66 research outputs found
Análise detalhada de abundâncias de estrelas gigantes de bário
No full textBolsa de Iniciação científica FAPERJEstrelas de bário são gigantes vermelhas de tipo espectral G e K que apresentam em suas atmosferas excessos de abundância dos elementos do processo-s de captura de nêutrons. Tais excessos são esperados se a estrela está atravessando a fase de pulsos térmicos do AGB (TP-AGB). As estrelas de bário são, no entanto, menos massivas e menos luminosas que as estrelas do AGB e não se espera, portanto, que elas tenham se auto enriquecido. A hipótese mais aceita para explicar tais anomalias químicas invoca a participação de uma estrela companheira inicialmente mais massiva. Esta companheira evoluiria pela fase TP-AGB e se auto enriqueceria com os elementos do processo-s e então, por processos de perda de massa, perderia material contaminado para a atmosfera da atual estrela de bário. A companheira evolui então para anã branca. As estrelas de bário são, portanto, úteis como testes observacionais para teorias de nucleossíntese pelo processo-s, convecção e perda de massa. Análises detalhadas de abundância com dados de alta qualidade na literatura são ainda escassas para estes objetos. Neste trabalho construímos modelos de atmosferas e, por meio de uma análise diferencial, determinamos parâmetros atmosféricos de uma amostra de dez gigantes de bário e quatro gigantes normais. Caracterizamos em detalhe seus padrões de abundância para os elementos Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr, Y, Zr, Ba, La, Ce, Nd, Sm, Eu e Gd e determinamos em detalhe as fontes de erro nestes resultados para sua melhor caracterização. Concluímos assim que alguns objetos classificados na literatura como estrelas de bário são na verdade gigantes normais. Posicionamos as estrelas no diagrama HR obtendo suas massas e idades. Construímos dois padrões médios de abundâncias, para estrelas com excessos de abundância moderados e com grandes excessos. Comparamos esses resultados com previsões teóricas e determinamos os melhores parâmetros de exposição de nêutrons que os ajustam. Concluímos que os mesmos valores de exposição de nêutrons podem reproduzir os excessos observados nos dois grupos de estrelas. Discutimos ainda efeitos nucleossintéticos, ligados ao processo-s, sugeridos na literatura para elementos como Cu, Mn, V e Sc
The relation between velocity dispersions and chemical abundances in RAVE giants
No full textAbstractWe developed a Bayesian framework to determine in a robust way the relation between velocity dispersions and chemical abundances in a sample of stars. Our modelling takes into account the uncertainties in the chemical and kinematic properties. We make use of RAVE DR5 radial velocities and abundances together with Gaia DR1 proper motions and parallaxes (when possible, otherwise UCAC4 data is used). We found that, in general, the velocity dispersions increase with decreasing [Fe/H] and increasing [Mg/Fe]. A possible decrease in velocity dispersion for stars with high [Mg/Fe] is a property of a negligible fraction of stars and hardly a robust result. At low [Fe/H] and high [Mg/Fe] the sample is incomplete, affected by biases, and likely not representative of the underlying stellar population.</jats:p
Exploring the chemodynamics of metal-poor stellar populations
No full textMetal-poor stars are key for studying the formation and evolution of the
Galaxy. Evidence of the early mergers that built up the Galaxy remains in the
distributions of abundances, kinematics, and orbital parameters of its stars.
Several substructures resulting from these mergers have been tentatively
identified in the literature. We conduct a global analysis of the chemodynamic
properties of metal-poor stars. Our aim is to identify signs of accreted and in
situ stars in different regions of the parameter space and to investigate their
differences and similarities. We selected a sample of about 6600 metal-poor
stars with [Fe/H] -0.8 from DR3 of the GALAH survey. We used
unsupervised machine learning to separate stars in a parameter space made of
two normalised orbital actions, plus [Fe/H] and [Mg/Fe], without additional a
priori cuts on stellar properties. We divided the halo stars in four main
groups. All groups exhibit a significant fraction of in situ contamination
(ISC). Accreted stars of these groups have very similar chemical properties,
except for those of the group of stars with very retrograde orbits. This points
to at most two main sources of accreted stars in the current sample, the major
one related to Gaia-Enceladus (GE) and the other possibly related to Thamnos
and/or Sequoia. Stars of GE are r-process enriched at low metallicities, but a
contribution of the s-process appears with increasing metallicity. A flat trend
of [Eu/Mg] as a function of [Fe/H] suggests that only core collapse supernovae
contributed to r-process elements in GE. To better characterise accreted stars
in the low metallicity regime, high precision abundances and guidance from
chemical evolution models are needed. It is possible that ISC in samples of
accreted stars has been underestimated. This can have important consequences
for attempts to estimate the properties of the original systems.Comment: 22 pages, 22 figures, Accepted for publication in A&
Chemical composition of giant stars in the open cluster IC 4756
No full textContext. Homogeneous investigations of red giant stars in open clusters contribute to studies of internal evolutionary mixing processes inside stars, which are reflected in abundances of mixing-sensitive chemical elements like carbon, nitrogen, and sodium, while α- and neutron-capture element abundances are useful in tracing the Galactic chemical evolution.
Aims. The main aim of this study is a comprehensive chemical analysis of red giant stars in the open cluster IC 4756, including determinations of 12C∕13C and C/N abundance ratios, and comparisons of the results with theoretical models of stellar and Galactic chemical evolution.
Methods. We used a classical differential model atmosphere method to analyse high-resolution spectra obtained with the FEROS spectrograph on the 2.2 m MPG/ESO Telescope. The carbon, nitrogen, and oxygen abundances, 12C∕13C ratios, and neutron-capture element abundances were determined using synthetic spectra, and the main atmospheric parameters and abundances of other chemical elements were determined from equivalent widths of spectral lines.
