn-off stars are more Li-poor, by up to 0.1 dex, than subgiants that have not yet undergone dredge-up. In addition, hotter dwarfs are slightly more Li-poor than cooler dwarfs, which may be a signature of the so-called Li dip in the cluster, commonly seen among Pop I stars. The feature is however weak. A considerably wide spread in Na abundance confirms that NGC 6397 has suffered from intracluster pollution in its infancy and a limited number of Na-enhanced and Li-deficient stars strongly contribute to forming a significant anti-correlation between the abundances of Na and Li. It is nevertheless seen that Li abundances are unaffected by relatively high degrees of pollution. Lithium abundance trends with effective temperature and stellar luminosity are compared to predictions from stellar structure models including atomic diffusion and ad-hoc turbulence below the convection zone. We confirm previous findings that some turbulence, with strict limits to its efficiency, is necessary for explaining the observations.
We present the high-resolution spectroscopic study of five −3.9 ≤ Fe/H ≤ −2.5 stars in the Local Group dwarf spheroidal, Sculptor, thereby doubling the number of stars with comparable observations in ...this metallicity range. We carry out a detailed analysis of the chemical abundances of α, iron peak, and light and heavy elements, and draw comparisons with the Milky Way halo and the ultra-faint dwarf stellar populations. We show that the bulk of the Sculptor metal-poor stars follow the same trends in abundance ratios versus metallicity as the Milky Way stars. This suggests similar early conditions of star formation and a high degree of homogeneity of the interstellar medium. We find an outlier to this main regime, which seems to miss the products of the most massive of the Type II supernovae. In addition to its help in refining galaxy formation models, this star provides clues to the production of cobalt and zinc. Two of our sample stars have low odd-to-even barium isotope abundance ratios, suggestive of a fair proportion of s-process. We discuss the implication for the nucleosynthetic origin of the neutron capture elements.
We present our analysis of the FLAMES dataset targeting the central 25′ region of the Sextans dwarf spheroidal galaxy (dSph). This dataset is the third major part of the high-resolution spectroscopic ...section of the ESO large program 171.B-0588(A) obtained by the Dwarf galaxy Abundances and Radial-velocities Team. Our sample is composed of red giant branch stars down to
V
∼ 20.5 mag, the level of the horizontal branch in Sextans, and allows users to address questions related to both stellar nucleosynthesis and galaxy evolution. We provide metallicities for 81 stars, which cover the wide Fe/H = −3.2 to −1.5 dex range. The abundances of ten other elements are derived: Mg, Ca, Ti, Sc, Cr, Mn, Co, Ni, Ba, and Eu. Despite its small mass, Sextans is a chemically evolved system, showing evidence of a contribution from core-collapse and Type Ia supernovae as well as low-metallicity asymptotic giant branch stars (AGBs). This new FLAMES sample offers a sufficiently large number of stars with chemical abundances derived with high accuracy to firmly establish the existence of a plateau in
α
/Fe at ∼0.4 dex followed by a decrease above Fe/H ∼ −2 dex. These features reveal a close similarity with the Fornax and Sculptor dSphs despite their very different masses and star formation histories, suggesting that these three galaxies had very similar star formation efficiencies in their early formation phases, probably driven by the early accretion of smaller galactic fragments, until the UV-background heating impacted them in different ways. The parallel between the Sculptor and Sextans dSph is also striking when considering Ba and Eu. The same chemical trends can be seen in the metallicity region common to both galaxies, implying similar fractions of SNeIa and low-metallicity AGBs. Finally, as to the iron-peak elements, the decline of Co/Fe and Ni/Fe above Fe/H ∼ −2 implies that the production yields of Ni and Co in SNeIa are lower than that of Fe. The decrease in Ni/Fe favours models of SNeIa based on the explosion of double-degenerate sub-Chandrasekhar mass white dwarfs.
This paper compares the chemical evolution of the Large Magellanic Cloud (LMC) to that of the Milky Way (MW) and investigates the relation between the bar and the inner disc of the LMC in the context ...of the formation of the bar. Our results show that the chemical history of the LMC experienced a strong contribution from type Ia supernovae as well as a strong s-process enrichment from metal-poor AGB winds. Massive stars made a smaller contribution to the chemical enrichment compared to the MW. The observed differences between the bar and the disc speak in favour of an episode of enhanced star formation a few Gyr ago, occurring in the central parts of the LMC and leading to the formation of the bar. This is in agreement with recently derived star formation histories.
