We present an overview of the distributions of 11 elemental abundances in the Milky Way's (MW) inner regions, as traced by APOGEE stars released as part of the Sloan Digital Sky Survey Data Release ...14/15 (DR14/DR15), including O, Mg, Si, Ca, Cr, Mn, Co, Ni, Na, Al, and K. This sample spans ∼4000 stars with RGC ≤ 4.0 kpc, enabling the most comprehensive study to date of these abundances and their variations within the innermost few kiloparsecs of the MW. We describe the observed abundance patterns (X/Fe-Fe/H), compare to previous literature results and to patterns in stars at the solar Galactocentric radius (RGC), and discuss possible trends with DR14/DR15 effective temperatures. We find that the position of the Mg/Fe-Fe/H "knee" is nearly constant with RGC, indicating a well-mixed star-forming medium or high levels of radial migration in the early inner Galaxy. We quantify the linear correlation between pairs of elements in different subsamples of stars and find that these relationships vary; some abundance correlations are very similar between the -rich and -poor stars, but others differ significantly, suggesting variations in the metallicity dependencies of certain supernova yields. These empirical trends will form the basis for more detailed future explorations and for the refinement of model comparison metrics. That the inner MW abundances appear dominated by a single chemical evolutionary track and that they extend to such high metallicities underscore the unique importance of this part of the Galaxy for constraining the ingredients of chemical evolution modeling and for improving our understanding of the evolution of the Galaxy as a whole.
We report on a detailed abundance analysis of HE 0107-5240, a halo giant with Fe/H sub(NLTE) =-5.3. This star was discovered in the course of follow-up medium-resolution spectroscopy of extremely ...metal-poor candidates selected from the digitized Hamburg/ESO objective-prism survey. On the basis of high- resolution VLT/UVES spectra, we derive abundances for eight elements (C, N, Na, Mg, Ca, Ti, Fe, and Ni) and upper limits for another 12 elements. A plane- parallel LTE model atmosphere has been specifically tailored for the chemical composition of HE 0107-5240. Scenarios of the origin of the abundance pattern observed in the star are discussed. We argue that HE 0107-5240 is most likely not a post-asymptotic giant branch star and that the extremely low abundances of the iron-peak and other elements are not due to selective dust depletion. The abundance pattern of HE 0107-5240 can be explained by preenrichment from a zero- metallicity Type II supernova (SN II) of 20-25 M sub(o), plus either self- enrichment with C and N or production of these elements in the asymptotic giant branch phase of a formerly more massive companion, which is now a white dwarf. However, significant radial velocity variations have not been detected within the 52 days covered by our moderate- and high-resolution spectra. Alternatively, the abundance pattern can be explained by enrichment of the gas cloud from which HE 0107-5240 formed by a 25 M sub(o) first-generation star exploding as a subluminous SN II, as proposed by Umeda & Nomoto. We discuss consequences of the existence of HE 0107-5240 for low-mass star formation in extremely metal-poor environments and for currently ongoing and future searches for the most metal- poor stars in the Galaxy.
The α-element knee of the Sagittarius stream de Boer, T. J. L; Belokurov, V; Beers, T. C ...
Monthly Notices of the Royal Astronomical Society,
09/2014, Letnik:
443, Številka:
1
Journal Article
Recenzirano
Odprti dostop
We employ abundances from the Sloan Digital Sky Survey (SDSS) and the Sloan Extension for Galactic Understanding and Exploration (SEGUE) to study the α-element distribution of the stellar members of ...the Sagittarius stream. To test the reliability of SDSS/SEGUE abundances for the study of Sagittarius, we select high-likelihood samples tracing the different components of the Milky Way, and recover known literature α-element distributions. Using selection criteria based on the spatial position, radial velocity, distance and colours of individual stars, we obtain a robust sample of Sagittarius-stream stars. The α-element distribution of the Sagittarius stream forms a narrow sequence at intermediate metallicities with a clear turn-down, consistent with the presence of an α-element ‘knee’. This is the first time that the α-element knee of the Sagittarius dwarf galaxy has been detected. Fitting a toy model to our data, we determine that the α-knee in Sagittarius takes place at Fe/H=−1.27±0.05, only slightly less metal-poor than the knee in the Milky Way. This indicates that a small number of Sagittarius-like galaxies could have contributed significantly to the build-up of the Milky Way's stellar halo system at ancient times.