Results. We have determined abundances of 23 chemical elements for 13 evolved stars and 12C∕13C ratios for six stars of IC 4756. The mean metallicity of this cluster, as determined from nine definite member stars, is very close to solar – [Fe/H] = − 0.02 ± 0.01. Abundances of carbon, nitrogen, and sodium exhibit alterations caused by extra-mixing: the mean 12C∕13C ratio is lowered to 19 ± 1.4, the C/N ratio is lowered to 0.79 ± 0.05, and the mean [Na/Fe] value, corrected for deviations from the local thermodynamical equilibrium encountered, is enhanced by 0.14 ± 0.05 dex. We compared our results to those by other authors and theoretical models.
Conclusions. Comparison of the α-element results with the theoretical models shows that they follow the thin disc α-element trends. Being relatively young (~ 800 Myr), the open cluster IC 4756 displays a moderate enrichment of s-process-dominated chemical elements compared to the Galactic thin disc model and confirms the enrichment of s-process-dominated elements in young open clusters compared to the older ones. The r-process-dominated element europium abundance agrees with the thin disc abundance. From the comparison of our results for mixing-sensitive chemical elements and the theoretical models, we can see that the mean values of 12C∕13C, C/N, and [Na/Fe] ratios lie between the model with only the thermohaline extra-mixing included and the model which also includes the rotation-induced mixing. The rotation was most probably smaller in the investigated IC 4756 stars than 30% of the critical rotation velocity when they were on the main sequence
Beryllium abundances along the evolutionary sequence of the open cluster IC 4651
No full textThe simultaneous investigation of Li and Be in stars is a powerful tool in the study of the evolutionary mixing processes. Here, we present beryllium abundances in stars along the whole evolutionary sequence of the open cluster IC 4651. This cluster has a metallicity of [Fe/H] = +0.11 and an age of 1.2 or 1.7 Gyr. Abundances have been determined from high-resolution, high signal-to-noise UVES spectra using spectrum synthesis and model atmospheres. Lithium abundances for the same stars were determined in a previous work. Confirming previous results, we find that the Li dip is also a Be dip. For post-main-sequence stars, the Be dilution starts earlier within the Hertzsprung gap than expected from classical predictions, as does the Li dilution. Theoretical hydrodynamical models are able to reproduce well all the observed feature
TITANS: the metal-poor reference stars
No full textTracing the history of chemical evolution of the Milky Way requires the determination of chemical abundances in a large assortment of stars with accurate stellar parameters. Nonetheless, the accuracy achieved in the derived astrophysical parameters is still insufficient, mainly because of the paucity of adequate calibrators, particularly in the metal-poor regime ([Fe/H] < -1.0). At present, the prime stellar calibration sample is that of the Gaia benchmark stars. Most of these stars have effective temperature constrained by measurements of their bolometric fluxes and angular diameters. However, only seven of them are metal-poor. To increase this number, we introduce the “Titans metal-poor reference stars”: stars with accurate model-dependent parameters that are "descendants" of the Gaia benchmarks
Beryllium abundances and the formation of the halo and the thick disk
No full textInternational audienceThe single stable isotope of beryllium is a pure product of cosmic-ray spallation in the ISM. Assuming that the cosmic-rays are globally transported across the Galaxy, the beryllium production should be a widespread process and its abundance should be roughly homogeneous in the early-Galaxy at a given time. Thus, it could be useful as a tracer of time. In an investigation of the use of Be as a cosmochronometer and of its evolution in the Galaxy, we found evidence that in a log(Be/H) vs. [alpha/Fe] diagram the halo stars separate into two components. One is consistent with predictions of evolutionary models while the other is chemically indistinguishable from the thick-disk stars. This is interpreted as a difference in the star formation history of the two components and suggests that the local halo is not a single uniform population where a clear age-metallicity relation can be defined. We also found evidence that the star formation rate was lower in the outer regions of the thick disk, pointing towards an inside-out formation
12C/13C ratios in giants of open clusters
No full textThe carbon isotopic ratio, 12C/13C, is a tracer of the mixing events during the evolution along the giant branch, due to the conversion of 12C into 13C (and 14N) via the CN cycle. A decrease of this ratio from 90, the solar value, to 20-25, is expected due to the first dredge-up. However, ratios down to 3-4, the CN cycle equilibrium value, have been observed in giants of the field, of globular and of open clusters. Observations seem to indicate a non-standard mixing in the RGB, probably beginning in the luminosity bump, when the outward moving hydrogen burning shell crosses the molecular weight barrier left by the convective layer in its maximum extent. We are currently analyzing a sample of 24 giants in 8 open clusters for which we determined 12C/13C from high resolution, high signal to noise spectra using spectrum synthesis. In this work we discuss the general characteristics of our results in comparison to previous analyses of giants in open clusters available in the literatur
Beryllium and the formation of the Thick Disk and of the Halo
No full textInternational audienceWe use Beryllium to investigate star formation in the early Galaxy. Be has been demonstrated to be a good indicator of time in these early epochs. By analyzing the so-far largest sample of halo and thick disk metal poor stars, we find a clear scatter in Be for a given value of [Fe/H] and [O/H]. The scatter is very pronounced for Halo stars, while it is marginal for thick disk stars. Our halo stars separate in the [alpha/Fe] - Be diagram, showing two main branches: one indistinguishable from the thick disk stars, and one with lower [alpha/Fe] ratio. The stars belonging to this branch are characterized by highly eccentric orbits and small perigalactic radius (Rmin). Their kinematics are consistent with an accreted component
- …