Context. Carbon-enhanced metal-poor (CEMP) stars are known to have properties that reflect the nucleosynthesis of the first low- and intermediate-mass stars, because most have been polluted by a ...now-extinct AGB star. Aims. By considering abundances in the various CEMP subclasses separately, we try to derive parameters (such as metallicity, mass, temperature, and neutron source) characterising AGB nucleosynthesis from the specific signatures imprinted on the abundances, and separate them from the impact of thermohaline mixing, first dredge-up, and dilution associated with the mass transfer from the companion. Methods. To place CEMP stars in a broader context, we collect abundances for about 180 stars of various metallicities (from solar to Fe/H $=-4$), luminosity classes (dwarfs and giants), and abundance patterns (e.g. C-rich and poor, Ba-rich and poor), from both our own sample and the literature. Results. We first show that there are CEMP stars that share the properties of CEMP-s stars and CEMP-no stars (which we refer to as CEMP-low-s stars). We also show that there is a strong correlation between Ba and C abundances in the s-only CEMP stars. This represents a strong detection of the operation of the $\rm^{13}$C neutron source in low-mass AGB stars. For the CEMP-rs stars (seemingly enriched with elements from both the s- and r-processes), the correlation of the N abundances with abundances of heavy elements from the 2nd and 3rd s-process peaks bears instead the signature of the $\rm^{22}$Ne neutron source. Since CEMP-rs stars also exhibit O and Mg enhancements, we conclude that extremely hot conditions prevailed during the thermal pulses of the contaminating AGB stars. We also note that abundances are not affected by the evolution of the CEMP-rs star itself (especially by the first dredge-up). This implies that mixing must have occurred while the star was on the main sequence, and that a large amount of matter must have been accreted so as to trigger thermohaline mixing. Finally, we argue that most CEMP-no stars (with neutron-capture element abundances comparable to non-CEMP stars) are likely the extremely metal-poor counterparts of CEMP neutron-capture-rich stars. We also show that the C enhancement in CEMP-no stars declines with metallicity at extremely low metallicity (Fe/H $< -3.2$). This trend is not predicted by any of the current AGB models.
Context. The study of the Milky Way relies on our ability to interpret the light from stars correctly. With the advent of the astrometric ESA mission Gaia we will enter a new era where the study of ...the Milky Way can be undertaken on much larger scales than currently possible. In particular we will be able to obtain full 3D space motions of red giant stars at large distances. This calls for a reinvestigation of how reliably we can determine, for example, iron abundances in such stars and how well they reproduce those of dwarf stars. Aims. Here we explore robust ways of determining the iron content of metal-rich giant stars. We aim to understand what biases and shortcomings the widely applied methods suffer from. Methods. In this study we were mainly concerned with standard methods of analysing stellar spectra. These include the analysis of individual lines to determine stellar parameters, and analysis of the broad wings of certain lines (e.g. Hα and calcium lines) to determine effective temperature and surface gravity for the stars. Results. For NGC 6528 we find that Fe/H = + 0.04 dex with a scatter of σ = 0.07 dex, which gives an error in the derived mean abundance of 0.02 dex. Conclusions. Our work has two important conclusions for analysis of metal-rich red giant branch stars. Firstly, for spectra with S/N of below about 35 per reduced pixel, Fe/H becomes too high. Secondly, determination of Teff using the wings of the Hα line results in Fe/H values about 0.1 dex higher than if excitational equilibrium is used. The last conclusion is perhaps unsurprising, as we expect the NLTE effect to become more prominent in cooler stars and we can not use the wings of the Hα line to determine Teff for the cool stars in our sample. We therefore recommend that in studies of metal-rich red giant stars care should be taken to obtain sufficient calibration data to enable use of the cooler stars.
