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Register a new userLi - O anti-correlation in NGC 6752: evidence for Li-enriched polluting gas
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Elemental correlations and anti-correlations are known to be present in globular clusters (GCs) owing to pollution by CNO cycled gas. Because of its fragility Li is destroyed at the temperature at which the CNO cycling occurs, and this makes Li a crucial study for the nature of the contaminating stars. We observed 112 un-evolved stars at the Turnoff of the NGC6752 cluster with FLAMES at the VLT to investigate the presence and the extent of a Li-O correlation. This correlation is expected if there is a simple pollution scenario. Li (670.8 nm) and O triplet (771 nm) abundances are derived in NLTE. All stars belong to a very narrow region of the color-magnitude diagram, so they have similar stellar parameters (Teff, log g). We find that O and Li correlate, with a high statistical significance that confirms the early results for this cluster. At first glance this is what is expected if a simple pollution of pristine gas with CNO cycled gas (O-poor, Li-poor) occurred. The slope of the relationship, however, is about 0.4, and differs from unity by over 7 Sigma. A slope of one is the value predicted for a pure contamination model. We confirm an extended Li-O correlation in non evolved stars of NGC 6752. At the same time the characteristic of the correlation shows that a simple pollution scenario is not sufficient to explain the observations. Within this scenario the contaminant gas must have been enriched in Li. This would rule out massive stars as main polluters, and favor the hypothesis that the polluting gas was enriched by intermediate or high-mass AGB stars, unless the former can be shown to be able to produce Li. According to our observations, the fraction of polluting gas contained in the stars observed is a considerable fraction of the stellar mass of the cluster.
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Li abundances for 9 Turnoff (TO) stars of the intermediate metallicity cluster ([Fe/H]=-1.4) NGC6752 are presented. The cluster is known to show abundance anomalies and anticorrelations observed in both evolved and main sequence stars. We find that Li abundance anticorrelates with Na (and N) and correlates with O in these Turn-Off stars. For the first time we observe Pop II hot dwarfs systematically departing from the Spite plateau. The observed anticorrelations are in qualitative agreement with what is expected if the original gas were contaminated by Intermediate Mass AGB - processed material. However, a quantitative comparison shows that none of the existing models can reproduce all the observations at once. The very large amount of processed gas present in the cluster does not imply a pollution, but rather that the whole protocluster cloud was enriched by a previous generation of stars. We finally note that the different abundance patterns in NGC 6397 and NGC 6752 imply different ejecta of the preenrichment composition for the two clusters.
High-dispersion spectra in the Li 6708 Angstrom region have been obtained and analyzed in the old, metal-deficient cluster, NGC 2243. From Hydra spectra for 29 astrometric and radial-velocity members, we derive rotational velocities, as well as [Fe/H], [Ca/H], [Si/H], and [Ni/H] based on 17, 1, 1, and 3 lines, respectively. Using ROBOSPECT, an automatic equivalent width measurement program, we derive [Fe/H] = -0.54 +/- 0.11 (MAD), for an internal precision for the cluster [Fe/H] below 0.03 dex. Given the more restricted line set, comparable values for [Ca/H], [Si/H], and [Ni/H] are -0.48 +/- 0.19, -0.44 +/- 0.11, and -0.61 +/- 0.06, respectively. With E(B-V) = 0.055, appropriate isochrones imply (m-M) = 13.2 +/- 0.1 and an age of 3.6 +/- 0.2 Gyr. Using available VLT spectra and published Li abundances, we construct a Li sample of over 100 stars extending from the tip of the giant branch to 0.5 mag below the Li-dip. The Li-dip is well populated and, when combined with results for NGC 6819 and Hyades/Praesepe, implies a mass/metallicity slope of 0.4 solar-mass/dex for the high mass edge of the Li-dip. The A(Li) distribution among giants reflects the degree of Li variation among the turnoff stars above the Li-dip, itself a function of stellar mass and metallicity and strongly anticorrelated with a v_rot distribution that dramatically narrows with age. Potential implications of these patterns for the interpretation of Li among dwarf and giant field populations, especially selection biases tied to age and metallicity, are discussed.
The Li-rich turn-off star in the globular cluster NGC 6397 could represent the smoking gun for some very rare episode of Li enrichment in globular clusters. We aim to understand the nature of the Li enrichment by performing a spectroscopic analysis of the star, in particular of its beryllium (Be) abundance, and by investigating its binary nature. We observe the near UV region where the Beii resonance doublet and the NH bands are located. We could not detect the Beii lines and derive an upper limit of log (Be/H)< -12.2, that is consistent with the Be observed in other stars of the cluster. We could detect a weak G-band, which implies a mild carbon enhancement [C/Fe]$+0.4pm0.2$. We could not detect the UV NH band, and we derive an upper limit [N/Fe]$< 0.0$. For oxygen we could notdetect any of the near UV OH lines, which implies that oxygen cannot be strongly enhanced in this star. This is consistent with the detection of the Oi triplet at 777nm, which is consistent with [O/Fe]~0.5. Combining the UVES and Mike data, we could not detect any variation in the radial velocity greater than 0.95 kms$^{-1}$ over 8 years. The chemical composition of the star strongly resembles that of `first generation NGC6397 stars, with the huge Li as the only deviating abundance. Not detecting Be rules out two possible explanations of the Li overabundance: capture of a substellar body and spallation caused by a nearby type II SNe. Discrepancies are also found with respect to other accretion scenarios,except for contamination by the ejecta of a star that has undergone the RGB Li-flash.
The present work aims to examine in detail the depletion of lithium in solar twins to better constrain stellar evolution models and investigate its possible connection with exoplanets. We employ spectral synthesis in the region of the asymmetric 6707.75 AA Li I line for a sample of 77 stars plus the Sun. As in previous works based on a smaller sample of solar twins, we find a strong correlation between Li depletion and stellar age. In addition, for the first time we show that the Sun has the lowest Li abundance in comparison with solar twins at similar age (4.6 $pm$ 0.5 Gyr). We compare the lithium content with the condensation temperature slope for a sub-sample of the best solar twins and determine that the most lithium depleted stars also have fewer refractory elements. We speculate whether the low lithium content in the Sun might be related to the particular configuration of our Solar system.
Optical emission lines are used to categorize galaxies into three groups according to their dominant central radiation source: active galactic nuclei, star formation, or low-ionization (nuclear) emission regions [LI(N)ERs] that may trace ionizing radiation from older stellar populations. Using the Wisconsin H-Alpha Mapper, we detect optical line emission in low-extinction windows within eight degrees of Galactic Center. The emission is associated with the 1.5-kiloparsec-radius Tilted Disk of neutral gas. We modify a model of this disk and find that the hydrogen gas observed is at least $48%$ ionized. The ratio [NII] $lambda$6584 $overset{lower.5emcirc}{mathrm{A}}$/H$alpha$ $lambda$6563 $overset{lower.5emcirc}{mathrm{A}}$ increases from 0.3 to 2.5 with Galactocentric radius; [OIII] $lambda$5007 $overset{lower.5emcirc}{mathrm{A}}$ and H$beta$ $lambda$4861 $overset{lower.5emcirc}{mathrm{A}}$ are also sometimes detected. The line ratios for most Tilted Disk sightlines are characteristic of LI(N)ER galaxies.
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