Abundances and possible diffusion of elements in M67 stars

We present a spectroscopic study at high resolution, R~50,000, of 14 stars located on the main sequence, at the turn-off point and on the early subgiant branch in the cluster M67 in order to investigate its detailed chemical composition, for comparison with the Sun and solar twins in the solar neighbourhood, and to explore selective atomic diffusion of chemical elements as predicted by stellar-structure theory. We have obtained VLT/FLAMES-UVES spectra and analysed these strictly differentially in order to explore chemical-abundance similarities and differences between the M67 stars and the Sun, and among the M67 stars themselves. Individual abundances of 19 different chemical elements are obtained for the stars. They are found to agree very well with solar abundances, with abundance ratios closer to solar than those of most solar twins in the solar neighbourhood. An exception is Li which shows a considerable scatter among the cluster stars. There is a tendency for the cluster-star abundances to be depleted relative to the abundances in the field stars in correlation with the condensation temperature of the elements, a tendency earlier found also for the Sun. The results support the hypothesis that the gas of the proto-cluster was depleted by formation and cleansing of dust before the stars formed. They also add support to the proposal that the Sun was once formed in a dense stellar environment. Moreover, the observed minor reductions of heavy elements in the atmospheres of the dwarfs and turn-off point stars relative to our standard star M67-1194 and the subgiants seem to suggest that diffusion processes are at work in these stars, although the evidence is not compelling. Based on theoretical models the diffusion-corrected initial metallicity of M67 is estimated to be [Fe/H]=+0.06.Comment: 14 pages, 9 figures, 5 tables. Accepted for publication in section 8. Stellar atmospheres of Astronomy and Astrophysics. Minor language corrections and an update of section 4.1 as compared to previous publicatio

Atomic Diffusion and Mixing in Old Stars V: A deeper look into the Globular Cluster NGC 6752

Abundance trends in heavier elements with evolutionary phase have been shown to exist in the globular cluster NGC 6752 [Fe/H]=-1.6. These trends are a result of atomic diffusion and additional (non-convective) mixing. Studying such trends can provide us with important constraints on the extent to which diffusion modifies the internal structure and surface abundances of solar-type, metal-poor stars. Taking advantage of a larger data sample, we investigate the reality and the size of these abundance trends and address questions and potential biases associated with the various stellar populations that make up NGC6752. Based on uvby Str\"omgren photometry, we are able to separate three stellar populations in NGC 6752 along the evolutionary sequence from the base of the red giant branch down to the turnoff point. We find weak systematic abundance trends with evolutionary phase for Ca, Ti, and Fe which are best explained by stellar-structure models including atomic diffusion with efficient additional mixing. We derive a new value for the initial lithium abundance of NGC 6752 after correcting for the effect of atomic diffusion and additional mixing which falls slightly below the predicted standard BBN value. We find three stellar populations by combining photometric and spectroscopic data of 194 stars in the globular cluster NGC 6752. Abundance trends for groups of elements, differently affected by atomic diffusion and additional mixing, are identified. Although the statistical significance of the individual trends is weak, they all support the notion that atomic diffusion is operational along the evolutionary sequence of NGC 6752.Comment: 15 pages, 11 figures, 2 online table

Fe I/Fe II ionization equilibrium in cool stars: NLTE versus LTE

Non-local thermodynamic equilibrium (NLTE) line formation for neutral and singly-ionized iron is considered through a range of stellar parameters characteristic of cool stars. A comprehensive model atom for Fe I and Fe II is presented. Our NLTE calculations support the earlier conclusions that the statistical equilibrium (SE) of Fe I shows an underpopulation of Fe I terms. However, the inclusion of the predicted high-excitation levels of Fe I in our model atom leads to a substantial decrease in the departures from LTE. As a test and first application of the Fe I/II model atom, iron abundances are determined for the Sun and four selected stars with well determined stellar parameters and high-quality observed spectra. Within the error bars, lines of Fe I and Fe II give consistent abundances for the Sun and two metal-poor stars when inelastic collisions with hydrogen atoms are taken into account in the SE calculations. For the close-to-solar metallicity stars Procyon and $\beta$ Vir, the difference (Fe II - Fe I) is about 0.1 dex independent of the line formation model, either NLTE or LTE. We evaluate the influence of departures from LTE on Fe abundance and surface gravity determination for cool stars.Comment: 4 pages, to appear in the proceedings of IAU Symposium 265, Chemical Abundances in the Universe: Connecting First Stars to Planets, K. Cunha, M. Spite & B. Barbuy, ed