We present homogeneous and accurate iron abundances for 42 Galactic Cepheids based on high resolution (R ~ 38 000) high signal-to-noise ratio (S/N ≥ 100) optical spectra collected with UVES at VLT ...(128 spectra). The above abundances were complemented with high-quality iron abundances provided either by our group (86) or available in the literature. We were careful to derive a common metallicity scale and ended up with a sample of 450 Cepheids. We also estimated accurate individual distances for the entire sample by using homogeneous near-infrared photometry and the reddening free period-Wesenheit relations. The new metallicity gradient is linear over a broad range of Galactocentric distances (RG ~ 5–19 kpc) and agrees quite well with similar estimates available in the literature (–0.060 ± 0.002 dex/kpc). We also uncover evidence that suggests that the residuals of the metallicity gradient are tightly correlated with candidate Cepheid groups (CGs). The candidate CGs have been identified as spatial overdensities of Cepheids located across the thin disk. They account for a significant fraction of the residual fluctuations, and also for the large intrinsic dispersion of the metallicity gradient. We performed a detailed comparison with metallicity gradients based on different tracers: OB stars and open clusters. We found very similar metallicity gradients for ages younger than 3 Gyr, while for older ages we found a shallower slope and an increase in the intrinsic spread. The above findings rely on homogeneous age, metallicity, and distance scales. Finally, by using a large sample of Galactic and Magellanic Cepheids for which accurate iron abundances are available, we found that the dependence of the luminosity amplitude on metallicity is vanishing.
We present the results of a dedicated search for extremely metal-poor stars in the Fornax, Sculptor, and Sextans dSphs. Five stars were selected from two earlier VLT/Giraffe and HET/HRS surveys and ...subsequently followed up at high spectroscopic resolution with VLT/UVES. All of them turned out to have Fe/H ≲ −3 and three stars are below Fe/H ~ −3.5. This constitutes the first evidence that the classical dSphs Fornax and Sextans join Sculptor in containing extremely metal-poor stars and suggests that all of the classical dSphs contain extremely metal-poor stars. One giant in Sculptor at Fe/H = −3.96 ± 0.06 is the most metal-poor star ever observed in an external galaxy. We carried out a detailed analysis of the chemical abundances of the α, iron peak, and the heavy elements, and we performed a comparison with the Milky Way halo and the ultra faint dwarf stellar populations. Carbon, barium, and strontium show distinct features characterized by the early stages of galaxy formation and can constrain the origin of their nucleosynthesis.
In the framework of the ESO Large Programme “First Stars”, very high-quality spectra of some 70 very metal-poor dwarfs and giants were obtained with the ESO VLT and UVES spectrograph. These stars are ...likely to have descended from the first generation(s) of stars formed after the Big Bang, and their detailed composition provides constraints on issues such as the nature of the first supernovae, the efficiency of mixing processes in the early Galaxy, the formation and evolution of the halo of the Galaxy, and the possible sources of reionization of the Universe. This paper presents the abundance analysis of an homogeneous sample of 35 giants selected from the HK survey of Beers et al. (CITE, CITE), emphasizing stars of extremely low metallicity: 30 of our 35 stars are in the range $-4.1 <{\rm Fe/H}< -2.7$, and 22 stars have ${\rm Fe/H} < -3.0$. Our new VLT/UVES spectra, at a resolving power of $R\sim45\,000$ and with signal-to-noise ratios of 100–200 per pixel over the wavelength range 330–1000 nm, are greatly superior to those of the classic studies of McWilliam et al. (CITE) and Ryan et al. (CITE). The immediate objective of the work is to determine precise, comprehensive, and homogeneous element abundances for this large sample of the most metal-poor giants presently known. In the analysis we combine the spectral line modeling code “Turbospectrum” with OSMARCS model atmospheres, which treat continuum scattering correctly and thus allow proper interpretation of the blue regions of the spectra, where scattering becomes important relative to continuous absorption ($\lambda < 400$ nm). We obtain detailed information on the trends of elemental abundance ratios and the star-to-star scatter around those trends, enabling us to separate the relative contributions of cosmic scatter and observational/analysis errors. Abundances of 17 elements from C to Zn have been measured in all stars, including K and Zn, which have not previously been detected in stars with Fe/H $< -$3.0. Among the key results, we discuss the oxygen abundance (from the forbidden OI line), the different and sometimes