44 research outputs found
Beryllium abundances along the evolutionary sequence of the open cluster IC 4651 - New test for hydrodynamical stellar models
[abridged] Previous analyses of lithium abundances in main sequence and red
giant stars have revealed the action of mixing mechanisms other than convection
in stellar interiors. Beryllium abundances in stars with lithium abundance
determinations can offer valuable complementary information on the nature of
these mechanisms. Our aim is to derive beryllium abundances along the whole
evolutionary sequence of an open cluster, IC 4651. These Be abundances are used
together with previously determined Li abundances, in the same sample stars, to
investigate the mixing mechanisms in a range of stellar masses and evolutionary
stages. New beryllium abundances are determined from high-resolution, high
signal-to-noise UVES spectra using spectrum synthesis and model atmospheres.
The careful synthetic modelling of the Be lines region is used to calculate
reliable abundances in rapidly rotating stars. The observed behavior of Be and
Li is compared to theoretical predictions from stellar models including
rotation-induced mixing, internal gravity waves, atomic diffusion, and
thermohaline mixing. Beryllium is detected in all the main sequence and
turn-off sample stars, both slow- and fast-rotating stars, including the Li-dip
stars, but was not detected in the red giants. Confirming previous results, we
find that the Li dip is also a Be dip, although the depletion of Be is more
modest than that of Li in the corresponding effective temperature range. For
post-main-sequence stars, the Be dilution starts earlier within the Hertzsprung
gap than expected from classical predictions as does the Li dilution. A clear
dispersion in the Be abundances is also observed. Theoretical stellar models
including the hydrodynamical transport processes mentioned above are able to
reproduce well all the observed features.Comment: 12 pages, accepted for publication in A&A, revised final versio
Exploring the chemodynamics of metal-poor stellar populations
Metal-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&
Beryllium abundances and the formation of the halo and the thick disk
The 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.Comment: 6 pages, 5 figures, To appear in the Proceedings of IAU Symp. 268 -
Light Elements in the Universe (C. Charbonnel, M. Tosi, F. Primas, C.
Chiappini, eds
A view of the Galactic halo using beryllium as a time scale
Beryllium stellar abundances were suggested to be a good tracer of time in
the early Galaxy. In an investigation of its use as a cosmochronometer, using a
large sample of local halo and thick-disk dwarfs, evidence was found 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.Comment: To appear in Proceedings of the International Astronomical Union, IAU
Symposium, Volume 265, Chemical abundances in the Universe: connecting first
stars to planets, K. Cunha, M. Spite and B. Barbuy, eds. 2 Pages, 2 figure
Beryllium abundances along the evolutionary sequence of the open cluster IC 4651
The 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
Consistent metallicity scale for cool dwarfs and giants. A benchmark test using the Hyades
In several instances chemical abundances of dwarf and giant stars are used
simultaneously under the assumption that they share the same abundance scale.
This assumption might have implications in different astrophysical contexts. We
aim to ascertain a methodology capable of producing a consistent metallicity
scale for giants and dwarfs. To achieve that, we analyzed giants and dwarfs in
the Hyades open cluster. All these stars have archival high-resolution
spectroscopic data obtained with HARPS and UVES. In addition, the giants have
interferometric measurements of the angular diameters. We analyzed the sample
with two methods. The first method constrains the atmospheric parameters
independently from spectroscopy. For that we present a novel calibration of
microturbulence based on 3D model atmospheres. The second method is the
classical spectroscopic based on Fe lines. We also tested two line lists in an
attempt to minimize possible non-LTE effects and to optimize the treatment of
the giants. We show that it is possible to obtain a consistent metallicity
scale between dwarfs and giants. The preferred method should constrain the
three parameters , , and independent of
spectroscopy. In particular, the lines should be chosen to be free of blends in
the spectra of giants. When attention is paid to the line list, the classical
spectroscopic method can also produce consistent results. The metallicities
derived with the well-constrained set of stellar parameters are consistent
independent of the line list used. Therefore, for this cluster we favor the
metallicity of +0.180.03 dex obtained with this method. The classical
spectroscopic analysis, using the line list optimized for the giants, provides
a metallicity of +0.140.03 dex, in agreement with previous works.Comment: 19 pages, 6 figures, 10 tables. Accepted for publication in A&