Observation of the γγ → ττ Process in Pb + Pb Collisions and Constraints on the τ-Lepton Anomalous Magnetic Moment with the ATLAS Detector

This Letter reports the observation of τ -lepton-pair production in ultraperipheral lead-lead collisions Pb + Pb → Pb ( γ γ → τ τ ) Pb and constraints on the τ -lepton anomalous magnetic moment a τ . The dataset corresponds to an integrated luminosity of 1.44     nb − 1 of LHC Pb + Pb collisions at √ s NN = 5.02     TeV recorded by the ATLAS experiment in 2018. Selected events contain one muon from a τ -lepton decay, an electron or charged-particle track(s) from the other τ -lepton decay, little additional central-detector activity, and no forward neutrons. The γ γ → τ τ process is observed in Pb + Pb collisions with a significance exceeding 5 standard deviations and a signal strength of μ τ τ = 1.0 3 + 0.06 − 0.05 assuming the standard model value for a τ . To measure a τ , a template fit to the muon transverse-momentum distribution from τ -lepton candidates is performed, using a dimuon ( γ γ → μ μ ) control sample to constrain systematic uncertainties. The observed 95% confidence-level interval for a τ is − 0.057 < a τ < 0.024

Stability and Maintenance of Foxp3+ Treg Cells in Non-lymphoid Microenvironments

Foxp3(+) Treg cells are indispensable for maintaining self-tolerance in secondary lymphoid organs (SLOs). However, Treg cells are also recruited to non-lymphoid tissues (NLTs) during inflammation. Recent advances in the understanding of Treg cell biology provided us with molecular mechanisms-both transcriptional and epigenetic-that enable Treg cells to retain their identity in an inflammatory milieu that is per se hostile to sustained expression of high levels of Foxp3. While Treg cells are recruited to sites of inflammation in order to resolve inflammation and re-establish appropriate organ function, it is increasingly recognized that a series of inflammatory (but also non-inflammatory) perturbations of organ function lead to the constitution of relatively long lived populations of Treg cells in NLTs. NLT Treg cells are heterogeneous according to their respective site of residence and it will be an important goal of future investigations to determine how these NLT Treg cells are maintained, e.g., what the role of antigen recognition by NLT Treg cells is and which growth factors are responsible for their self-renewal in the relative deficiency of IL-2. Finally, it is an open question what functions NLT Treg cells have besides their role in maintaining immunologic tolerance. In this review, we will highlight and summarize major ideas on the biology of NLT Treg cells (in the central nervous system but also at other peripheral sites) during inflammation and in steady state

Optimizing the shape of photometric redshift distributions with clustering cross-correlations

We present an optimization method for the assignment of photometric galaxies to a chosen set of redshift bins. This is achieved by combining simulated annealing, an optimization algorithm inspired by solid-state physics, with an unsupervised machine learning method, a self-organizing map (SOM) of the observed colours of galaxies. Starting with a sample of galaxies that is divided into redshift bins based on a photometric redshift point estimate, the simulated annealing algorithm repeatedly reassigns SOM-selected subsamples of galaxies, which are close in colour, to alternative redshift bins. We optimize the clustering cross-correlation signal between photometric galaxies and a reference sample of galaxies with well-calibrated redshifts. Depending on the effect on the clustering signal, the reassignment is either accepted or rejected. By dynamically increasing the resolution of the SOM, the algorithm eventually converges to a solution that minimizes the number of mismatched galaxies in each tomographic redshift bin and thus improves the compactness of their corresponding redshift distribution. This method is demonstrated on the synthetic Legacy Survey of Space and Time cosmoDC2 catalogue. We find a significant decrease in the fraction of catastrophic outliers in the redshift distribution in all tomographic bins, most notably in the highest redshift bin with a decrease in the outlier fraction from 57 per cent to 16 per cent