75 research outputs found
Stellar Abundances in Giant Stars in the Metal-Rich Globular Cluster NGC6528
We present the first results of a detailed abundance analysis, based on VLT
observations, of giant stars in the very metal-rich globular cluster NGC 6528.
We will be able to tie the horizontal branch abundances (see e.g. Carretta et
al. 2001) to those of the more luminous giants (see CMDs). For the very similar
cluster NGC 6553 studies of different types of stars have yielded very
disparate results. Our first analysis of three of our stars seem to indicate
that indeed the different sorts of stars do show similar abundances if one
homogeneous set of models and parameters are being used.Comment: 2 pages, 2 diagrams, to be published in the proceedings of "New
Horizons in Globular Cluster Astronomy" held in Padua 2002, eds. Piotto,
Meylan, Djorgovsk
Observational signatures of lithium depletion in the metal-poor globular cluster NGC6397
The "stellar” solution to the cosmological lithium problem proposes that surface depletion of lithium in low-mass, metal-poor stars can reconcile the lower abundances found for Galactic halo stars with the primordial prediction. Globular clusters are ideal environments for studies of the surface evolution of lithium, with large number statistics possible to obtain for main sequence stars as well as giants. We discuss the Li abundances measured for >450 stars in the globular cluster NGC 6397, focusing on the evidence for lithium depletion and especially highlighting how the inferred abundances and interpretations are affected by early cluster self-enrichment and systematic uncertainties in the effective temperature determinatio
The lithium history of NGC 6397
The primordial lithium abundance inferred from WMAP and standard Big Bang nucleosysnthesis is approximately three times higher than the plateau value measured in old metal-poor Population II stars, suggesting that these stars have undergone atmospheric Li depletion. To constrain the physics responsible for such depletion, we conducted a homogeneous analysis of a large sample of stars in the metal-poor globular cluster NGC 6397, covering all evolutionary phases from below the main-sequence turnoff to high up the red-giant branch (RGB). The dwarf, turnoff, and early subgiant stars form a thin abundance plateau, with a sharpe edge in the middle of the subgiant branch, where Li dilution caused by the inward extension of the convective envelope starts (the beginning of the so-called first dredge up). A second steep abundance drop is seen at the RGB bump, again highlighting the need for the onset of nonstandard mixing in this evolutionary phase. Moreover, by also measuring the sodium abundances of the targets, we have gained insight into the degree of pollution by early cluster self-enrichement, and may separate highly polluted, Li-poor and Na-rich stars from stars formed from pristine material. Our observational findings strictly limit both the extent of lithium surface depletion, which in turn constrains the efficiency of mixing below the outer convection zone, and the resulting spread in lithium abundance in metal-poor turn-off star
Atomic diffusion and mixing in old stars VI: The lithium content of M30
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. 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. 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. NLTE
abundances for Li, Ca, and Fe were derived where possible. 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_eff calibrations
to the broadband photometry. 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. After applying a conservative correction for atomic
diffusion, we find an initial Li abundance of A(Li) = for the
globular cluster M30.Comment: 13 pages, 7 tables (Tab. 1 and 5 can be obtained at ADS) and 8
figure
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