Context.The primordial lithium abundance is a key prediction of models of big bang nucleosynthesis, and its abundance in metal-poor dwarfs (the Spite plateau) is an important, independent ...observational constraint on such models. Aims.This study aims to determine the level and constancy of the Spite plateau as definitively as possible from homogeneous high-quality VLT-UVES spectra of 19 of the most metal-poor dwarf stars known. Methods.Our high-resolution ($R\sim 43\,000$), high $S/N$ spectra are analysed with OSMARCS 1D LTE model atmospheres and turbospectrum synthetic spectra to determine effective temperatures, surface gravities, and metallicities, as well as Li abundances for our stars. Results.Eliminating a cool subgiant and a spectroscopic binary, we find 8 stars to have $-3.5 < \rm Fe/H < -3.0$ and 9 stars with $-3.0 < \rm Fe/H < -2.5$. Our best value for the mean level of the plateau is $A({\rm Li}) =2.10\pm 0.09$. The scatter around the mean is entirely explained by our estimate of the observational error and does not allow for any intrinsic scatter in the Li abundances. In addition, we conclude that a systematic error of the order of 200 K in any of the current temperature scales remains possible. The iron excitation equilibria in our stars support our adopted temperature scale, which is based on a fit to wings of the Hα line, and disfavour hotter scales, which would lead to a higher Li abundance, but fail to achieve excitation equilibrium for iron. Conclusions.We confirm the previously noted discrepancy between the Li abundance measured in extremely metal-poor turnoff stars and the primordial Li abundance predicted by standard Big-Bang nucleosynthesis models adopting the baryonic density inferred from WMAP. We discuss recent work explaining the discrepancy in terms of diffusion and find that uncertain temperature scales remain a major question.
We present the quantitative methods used for selecting candidate metal-poor stars in the Hamburg/ESO objective-prism survey (HES). The selection is based on the strength of the Ca II K line, B-V ...colors (both measured directly from the digital HES spectra), as well as J-K colors from the 2 Micron All Sky Survey. The KP index for Ca II K can be measured from the HES spectra with an accuracy of 1.0 Aa, and a calibration of the HES B-V colors, using CCD photometry, yields a 1-\sigma uncertainty of 0.07 mag for stars in the color range 0.3 < B-V < 1.4. These accuracies make it possible to reliably reject stars with Fe/H >-2.0 without sacrificing completeness at the lowest metallicities. A test of the selection using 1121 stars of the HK survey of Beers, Preston, and Shectman present on HES plates suggests that the completeness at Fe/H <-3.5 is close to 100% and that, at the same time, the contamination of the candidate sample with false positives is low: 50% of all stars with Fe/H >-2.5 and 97% of all stars with Fe/H >-2.0 are rejected. The selection was applied to 379 HES fields, covering a nominal area of 8853 deg super(2) of the southern high Galactic latitude sky. The candidate sample consists of 20 271 stars in the magnitude range 10 lesssim B lesssim 18. A comparison of the magnitude distribution with that of the HK survey shows that the magnitude limit of the HES sample is about 2 mag fainter. Taking the overlap of the sky areas covered by both surveys into account, it follows that the survey volume for metal-poor stars has been increased by the HES by about a factor of 10 with respect to the HK survey. We have already identified several very rare objects with the HES, including, e.g., the three most heavy-element deficient stars currently known.
Context. Extremely metal-poor (EMP) stars in the halo of the Galaxy are sensitive probes of the production of the first heavy elements and the efficiency of mixing in the early interstellar medium. ...The heaviest measurable elements in such stars are our main guides to understanding the nature and astrophysical site(s) of early neutron-capture nucleosynthesis. Aims. Our aim is to measure accurate, homogeneous neutron-capture element abundances for the sample of 32 EMP giant stars studied earlier in this series, including 22 stars with \rm Fe/H<-3.0. Methods. Based on high- resolution, high S/N spectra from the ESO VLT/UVES, 1D, LTE model atmospheres, and synthetic spectrum fits, we determine abundances or upper limits for the 16 elements Sr, Y, Zr, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, and Yb in all stars. Results. As found earlier, Sr/Fe, Y/Fe, Zr/Fe and Ba/Fe are below Solar in the EMP stars, with very large scatter. However, we find a tight anti-correlation of Sr/Ba, Y/Ba, and Zr/Ba with Ba/H for-4.5 <\rm Ba/H <-2.5, also when subtracting the contribution of the main r-process as measured by Ba/H. Spectra of even higher S/N ratio are needed to confirm and extend these results below \rm Fe/H \simeq-3.5. The huge, well-characterised scatter of the n-capture/Fe ratios in our EMP stars is in stark contrast to the negligible dispersion in the \alpha/Fe and Fe-peak/Fe ratios for the same stars found in Paper V. Conclusions. These results demonstrate that a second ("weak" or LEPP) r-process dominates the production of the lighter neutron-capture elements for \rm Ba/H <-2.5. The combination of very consistent \alpha/Fe and erratic n-capture/Fe ratios indicates that inhomogeneous models for the early evolution of the halo are needed. Our accurate data provide strong constraints on future models of the production and mixing of the heavy elements in the early Galaxy.
