44 research outputs found
Galactic globular cluster 47 Tucanae: new ties between the chemical and dynamical evolution of globular clusters?
It is generally accepted today that Galactic globular clusters (GGCs) consist
of at least two generations of stars that are different in their chemical
composition and perhaps age. However, knowledge about the kinematical
properties of these stellar generations, which may provide important
information for constraining evolutionary scenarios of the GGCs, is still
limited. We therefore study the connections between chemical and kinematical
properties of different stellar generations in the Galactic globular cluster 47
Tuc. To achieve this goal, we used abundances of Li, O, and Na determined in
101 main sequence turn-off (TO) stars with the aid of 3D hydrodynamical model
atmospheres and NLTE abundance analysis methodology. We divided our sample TO
stars into three groups according to their position in the [Li/Na]-[Na/O] plane
to study their spatial distribution and kinematical properties. We find that
there are statistically significant radial dependencies of lithium and oxygen
abundances, A(Li) and A(O), as well as that of [Li/Na] abundance ratio. Our
results show that first-generation stars are less centrally concentrated and
dynamically hotter than stars belonging to subsequent generations. We also find
a significant correlation between the velocity dispersion and O and Na
abundance, and between the velocity dispersion and the [Na/O] abundance ratio.Comment: 4 pages, 6 figures, accepted for publication in A&
Abundance of zirconium in the globular cluster 47 Tuc: a possible Zr-Na correlation?
We determined abundances of Na and Zr in the atmospheres of 237 RGB stars in
Galactic globular cluster (GGC) 47 Tuc (NGC 104), with a primary objective of
investigating possible differences between the abundances of Zr in the first
generation (1P) and second generation (2P) stars. For the abundance analysis,
we used archival UVES/GIRAFFE spectra obtained during three different observing
programmes. Abundances were determined from two Na I and three Zr I lines,
using 1D hydrostatic ATLAS model atmospheres. The target stars for the
abundance analysis were limited to those with 4200
4800 K. This is the largest sample of GGC stars in which Na and Zr abundances
have been studied so far. While our mean [Na/Fe] and [Zr/Fe] ratios agree well
with those determined in the earlier studies, we find a weak but statistically
significant correlation in the plane. A
comparison of the mean [Zr/Fe] abundance ratios in the 1P and 2P stars suggests
a small but statistically significant Zr over-abundance in the 2P stars,
dex. Also, our analysis shows
that stars enriched in both Zr and Na are more centrally concentrated. However,
we find no correlation between their distance from the cluster centre and their
full spatial velocity. While there may be some influence of CN line blends on
the determined Zr abundances, it seems very unlikely that the detected Zr-Na
correlation, for the slightly higher Zr abundances in the 2P stars, would be
caused by the CN blending alone. The obtained results indicate that, in 47 Tuc,
some amount of Zr should have been synthesised by the same polluters that
enriched 2P stars with the light elements. Unfortunately, our data alone do not
allow us to distinguish which of the two scenarios, or whether or not a
combination of both, could have operated in this GGC
Can we trust elemental abundances derived in late-type giants with the classical 1D stellar atmosphere models?
We compare the abundances of various chemical species as derived with 3D
hydrodynamical and classical 1D stellar atmosphere codes in a late-type giant
characterized by T_eff=3640K, log g = 1.0, [M/H] = 0.0. For this particular set
of atmospheric parameters the 3D-1D abundance differences are generally small
for neutral atoms and molecules but they may reach up to 0.3-0.4 dex in case of
ions. The 3D-1D differences generally become increasingly more negative at
higher excitation potentials and are typically largest in the optical
wavelength range. Their sign can be both positive and negative, and depends on
the excitation potential and wavelength of a given spectral line. While our
results obtained with this particular late-type giant model suggest that 1D
stellar atmosphere models may be safe to use with neutral atoms and molecules,
care should be taken if they are exploited with ions.Comment: Poster presented at the IAU Symposium 265 "Chemical Abundances in the
Universe: Connecting First Stars to Planets", Rio de Janeiro, 10-14 August
2009; 2 pages, 1 figur
Abundances of lithium, oxygen, and sodium in the turn-off stars of Galactic globular cluster 47 Tuc
We aim to determine abundances of Li, O and Na in a sample of of 110 turn-off
(TO) stars, in order to study the evolution of light elements in this cluster
and to put our results in perspective with observations of other globular and
open clusters, as well as with field stars. We use medium resolution spectra
obtained with the GIRAFFE spectrograph at the ESO 8.2m Kueyen VLT telescope and
use state of the art 1D model atmospheres and NLTE line transfer to determine
the abundances. We also employ CO5BOLD hydrodynamical simulations to assess the
impact of stellar granulation on the line formation and inferred abundances.
