553 research outputs found
Stellar abundances of beryllium and CUBES
Stellar abundances of beryllium are useful in different areas of
astrophysics, including studies of the Galactic chemical evolution, of stellar
evolution, and of the formation of globular clusters. Determining Be abundances
in stars is, however, a challenging endeavor. The two Be II resonance lines
useful for abundance analyses are in the near UV, a region strongly affected by
atmospheric extinction. CUBES is a new spectrograph planned for the VLT that
will be more sensitive than current instruments in the near UV spectral region.
It will allow the observation of fainter stars, expanding the number of targets
where Be abundances can be determined. Here, a brief review of stellar
abundances of Be is presented together with a discussion of science cases for
CUBES. In particular, preliminary simulations of CUBES spectra are presented,
highlighting its possible impact in investigations of Be abundances of
extremely metal-poor stars and of stars in globular clusters.Comment: 11 pages, 8 figures. To appear in Astrophysics and Space Science
"Special Issue: UV Astronomy 3", Proceedings of the ESO/NUVA/IAG Workshop on
Challenges in UV Astronomy, ESO Garching, 7-11 October 201
CNONa and 12C/13C in giants of 10 open clusters
Evolved low-mass stars of a wide range of metallicity bear signatures of a
non-standard mixing event in their surface abundances of Li, C, and N, and in
their 12C/13C ratio. A Na overabundance has also been reported in some giants
of open clusters but remains controversial. The cause of the extra-mixing has
been attributed to thermohaline convection that should take place after the RGB
bump for low-mass stars and on the early-AGB for more massive objects. To track
the occurrence of this process over a wide mass range, we derive in a
homogeneous way the abundances of C, N, O, and Na, as well as the 12C/13C ratio
in a sample of 31 giants of 10 open clusters with turn-off masses from 1.7 to
3.1 Msun. A group of first ascent red giants with M/Msun \leq 2.5 exhibits
lower [N/C] ratios than those measured in clump giants of the same mass range,
suggesting an additional increase in the [N/C] ratio after the first dredge-up.
The sodium abundances corrected from NLTE are found to be about solar. [Na/Fe]
shows a slight increase of 0.10 dex as a function of stellar mass in the 1.8 to
3.2 Msun range covered by our sample, in agreement with standard first
dredge-up predictions. Our results do not support previous claims of sodium
overabundances as high as +0.60 dex. An anti-correlation between 12C/13C and
turn-off mass is identified and interpreted as being caused by a post-bump
thermohaline mixing. Moreover, we find low 12C/13C ratios in a few
intermediate-mass early-AGB stars, confirming that an extra-mixing process also
operates in stars that do not experienced the RGB bump. In this case, the
extra-mixing possibly acts on the early-AGB, in agreement with theoretical
expectations for thermohaline mixing. [abridged]Comment: A&A accepted, revised versio
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
The Be-test in the Li-rich star \#1657 of NGC 6397: evidence for Li-flash in RGB stars?
The Li-rich turn-off star in the globular cluster NGC 6397 could represent
the smoking gun for some very rare episode of Li enrichment in globular
clusters. We aim to understand the nature of the Li enrichment by performing a
spectroscopic analysis of the star, in particular of its beryllium (Be)
abundance, and by investigating its binary nature. We observe the near UV
region where the Beii resonance doublet and the NH bands are located. We could
not detect the Beii lines and derive an upper limit of log (Be/H)< -12.2, that
is consistent with the Be observed in other stars of the cluster. We could
detect a weak G-band, which implies a mild carbon enhancement
[C/Fe]. We could not detect the UV NH band, and we derive an upper
limit [N/Fe]. For oxygen we could notdetect any of the near UV OH lines,
which implies that oxygen cannot be strongly enhanced in this star. This is
consistent with the detection of the Oi triplet at 777nm, which is consistent
with [O/Fe]~0.5. Combining the UVES and Mike data, we could not detect any
variation in the radial velocity greater than 0.95 kms over 8 years. The
chemical composition of the star strongly resembles that of `first generation'
NGC6397 stars, with the huge Li as the only deviating abundance. Not detecting
Be rules out two possible explanations of the Li overabundance: capture of a
substellar body and spallation caused by a nearby type II SNe. Discrepancies
are also found with respect to other accretion scenarios,except for
contamination by the ejecta of a star that has undergone the RGB Li-flash.Comment: Accepted for publication in A&
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