The stellar population in the Galactic halo is characterised by a large fraction of carbon-enhanced metal-poor (CEMP) stars. Most CEMP stars have enhanced abundances of s-process elements (CEMP-s ...stars), and some of these are also enriched in r-process elements (CEMP-s/r stars). In one formation scenario proposed for CEMP stars, the observed carbon excess is explained by invoking wind mass transfer in the past from a more massive thermally-pulsing asymptotic giant branch (AGB) primary star in a binary system.In this work we generate synthetic populations of binary stars at metallicity Z = 0.0001 (Fe/H ≈ − 2.3), with the aim of reproducing the observed fraction of CEMP stars in the halo. In addition, we aim to constrain our model of the wind mass-transfer process, in particular the wind-accretion efficiency and angular-momentum loss, and investigate under which conditions our model populations reproduce observed distributions of element abundances.We compare the CEMP fractions determined from our synthetic populations and the abundance distributions of many elements with observations. Several physical parameters of the binary stellar population of the halo are uncertain, in particular the initial mass function, the mass-ratio distribution, the orbital-period distribution, and the binary fraction. We vary the assumptions in our model about these parameters, as well as the wind mass-transfer process, and study the consequent variations of our synthetic CEMP population.The CEMP fractions calculated in our synthetic populations vary between 7% and 17%, a range consistent with the CEMP fractions among very metal-poor stars recently derived from the SDSS/SEGUE data sample. The resulting fractions are more than a factor of three higher than those determined with default assumptions in previous population-synthesis studies, which typically underestimated the observed CEMP fraction. We find that most CEMP stars in our simulations are formed in binary systems with periods longer than 10 000 days. Few CEMP stars have measured orbital periods, but all that do have periods up to a few thousand days. Our results are consistent only if this small subpopulation represents the short-period tail of the underlying period distribution. The results of our comparison between the modelled and observed abundance distributions are significantly different for CEMP-s/r stars and for CEMP-s stars without strong enrichment in r-process elements. For the latter, our simulations qualitatively reproduce the observed distributions of carbon, sodium, and heavy elements such as strontium, barium, europium, and lead. Contrarily, for CEMP-s/r stars our model cannot reproduce the large abundances of neutron-rich elements such as barium, europium, and lead. This result is consistent with previous studies, and suggests that CEMP-s/r stars experienced a different nucleosynthesis history to CEMP-s stars.
Regulatory T cells (Tregs) play a neuroprotective role by suppressing microglia and macrophage-mediated inflammation and modulating adaptive immune reactions. We previously documented that Treg ...immunomodulatory mechanisms are compromised in Alzheimer's disease (AD). Ex vivo expansion of Tregs restores and amplifies their immunosuppressive functions in vitro. A key question is whether adoptive transfer of ex vivo expanded human Tregs can suppress neuroinflammation and amyloid pathology in a preclinical mouse model.
An immunodeficient mouse model of AD was generated by backcrossing the 5xFAD onto Rag2 knockout mice (5xFAD-Rag2KO). Human Tregs were expanded ex vivo for 24 days and administered to 5xFAD-Rag2KO. Changes in amyloid burden, microglia characteristics and reactive astrocytes were evaluated using ELISA and confocal microscopy. NanoString Mouse AD multiplex gene expression analysis was applied to explore the impact of ex vivo expanded Tregs on the neuroinflammation transcriptome.
Elimination of mature B and T lymphocytes and natural killer cells in 5xFAD-Rag2KO mice was associated with upregulation of 95 inflammation genes and amplified number of reactive microglia within the dentate gyrus. Administration of ex vivo expanded Tregs reduced amyloid burden and reactive glial cells in the dentate gyrus and frontal cortex of 5xFAD-Rag2KO mice. Interrogation of inflammation gene expression documented down-regulation of pro-inflammatory cytokines (IL1A&B, IL6), complement cascade (C1qa, C1qb, C1qc, C4a/b), toll-like receptors (Tlr3, Tlr4 and Tlr7) and microglial activations markers (CD14, Tyrobp,Trem2) following Treg administration.
Ex vivo expanded Tregs with amplified immunomodulatory function, suppressed neuroinflammation and alleviated AD pathology in vivo. Our results provide preclinical evidences for Treg cell therapy as a potential treatment strategy in AD.
The chemical composition of the most metal-deficient stars largely reflects the composition of the gas from which they formed. These old stars provide crucial clues to the star formation history and ...the synthesis of chemical elements in the early Universe. They are the local relics of epochs otherwise observable only at very high redshifts; if totally metal-free ('population III') stars could be found, this would allow the direct study of the pristine gas from the Big Bang. Earlier searches for such stars found none with an iron abundance less than 1/10,000 that of the Sun, leading to the suggestion that low-mass stars could form from clouds above a critical iron abundance. Here we report the discovery of a low-mass star with an iron abundance as low as 1/200,000 of the solar value. This discovery suggests that population III stars could still exist-that is, that the first generation of stars also contained long-lived low-mass objects. The previous failure to find them may be an observational selection effect.
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