complex trends of the abundance ratios with metallicity, the very tight relationship between the abundances of certain elements (e.g., Fe and Cr), and the high Zn/Fe ratio in the most metal-poor stars. Within the error bars, the trends of the abundance ratios with metallicity are consistent with those found in earlier literature, but in many cases the scatter around the average trends is much smaller than found in earlier studies, which were limited to lower-quality spectra. We find that the cosmic scatter in several element ratios may be as low as 0.05 dex. The evolution of the abundance trends and scatter with declining metallicity provides strong constraints on the yields of the first supernovae and their mixing into the early ISM. The abundance ratios found in our sample do not match the predicted yields from pair-instability hypernovae, but are consistent with element production by supernovae with progenitor masses up to 100 $M_{\sun}$. Moreover, the composition of the ejecta that have enriched the matter
Thanks to the accurate determination of the baryon density of the universe by the recent cosmic microwave background experiments, updated predictions of the standard model of Big Bang nucleosynthesis ...now yield the initial abundance of the primordial light elements with unprecedented precision. In the case of 7Li, the CMB+SBBN value is significantly higher than the generally reported abundances for Pop II stars along the so-called Spite plateau. In view of the crucial importance of this disagreement, which has cosmological, galactic and stellar implications, we decided to tackle the most critical issues of the problem by revisiting a large sample of literature Li data in halo stars that we assembled following some strict selection criteria on the quality of the original analyses. In the first part of the paper we focus on the systematic uncertainties affecting the determination of the Li abundances, one of our main goal being to look for the “highest observational accuracy achievable" for one of the largest sets of Li abundances ever assembled. We explore in great detail the temperature scale issue with a special emphasis on reddening. We derive four sets of effective temperatures by applying the same colour–${T}_{\rm eff}$ calibration but making four different assumptions about reddening and determine the LTE lithium values for each of them. We compute the NLTE corrections and apply them to the LTE lithium abundances. We then focus on our “best” (i.e. most consistent) set of temperatures in order to discuss the inferred mean Li value and dispersion in several ${T}_{\rm eff}$ and metallicity intervals. The resulting mean Li values along the plateau for Fe/H ≤ –1.5 are $A({\rm Li})_{\rm NLTE} = 2.214\pm0.093$ and $2.224\pm0.075$ when the lowest effective temperature considered is taken equal to 5700 K and 6000 K respectively. This is a factor of ~2.48 to 2.81 (depending on the adopted SBBN model and on the effective temperature range chosen to delimit the plateau) lower than the CMB+SBBN determination. We find no evidence of intrinsic dispersion. Assuming the correctness of the CMB+SBBN prediction, we are then left with the conclusion that the Li abundance along the plateau is not the pristine one, but that halo stars have undergone surface depletion during their evolution. In the second part of the paper we further dissect our sample in search of new constraints on Li depletion in halo stars. By means of the Hipparcos parallaxes, we derive the evolutionary status of each of our sample stars, and re-discuss our derived Li abundances. A very surprising result emerges for the first time from this examination. Namely, the mean Li value as well as the dispersion appear to be lower (although fully compatible within the errors) for the dwarfs than for the turnoff and subgiant stars. For our most homogeneous dwarfs-only sample with Fe/H ≤ –1.5, the mean Li abundances are $A({\rm L})_{\rm NLTE} = 2.177\pm 0.071$ and $2.215\pm0.074$ when the lowest effective temperature considered is taken equal to 5700 K and 6000 K respectively. This is a factor of 2.52 to 3.06 (depending on the selected range in ${T}_{\rm eff}$ for the plateau and on the SBBN predictions we compare to) lower than the CMB+SBBN primordial value. Instead, for the post-main sequence stars the corresponding values are $2.260\pm0.1$ and $2.235\pm0.077$, which correspond to a depletion factor of 2.28 to 2.52. These results, together with the finding that all the stars with Li abnormalities (strong deficiency or high content) lie on or originate from the hot side of the plateau, lead us to suggest that the most massive of the halo stars have had a slightly different Li history than their less massive contemporaries. In turn, this puts strong new constraints on the possible depletion mechanisms and reinforces Li as a stellar tomographer.
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