The Milky Way is a unique laboratory in which stellar properties can be measured and analyzed in detail. In particular, stars in the older populations encode information on the mechanisms that led to ...the formation of our Galaxy. In this article, we analyze the kinematics, spatial distribution, and chemistry of a large number of stars in the solar neighborhood, where all of the main Galactic components are well represented. We find that the thick disk comprises two distinct and overlapping stellar populations with different kinematic properties and chemical compositions. The metal-weak thick disk (MWTD) contains two-times less metal content than the canonical thick disk, and exhibits enrichment of light elements typical of the oldest stellar populations of the Galaxy. The rotational velocity of the MWTD around the Galactic center is ∼150 km s−1, corresponding to a rotational lag of 30 km s−1 relative to the canonical thick disk (∼180 km s−1), with a velocity dispersion of 60 km s−1. This stellar population likely originated from the merger of a dwarf galaxy during the early phases of our Galaxy's assembly, or it is a precursor disk, formed in the inner Galaxy and brought into the solar neighborhood by bar instability or spiral-arm formation mechanisms.
Very high quality spectra of 24 metal-poor halo dwarfs and subgiants have been acquired with ESO's VLT/UVES for the purpose of determining Li isotopic abundances. The derived one-dimensional, non-LTE ...super(7)Li abundances from the Li I 670.8 nm line reveal a pronounced dependence on metallicity but with negligible scatter around this trend. Very good agreement is found between the abundances from the Li I 670.8 nm line and the Li I 610.4 nm line. The estimated primordial super(7)Li abundance is super(7)Li/H = (1.1-1.5) x 10 super(-10), which is a factor of 3-4 lower than predicted from standard big bang nucleosynthesis with the baryon density inferred from the cosmic microwave background. Interestingly, super(6)Li is detected in 9 of our 24 stars at the .2 s significance level. Our observations suggest the existence of a super(6)Li plateau at the level of log super(6)Li - 0.8; however, taking into account predictions for super(6)Li destruction during the pre-main-sequence evolution tilts the plateau such that the super(6)Li abundances apparently increase with metallicity. Our most noteworthy result is the detection of super(6)Li in the very metal-poor star LP 815-43. Such a high super(6)Li abundance during these early Galactic epochs is very difficult to achieve by Galactic cosmic-ray spallation and a-fusion reactions. It is concluded that both Li isotopes have a pre-Galactic origin. Possible super(6)Li production channels include protogalactic shocks and late-decaying or annihilating supersymmetric particles during the era of big bang nucleosynthesis. The presence of super(6)Li limits the possible degree of stellar super(7)Li depletion and thus sharpens the discrepancy with standard big bang nucleosynthesis.
In today professional world and especially in STEM related fields, gender balance (or the lack thereof) remains a hotly debated topic. Gender equality and the empowerment of all women and girls is ...the fifth goal set by the United Nations in 2015 (out of 17 Sustainable Development Goals in total) that is intended to be achieved by the year 2030. It is in fact fully understood that we cannot simply afford missing out on half (actually more) of the human capital and potential and one of the key factors to achieve this is education for all. Together with broader themes like diversity and inclusion, gender equality has also made it to the top of many institutions/organizations agendas, because diverse teams are more creative, more productive and improve the working experience and environment for everybody. To this end, a significant push has also come from regular reports published by international or consultancy agencies like the World Economic Forum (WEF) or Mc Kinsey that have often made the news headlines with highly visible and provocative titles. The most recent WEF Global Gender Gap Report 2020 clearly states that “It will take us another 99.5 years to reach gender parity” and that “It will be the year 2277 when the gender pay gap will close”. In this review, the focus will be on a specific STEM field that has always fascinated generations of people, just by looking up at the peaceful beauty of a night sky: astronomy.
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.
Atomic diffusion and mixing in old stars Gruyters, Pieter; Lind, Karin; Richard, Olivier ...
Astronomy and astrophysics (Berlin),
05/2016, Letnik:
589
Journal Article
Recenzirano
Context. The prediction of the Planck-constrained primordial lithium abundance in the Universe is in discordance with the observed Li abundances in warm Population II dwarf and subgiant stars. Among ...the physically best motivated ideas, it has been suggested that this discrepancy can be alleviated if the stars observed today had undergone photospheric depletion of lithium. Aims. The cause of this depletion is investigated by accurately tracing the behaviour of the lithium abundances as a function of effective temperature. Globular clusters are ideal laboratories for such an abundance analysis as the relative stellar parameters of their stars can be precisely determined. Methods. We performed a homogeneous chemical abundance analysis of 144 stars in the metal-poor globular cluster M30, ranging from the cluster turnoff point to the tip of the red giant branch. Non-local thermal equilibrium (NLTE) abundances for Li, Ca, and Fe were derived where possible by fitting spectra obtained with VLT/FLAMES-GIRAFFE using the quantitative-spectroscopy package SME. Stellar parameters were derived by matching isochrones to the observed V vs. V-I colour-magnitude diagram. Independent effective temperatures were obtained from automated profile fitting of the Balmer lines and by applying colour-T sub(eff) calibrations to the broadband photometry. Results. Li abundances of the turnoff and early subgiant stars form a thin plateau that is broken off abruptly in the middle of the SGB as a result of the onset of Li dilution caused by the first dredge-up. Abundance trends with effective temperature for Fe and Ca are observed and compared to predictions from stellar structure models including atomic diffusion and ad hoc additional mixing below the surface convection zone. The comparison shows that the stars in M30 are affected by atomic diffusion and additional mixing, but we were unable to determine the efficiency of the additional mixing precisely. This is the fourth globular cluster (after NGC6397, NGC6752, and M4) in which atomic diffusion signatures are detected. After applying a conservative correction (T6.0 model) for atomic diffusion, we find an initial Li abundance of A(Li) = 2.48 + or - 0.10 for the globular cluster M30. We also detected a Li-rich SGB star with a Li abundance of A(Li) = 2.39. The finding makes Li-rich mass transfer a likely scenario for this star and rules out models in which its Li enhancement is created during the RGB bump phase.
Context. Several models compete to explain the abundance properties of stellar populations in globular clusters. One of the main constraints is the present-day ratio of first- and second-generation ...stars that are currently identified based on their sodium content. Aims. We propose an alternative interpretation of the observed sodium distribution, and suggest that stars with low sodium abundance that are counted as members of the first stellar generation could actually be second-generation stars. Methods. We compute the number ratio of second-generation stars along the Na distribution following the fast rotating massive star model using the same constraints from the well-documented case of NGC 6752 as in our previous developments. Results. We reproduce the typical percentage of low-sodium stars usually classified as first-generation stars by invoking only secondary star formation from material ejected by massive stars and mixed with original globular cluster material in proportions that account for the Li-Na anti-correlation in this cluster. Conclusions. Globular clusters could be totally devoid of first-generation low-mass stars today. This can be tested with the determination of the carbon isotopic ratio and nitrogen abundance in turn-off globular cluster stars. Consequences and related issues are briefly discussed.
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