Our results confirm the existence of Na-O abundance anti-correlation and hint
towards a possible Li-O anti-correlation in the TO stars of 47 Tuc. We find no
convincing evidence supporting the existence of Li-Na correlation. The obtained
3D NLTE mean lithium abundance in a sample of 94 TO stars where Li lines were
detected reliably,
dex, appears to be significantly lower than what is observed in other globular
clusters. At the same time, star-to-star spread in Li abundance is also larger
than seen in other clusters. The highest Li abundance observed in 47 Tuc is
about 0.1 dex lower than the lowest Li abundance observed among the un-depleted
stars of the metal-poor open cluster NGC 2243. The lithium abundances in 47
Tuc, when put into context with observations in other clusters and field stars,
suggest that stars that are more metal-rich than [FeH] \sim -1.0 experience
significant lithium depletion during their lifetime on the main sequence, while
the more metal-poor stars do not. Rather strikingly, our results suggest that
initial lithium abundance with which the star was created may only depend on
its age (the younger the star, the higher its Li content) and not on its
metallicity.Comment: 24 pages, 13 figures; discussion and conclusions expanded. Accepted
for publication in A&
Abell 43: Longest period Planetary Nebula Nucleus variable
Based on 24h high speed photometry of the hybrid PG 1159 star Abell 43, we
have detected 6 sighificant pulsations with periods between 2380 s and 6075 s.
A short (4h) run on the almost spectroscopic twin NGC 7094 central star
resulted in detection of 3 low amplitude pulsations with periods between 2000 s
and 5000 s. The results are close to predictions for g-mode pulsations driven
by the kappa-mechanism induced by the partial ionization of carbon and oxygen.Comment: 5 pages, 5 figures, to be published in Astronomy and Astrophysic
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The solar photospheric silicon abundance according to CO5BOLD: Investigating line broadening, magnetic fields, and model effects
Context. In this work, we present a photospheric solar silicon abundance derived using CO5BOLD model atmospheres and the LINFOR3D spectral synthesis code. Previous works have differed in their choice of a spectral line sample and model atmosphere as well as their treatment of observational material, and the solar silicon abundance has undergone a downward revision in recent years. We additionally show the effects of the chosen line sample, broadening due to velocity fields, collisional broadening, model spatial resolution, and magnetic fields. Aims. Our main aim is to derive the photospheric solar silicon abundance using updated oscillator strengths and to mitigate model shortcomings such as over-broadening of synthetic spectra. We also aim to investigate the effects of different line samples, fitting configurations, and magnetic fields on the fitted abundance and broadening values. Methods. CO5BOLD model atmospheres for the Sun were used in conjunction with the LINFOR3D spectral synthesis code to generate model spectra, which were then fit to observations in the Hamburg solar atlas. We took pixel-to-pixel signal correlations into account by means of a correlated noise model. The choice of line sample is crucial to determining abundances, and we present a sample of 11 carefully selected lines (from an initial choice of 39 lines) in both the optical and infrared, which has been made possible with newly determined oscillator strengths for the majority of these lines. Our final sample includes seven optical Si i lines, three infrared Si i lines, and one optical Si ii line. Results. We derived a photospheric solar silicon abundance of log ϔSi = 7.57 ± 0.04, including a - 0.01 dex correction from Non-Local Thermodynamic Equilibrium (NLTE) effects. Combining this with meteoritic abundances and previously determined photospheric abundances results in a metal mass fraction Z/X = 0.0220 ± 0.0020. We found a tendency of obtaining overly broad synthetic lines. We mitigated the impact of this by devising a de-broadening procedure. The over-broadening of synthetic lines does not substantially affect the abundance determined in the end. It is primarily the line selection that affects the final fitted abundance