241 research outputs found
Cu I resonance lines in turn-off stars of NGC 6752 and NGC 6397. Effects of granulation from CO5BOLD models
Context. Copper is an element whose interesting evolution with metallicity is
not fully understood. Observations of copper abundances rely on a very limited
number of lines, the strongest are the Cu I lines of Mult. 1 at 324.7 nm and
327.3 nm which can be measured even at extremely low metallicities. Aims. We
investigate the quality of these lines as abundance indicators. Method. We
measure these lines in two turn-off (TO) stars in the Globular Cluster NGC 6752
and two TO stars in the Globular Cluster NGC 6397 and derive abundances with 3D
hydrodynamical model atmospheres computed with the CO5BOLD code. These
abundances are compared to the Cu abundances measured in giant stars of the
same clusters, using the lines of Mult. 2 at 510.5 nm and 578.2 nm. Results.
The abundances derived from the lines of Mult. 1 in TO stars differ from the
abundances of giants of the same clusters. This is true both using CO5BOLD
models and using traditional 1D model atmospheres. The LTE 3D corrections for
TO stars are large, while they are small for giant stars. Conclusions. The Cu I
resonance lines of Mult. 1 are not reliable abundance indicators. It is likely
that departures from LTE should be taken into account to properly describe
these lines, although it is not clear if these alone can account for the
observations. An investigation of these departures is indeed encouraged for
both dwarfs and giants. Our recommendation to those interested in the study of
the evolution of copper abundances is to rely on the measurements in giants,
based on the lines of Mult. 2. We caution, however, that NLTE studies may imply
a revision in all the Cu abundances, both in dwarfs and giants.Comment: to be published on A\&
Lithium abundance in a turnoff halo star on an extreme orbit
The lithium abundance in turnoff stars of the old population of our Galaxy is
remarkably constant in the metallicity interval -2.8\textless{}[Fe/H]
\textless{}-2.0, defining a plateau. The Li abundance of these turnoff stars is
clearly lower than the abundance predicted by the primordial nucleosynthesis in
the frame of the standard Big Bang nucleosynthesis. Different scenarios have
been proposed for explaining this discrepancy, along with the very low scatter
of the lithium abundance around the plateau. The recently identified very high
velocity star, WISE J072543.88-235119.7 appears to belong to the old Galactic
population, and appears to be an extreme halo star on a bound, retrograde
Galactic orbit. In this paper, we study the abundance ratios and, in particular
the lithium abundance, in this star. The available spectra (ESO-Very Large
Telescope) are analyzed and the abundances of Li, C, Na, Mg, Al, Si, Ca, Sc,
Ti, Cr, Mn, Fe, Co, Ni, Sr and Ba are determined.The abundance ratios in WISE
J072543.88-235119.7 are those typical of old turnoff stars. The lithium
abundance in this star ~is in close agreement with the lithium abundance found
in the metal-poor turnoff stars located at moderate distance from the Sun. This
high velocity star confirms, in an extreme case, that the very small scatter of
the lithium plateau persists independent of the dynamic and kinematic
properties of the stars
Overview of the lithium problem in metal-poor stars and new results on 6Li
Two problems are discussed here. The first one is the 0.4 dex discrepancy
between the 7Li abundance derived from the spectra of metal-poor halo stars on
the one hand, and from Big Bang nucleosynthesis, based on the cosmological
parameters constrained by the WMAP measurements, on the other hand. Lithium,
indeed, can be depleted in the convection zone of unevolved stars. The
understanding of the hydrodynamics of the crucial zone near the bottom of the
convective envelope in dwarfs or turn-off stars of solar metallicity has
recently made enormous progress with the inclusion of internal gravity waves.
