Looking for imprints of the first stellar generations in metal-poor bulge field stars

© 2016 ESO. Context. Efforts to look for signatures of the first stars have concentrated on metal-poor halo objects. However, the low end of the bulge metallicity distribution has been shown to host some of the oldest objects in the Milky Way and hence this Galactic component potentially offers interesting targets to look at imprints of the first stellar generations. As a pilot project, we selected bulge field stars already identified in the ARGOS survey as having [Fe/H] 1 and oversolar [α/Fe] ratios, and we used FLAMES-UVES to obtain detailed abundances of key elements that are believed to reveal imprints of the first stellar generations. Aims. The main purpose of this study is to analyse selected ARGOS stars using new high-resolution (R ∼ 45 000) and high-signal-tonoise (S=N > 100) spectra. We aim to derive their stellar parameters and elemental ratios, in particular the abundances of C, N, the α-elements O, Mg, Si, Ca, and Ti, the odd-Z elements Na and Al, the neutron-capture s-process dominated elements Y, Zr, La, and Ba, and the r-element Eu. Methods. High-resolution spectra of five field giant stars were obtained at the 8 m VLT UT2-Kueyen telescope with the UVES spectrograph in FLAMES-UVES configuration. Spectroscopic parameters were derived based on the excitation and ionization equilibrium of Fe i and Fe ii. The abundance analysis was performed with a MARCS LTE spherical model atmosphere grid and the Turbospectrum spectrum synthesis code. Results.We confirm that the analysed stars are moderately metal-poor (-1:04≤[Fe/H]≤-0:43), non-carbon-enhanced (non-CEMP) with [C/Fe] ≤+0:2, and α-enhanced.We find that our three most metal-poor stars are nitrogen enhanced. The α-enhancement suggests that these stars were formed from a gas enriched by core-collapse supernovae, and that the values are in agreement with results in the literature for bulge stars in the same metallicity range. No abundance anomalies (Na-O, Al-O, Al-Mg anti-correlations) were detected in our sample. The heavy elements Y, Zr, Ba, La, and Eu also exhibit oversolar abundances. Three out of the five stars analysed here show slightly enhanced [Y/Ba] ratios similar to those found in other metal-poor bulge globular clusters (NGC 6522 and M 62). Conclusions. This sample shows enhancement in the first-to-second peak abundance ratios of heavy elements, as well as dominantly s-process element excesses. This can be explained by different nucleosynthesis scenarios: (a) the main r-process plus extra mechanisms, such as the weak r-process; (b) mass transfer from asymptotic giant branch stars in binary systems; (c) an early generation of fast-rotating massive stars. Larger samples of moderately metal-poor bulge stars, with detailed chemical abundances, are needed to better constrain the source of dominantly s-process elements in the early Universe

Oxygen Abundance in the Template Halo Giant HD 122563

HD 122563 is a well-known bright (V ¼ 6:2) halo giant of low metallicity ((Fe/H ��� 2:7). We have observed HD 122563 for infrared OH lines at 1.5-1.7 lm in the H band with the NIRSPEC high-resolution spectrograph at the 10 m Keck Telescope. Optical spectra were obtained with the UVES spectrograph at the 8 m VLT UT2 telescope at ESO (Paranal) and the FEROS spectrograph at ESO (La Silla). Based on the opti- cal high-resolution data, a detailed analysis has been carried out, and data on the forbidden (O i) 6300 Aline, unblended by telluric or sky lines, was obtained with the FEROS spectrograph. Signal-to-noise ratios of 200- 400 were obtained at resolutions of 37,000 in the H band and 45,000 in the optical. For the analysis we have adopted a photometric effective temperature Teff ¼ 4600 K. Two values for the gravity were adopted: a value deduced from ionization equilibrium, log g ¼ 1:1, with corresponding metallicity (Fe/H �¼� 2:8 and micro- turbulence velocity vt ¼ 2: 0k m s � 1 ; and log g ¼ 1:5, derived from the Hipparcos parallax, implying (Fe/H �¼� 2:71 and vt ¼ 2: 0k m s � 1 . The forbidden (O i) 6300 Aand the permitted O i 7771 Alines give O/Fe ratios essentially insensitive to model parameter variations, whereas the oxygen abundances from OH lines are sensitive to gravity, giving (O/Fe �¼þ 0:9 and +0.7, respectively, for log g ¼ 1:1 and 1.5. We derive the following oxygen abundances: for model 1, (O/Fe �¼þ 0:6, +1.1, and +0.9; and for model 2, (O/Fe �¼þ 0:6, +1.1, and +0.7, based on the (O i) 6300 A ˚ ,O i 7771 A ˚ , and IR OH 1.6 lm lines, respectively. The different oxygen abundance indicators give different oxygen abundances, illustrating the problem of oxy- gen abundance derivation in metal-poor giants. This is important because the age of globular clusters and the production of Li, Be, and B from spallation of C, N, and O atoms in the early Galaxy depend on the oxygen abundance adopted for the metal-poor stars. Subject headings: stars: abundances — stars: individual (HD 122563) — stars: Population II On-line material: machine-readable tabl