However, similar work for metal-poor stars is still lacking. Therefore it is
not yet clear whether the depletion occurring in the metal-poor stars
themselves is adequate to produce a 7Li plateau. The second problem pertains to
the large amount of 6Li recently found in metal-poor halo stars. The
convection-related asymmetry of the 7Li line could mimic the signal attributed
so far to the weak blend of 6Li in the red wing of the 7Li line. Theoretical
computations show that the signal generated by the asymmetry of 7Li is 2.0,
2.1, and 3.7 per cent for [Fe/H]= -3.0, -2.0, -1.0, respectively (Teff =6250 K
and log g=4.0 [cgs]). In addition we re-investigate the statistical properties
of the 6Li plateau and show that previous analyses were biased. Our conclusion
is that the 6Li plateau can be reinterpreted in terms of intrinsic line
asymmetry, without the need to invoke a contribution of 6Li. (abridged)Comment: Invited talk at the 10th Symposium on Nuclei in the Cosmos - July 27
- August 1 2008 - Mackinac Island, Michigan, USA, Accepted version. Minor
changes following referee's suggestion
Intrinsic colour calibration for F, G, K stars
We derive an intrinsic colour calibration for F--K stars using broad band
Johnson colours and line indices KP and HP2. Through this calibration we can
determine E(B-V) of an individual star within 0.03 mag. The E(B-V) values thus
derived are in excellent agreement with those derived from Stromgren photometry
through the Schuster & Nissen (1989) calibration. The agreement is also good
with the reddening maps of Burstein & Heiles (1982) and Schlegel et al (1998),
although in this case there exists a small offset of about 0.01 mag. This
calibration may be applied to the large body of data of the HK survey extension
which will be published in the near future.Comment: To be published in A&A
The photospheric solar oxygen project: II. Non-concordance of the oxygen abundance derived from two forbidden lines
In the Sun, the two forbidden [OI] lines at 630 and 636 nm were previously
found to provide discrepant oxygen abundances. aims: We investigate whether
this discrepancy is peculiar to the Sun or whether it is also observed in other
stars. method: We make use of high-resolution, high signal-to-noise ratio
spectra of four dwarf to turn-off stars, five giant stars, and one sub-giant
star observed with THEMIS, HARPS, and UVES to investigate the coherence of the
two lines. results: The two lines provide oxygen abundances that are
consistent, within observational errors, in all the giant stars examined by us.
On the other hand, for the two dwarf stars for which a measurement was
possible, for Procyon, and for the sub-giant star Capella, the 636 nm line
provides systematically higher oxygen abundances, as already seen for the Sun.
conclusions: The only two possible reasons for the discrepancy are a serious
error in the oscillator strength of the NiI line blending the 630 nm line or
the presence of an unknown blend in the 636 nm line, which makes the feature
stronger. The CN lines blending the 636 nm line cannot be responsible for the
discrepancy. The CaI autoionisation line, on the red wing of which the 636 nm
line is formed, is not well modelled by our synthetic spectra. However, a
better reproduction of this line would result in even higher abundances from
the 636 nm, thus increasing the discrepancy.Comment: A&A accepte
An astrophysical oscillator strength for the SII 94.7 nm resonance line and S abundances in DLAs
By using UV spectra for the O star HD 93521 taken with the ORFEUS II echelle
spectrograph, we determine an ``astrophysical'' f value for the SII 94.7 nm
line: f = 0.00498 (+0.00172)(-0.00138), error at 1 sigma level.This is almost a
factor of 30 smaller than the guessed value found in the Kurucz database
(f=0.1472), which was up to now the only one available for this transition. We
use our ``astrophysical'' f to investigate the S abundance in two Damped
Ly-alpha absorption systems (DLAs) observed with UVES at the ESO 8.2m Kueyen
telescope. In the case of the absorber at zabs=3.02486 towards QSO 0347-3819 we
find a sulphur column density which is consistent, within errors, with that
determined by Centurion et al by means of the 125.9 nm line, thus providing an
external check on the accuracy of our f value. For the damped absorber at
zabs=4.4680 towards BR J0307-4945 we determine a high value of the S abundance,
which, however, is probably the result of blending with Ly-alpha forest lines.Comment: Accepted by MNRA
An investigation of the formation and line properties of MgH in 3D hydrodynamical model stellar atmospheres
Studies of the isotopic composition of magnesium in cool stars have so far
relied upon the use of one-dimensional (1D) model atmospheres. Since the
isotopic ratios derived are based on asymmetries of optical MgH lines, it is
important to test the impact from other effects affecting line asymmetries,
like stellar convection. Here, we present a theoretical investigation of the
effects of including self-consistent modeling of convection. Using spectral
syntheses based on 3D hydrodynamical COBOLD models of dwarfs
(4000K, log(g),
) and giants (K,
log(g), ), we perform a detailed
analysis comparing 3D and 1D syntheses.
We describe the impact on the formation and behavior of MgH lines from using
3D models, and perform a qualitative assessment of the systematics introduced
by the use of 1D syntheses.
Using 3D model atmospheres significantly affect the strength of the MgH
lines, especially in dwarfs, with 1D syntheses requiring an abundance
correction of up to +0.69 dex largest for our 5000K models. The corrections are
correlated with and are also affected by the metallicity. The
shape of the strong MgH component in the 3D syntheses is poorly
reproduced in 1D. This results in 1D syntheses underestimating MgH by up
to percentage points and overestimating MgH by a similar amount
for dwarfs. This discrepancy increases with decreasing metallicity. MgH
is recovered relatively well, with the largest difference being
percentage points. The use of 3D for giants has less impact, due to smaller
differences in the atmospheric structure and a better reproduction of the line
shape in 1D.Comment: 20 pages, 15 figures, accepted for publication in Ap
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