UVES analysis of red giants in the bulge globular cluster NGC 6522

Context. NGC 6522 is a moderately metal-poor bulge globular cluster ([Fe/H] ~ −1.0), and it is a well-studied representative among a number of moderately metal-poor blue horizontal branch clusters located in the bulge. The NGC 6522 abundance pattern can give hints on the earliest chemical enrichment in the central Galaxy. Aims. The aim of this study is to derive abundances of the light elements C and N; alpha elements O, Mg, Si, Ca, and Ti; odd-Z elements Na and Al; neutron-capture elements Y, Zr, Ba, La, and Nd; and the r-process element Eu. We verify if there are first- and second-generation stars: we find clear evidence of Na–Al, Na–N, and Mg–Al correlations, while we cannot identify the Na–O anti-correlation from our data. Methods. High-resolution spectra of six red giants in the bulge globular cluster NGC 6522 were obtained at the 8m VLT UT2-Kueyen telescope with both the UVES and GIRAFFE spectrographs in FLAMES+UVES configuration. In light of Gaia data, it turned out that two of them are non-members, but these were also analysed. Spectroscopic parameters were derived through the excitation and ionisation equilibrium of Fe I and Fe II lines from UVES spectra. The abundances were obtained with spectrum synthesis. Comparisons of abundances derived from UVES and GIRAFFE spectra were carried out. Results. The present analysis combined with previous UVES results gives a mean radial velocity of vrhel = −15.62±7.7 km s−1 and a metallicity of [Fe/H] = −1.05 ± 0.20 for NGC 6522. Mean abundances of alpha elements for the present four member stars are enhanced with [O/Fe] = +0.38, [Mg/Fe] = ≈+0.28, [Si/Fe] ≈ +0.19, and [Ca/Fe] ≈ +0.13, together with the iron-peak element [Ti/Fe] ≈ +0.13, and the r-process element [Eu/Fe] = +0.40. The neutron-capture elements Y, Zr, Ba, and La show enhancements in the +0.08 < [Y/Fe] < +0.90, 0.11 < [Zr/Fe] < +0.50, 0.00 < [Ba/Fe] < +0.63, 0.00 < [La/Fe] < +0.45, and −0.10 < [Nd/Fe] < +0.70 ranges. We also discuss the spread in heavy-element abundances

The Astropy Problem

The Astropy Project (http://astropy.org) is, in its own words, "a community effort to develop a single core package for Astronomy in Python and foster interoperability between Python astronomy packages." For five years this project has been managed, written, and operated as a grassroots, self-organized, almost entirely volunteer effort while the software is used by the majority of the astronomical community. Despite this, the project has always been and remains to this day effectively unfunded. Further, contributors receive little or no formal recognition for creating and supporting what is now critical software. This paper explores the problem in detail, outlines possible solutions to correct this, and presents a few suggestions on how to address the sustainability of general purpose astronomical software

NOMINAL VALUES FOR SELECTED SOLAR AND PLANETARY QUANTITIES: IAU 2015 RESOLUTION B3

In this brief communication we provide the rationale for and the outcome of the International Astronomical Union (IAU) resolution vote at the XXIXth General Assembly in Honolulu, Hawaii, in 2015, on recommended nominal conversion constants for selected solar and planetary properties. The problem addressed by the resolution is a lack of established conversion constants between solar and planetary values and SI units: a missing standard has caused a proliferation of solar values (e.g., solar radius, solar irradiance, solar luminosity, solar effective temperature, and solar mass parameter) in the literature, with cited solar values typically based on best estimates at the time of paper writing. As precision of observations increases, a set of consistent values becomes increasingly important. To address this, an IAU Working Group on Nominal Units for Stellar and Planetary Astronomy formed in 2011, uniting experts from the solar, stellar, planetary, exoplanetary, and fundamental astronomy, as well as from general standards fields to converge on optimal values for nominal conversion constants. The effort resulted in the IAU 2015 Resolution B3, passed at the IAU General Assembly by a large majority. The resolution recommends the use of nominal solar and planetary values, which are by definition exact and are expressed in SI units. These nominal values should be understood as conversion factors only, not as the true solar/planetary properties or current best estimates. Authors and journal editors are urged to join in using the standard values set forth by this resolution in future work and publications to help minimize further confusion

Abondance des éléments plus légers que le zinc dans les premières étoiles galactiques - Implications sur la nature des premières supernovæ

During my Ph. D. I have analyzed $33$ extremely metal-deficient galactic halo stars (stars having less than $500$ times less metals than the Sun) observed at the VLT, using the high resolution spectrograph UVES. These stars are relics from the very first ages of our Galaxy, and thus provide useful constraints on both the formation and on the evolution models of our Galaxy.I determined the abundances for $17$ elements from carbon to zinc with an unprecedented accuracy, including the key elements oxygen and zinc, tounderstand which kind of supernova had enriched the interstellar medium during the early times of the Galaxy.I have shown in this work that we could explain the observed abundance ratio witout including very massive supernova (stars whose mass is greater then $100$\Msol).In addition, the abundance trends are compared with Galactic chemical evolution models. As the study is based on very metal poor stars that are supposed to be born during the first ages of our Galaxy, my work brings strong new observational constraints to these models.Au cours de cette thèse, j'ai étudié $33$ étoiles extrêmement déficientes du halo galactique (ayant moins de $500$ fois moins de métaux que le Soleil) observées au VLT avec le spectrographe à haute résolution UVES. Ces étoiles sont les témoins des tous premiers âges de notre Galaxie. La connaissance détaillée de leur composition chimique permet de contraindre les modèles de formation et d'évolution de notre galaxie.J'ai pu déterminer dans ces étoiles les abondances de $17$ éléments avec une précision inégalée, allant du carbone au zinc, et en particulier, celles d'éléments «clés» comme l'oxygène et le zinc, pour comprendre quel type de supernova a enrichi la matière au début de la vie de la Galaxie.J'ai montré en particulier que l'on peut expliquer les rapports d'abondance observés sans faire intervenir de supernova supermassive (dont la masse dépasse $100$\Msol~).Par ailleurs, l'évolution des abondances en fonction de la métallicité moyenne est comparée aux modèles d'évolution chimique de la Galaxie. Notre étude portant sur des étoiles réputées être nées au tout début de la vie de notre Galaxie, nous apportons des contraintes observationnelles fortes à ces modèles

Abondance des éléments plus légers que le zinc dans les premières étoiles galactiques (implications sur la nature des premières supernovae)

PARIS-BIUSJ-Thèses (751052125) / SudocMEUDON-Observatoire (920482302) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Metal-poor Stars Observed with the Southern African Large Telescope

© 2020. The American Astronomical Society. All rights reserved.. We present the first release of a large-scale study of relatively bright (V +0.70), and that a large fraction (26 of 50, 52%) are enhanced in r-process elements. Among the r-process-enhanced stars, five are strongly enhanced r-II ([Eu/Fe] > +1.0) stars (two of which are newly discovered) and 21 are newly discovered moderately enhanced r-I (+0.3 ≤ [Eu/Fe] ≤ +1.0) stars. There are eight stars in our sample that, on the basis of their abundances and kinematics, are possible members of the metal-weak thick-disk population. We also compare our measured abundances to progenitor-enrichment models, and find that the abundance patterns for the majority of our stars can be attributed to a single (rather than multiple) enrichment event

High-resolution abundance analysis of red giants in the globular cluster NGC 6522

Context. The [Sr/Ba] and [Y/Ba] scatter observed in some galactic halo stars that are very metal-poor and in a few individual stars of the oldest known Milky Way globular cluster NGC 6522 have been interpreted as evidence of early enrichment by massive fastrotating stars (spinstars). Because NGC 6522 is a bulge globular cluster, the suggestion was that not only the very-metal poor halo stars, but also bulge stars at [Fe/H] ∼−1 could be used as probes of the stellar nucleosynthesis signatures from the earlier generations of massive stars, but at much higher metallicity. For the bulge the suggestions were based on early spectra available for stars in NGC 6522, with a medium resolution of R ∼ 22 000 and a moderate signal-to-noise ratio. Aims. The main purpose of this study is to re-analyse the NGC 6522 stars reported previously by using new high-resolution (R ∼ 45 000) and high signal-to-noise spectra (S /N > 100).We aim at re-deriving their stellar parameters and elemental ratios, in particular the abundances of the neutron-capture s-process-dominated elements such as Sr, Y, Zr, La, and Ba, and of the r-element Eu. Methods. High-resolution spectra of four giants belonging to the bulge globular cluster NGC 6522 were obtained at the 8m VLT UT2-Kueyen telescope with the UVES spectrograph in FLAMES-UVES configuration. The spectroscopic parameters were derived based on the excitation and ionization equilibrium of Fe i and Fe ii. Results. Our analysis confirms a metallicity [Fe/H] = −0.95 ± 0.15 for NGC 6522 and the overabundance of the studied stars in Eu (with +0.2 < [Eu/Fe] < +0.4) and alpha-elements O and Mg. The neutron-capture s-element-dominated Sr, Y, Zr, Ba, and La now show less pronounced variations from star to star. Enhancements are in the range 0.0 < [Sr/Fe] < +0.4, +0.23 < [Y/Fe] < +0.43, 0.0 < [Zr/Fe] < +0.4, 0.0 < [La/Fe] < +0.35, and 0.05 < [Ba/Fe] < +0.55. Conclusions. The very high overabundances of [Y/Fe] previously reported for the four studied stars is not confirmed with the new high-quality spectra. The moderate enhancement in [Sr/Fe] previously reported for one of the re-studied stars is confirmed, but the strong enhancements of this ratio for the other two stars are not confirmed. Despite the moderate enhancements found for the neutroncapture s-element-dominated species, none of the four stars studied here show positive values for all [Sr/Ba], [Y/Ba] and [Zr/Ba] ratios. The re-studied stars are now compatible not only with the interpretation that the s-process enhancements in these very old stars are due to spinstars, but also with alternative models such as mass transfer from s-process-rich AGB stars. Note, however, that when our results are interpreted in the context of more extended datasets from the literature, the spinstar scenario still seems to be favoured