732 research outputs found
MyGIsFOS: an automated code for parameter determination and detailed abundance analysis in cool stars
The current and planned high-resolution, high-multiplexity stellar
spectroscopic surveys, as well as the swelling amount of under-utilized data
present in public archives have led to an increasing number of efforts to
automate the crucial but slow process to retrieve stellar parameters and
chemical abundances from spectra. We present MyGIsFOS, a code designed to
derive atmospheric parameters and detailed stellar abundances from medium -
high resolution spectra of cool (FGK) stars. We describe the general structure
and workings of the code, present analyses of a number of well studied stars
representative of the parameter space MyGIsFOS is designed to cover, and
examples of the exploitation of MyGIsFOS very fast analysis to assess
uncertainties through Montecarlo tests. MyGIsFOS aims to reproduce a
``traditional'' manual analysis by fitting spectral features for different
elements against a precomputed grid of synthetic spectra. Fe I and Fe II lines
can be employed to determine temperature, gravity, microturbulence, and
metallicity by iteratively minimizing the dependence of Fe I abundance from
line lower energy and equivalent width, and imposing Fe I - Fe II ionization
equilibrium. Once parameters are retrieved, detailed chemical abundances are
measured from lines of other elements. MyGIsFOS replicates closely the results
obtained in similar analyses on a set of well known stars. It is also quite
fast, performing a full parameter determination and detailed abundance analysis
in about two minutes per star on a mainstream desktop computer. Currently, its
preferred field of application are high-resolution and/or large spectral
coverage data (e.g UVES, X-Shooter, HARPS, Sophie).Comment: 15 pages, 14 figures, accepted for publication by A&
A search for Interstellar absorptions towards Sgr dSph
We searched for Na I D1,D2 interstellar absorption lines towards 12 giant
stars of the Sgr dwarf spheroidal and 3 stars of the associated globular
cluster Terzan 7, observed at high resolution with the UVES spectrograph at the
8.2 m Kueyen VLT telescope. These stars are not an ideal background source for
the analysis of interstellar (IS) absorption lines since they have been
observed in the framework of a stellar abundances study. However they are
sufficiently luminous to allow a decent S/N ratio at a resolution of about
43000, i.e. ~7 km/s, which allows an exploratory study. We detected two
distinct groups of IS absorptions, a ``local'' group, with radial velocities
ranging from -43 km/s to 50 km/s, and a high velocity group with radial
velocities ranging from 150 km/s to 165 km/s. Likely the high velocity
components are due to gas falling on the Sgr dSph since have been observed only
in 2 stars which are separated by only 1.5' in the sky. We argue that the
observations suggest a stripping of gas due to the passage of Sgr through the
Galactic disc.Comment: accepted by Astronomy & Astrophysic
Chemical abundances of the metal-poor horizontal-branch stars CS 22186-005 and CS 30344-033
We report on a chemical-abundance analysis of two very metal-poor
horizontal-branch stars in the Milky Way halo: CS 22186-005 ([Fe/H]=-2.70) and
CS 30344-033 ([Fe/H]=-2.90). The analysis is based on high-resolution spectra
obtained at ESO, with the spectrographs HARPS at the 3.6 m telescope, and UVES
at the VLT. We adopted one-dimensional, plane-parallel model atmospheres
assuming local thermodynamic equilibrium. We derived elemental abundances for
13 elements for CS 22186-005 and 14 elements for CS 30344-033. This study is
the first abundance analysis of CS 30344-033. CS 22186-005 has been analyzed
previously, but we report here the first measurement of nickel (Ni; Z = 28) for
this star, based on twenty-two NiI lines ([Ni/Fe]=-0.210.02); the
measurement is significantly below the mean found for most metal-poor stars.
Differences of up to 0.5 dex in [Ni/Fe] ratios were determined by different
authors for the same type of stars in the literature, which means that it is
not yet possible to conclude that there is a real intrinsic scatter in the
[Ni/Fe] ratios. For the other elements for which we obtained estimates, the
abundance patterns in these two stars match the Galactic trends defined by
giant and turnoff stars well. This confirms the value of horizontal-branch
stars as tracers of the chemical properties of stellar populations in the
Galaxy. Our radial velocities measurements for CS 22186-005 differ from
previously published measurements by more than the expected statistical errors.
More measurements of the radial velocity of this star are encouraged to confirm
or refute its radial velocity variability
Clues on the Galactic evolution of sulphur from star clusters
(Abridged) The abundances of alpha-elements are a powerful diagnostic of the
star formation history and chemical evolution of a galaxy. Sulphur, being
moderately volatile, can be reliably measured in the interstellar medium (ISM)
of damped Ly-alpha galaxies and extragalactic HII regions. Measurements in
stars of different metallicity in our Galaxy can then be readily compared to
the abundances in external galaxies. Such a comparison is not possible for Si
or Ca that suffer depletion onto dust in the ISM. Furthermore, studying sulphur
is interesting because it probes nucleosynthetic conditions that are very
different from those of O or Mg. The measurements in star clusters are a
reliable tracers of the Galactic evolution of sulphur. We find
NLTE=6.11+/-0.04 for M 4, NLTE=7.17+/-0.02 for NGC 2477, and
NLTE=7.13+/-0.06 for NGC 5822. For the only star studied in Trumpler 5 we
find A(S)NLTE=6.43+/-0.03 and A(S)LTE=6.94+/-0.05. Our measurements show that,
by and large, the S abundances in Galactic clusters trace reliably those in
field stars. The only possible exception is Trumpler 5, for which the NLTE
sulphur abundance implies an [S/Fe] ratio lower by roughly 0.4 dex than
observed in field stars of comparable metallicity, even though its LTE sulphur
abundance is in line with abundances of field stars. Moreover the LTE sulphur
abundance is consistent only with the abundance of another alpha-element, Mg,
in the same star, while the low NLTE value is consistent with Si and Ca. The S
abundances in our sample of stars in clusters imply that the clusters are
chemically homogeneous for S within 0.05 dex.Comment: A&A in pres
Detailed abundances in stars belonging to ultra-faint dwarf spheroidal galaxies
We report preliminary results concerning the detailed chemical composition of
metal poor stars belonging to close ultra-faint dwarf galaxies (hereafter
UfDSphs). The abundances have been determined thanks to spectra obtained with
X-Shooter, a high efficiency spectrograph installed on one of the ESO VLT
units. The sample of ultra-faint dwarf spheroidal stars have abundance ratios
slightly lower to what is measured in field halo star of the same
metallicity.We did not find extreme abundances in our Hercules stars as the one
found by Koch for his 2 Hercules stars. The synthesis of the neutron capture
elements Ba and Sr seems to originate from the same nucleosynthetic process in
operation during the early stages of the galactic evolution.Comment: 3 pages, 1 figure; OMEG11 conference (Tokyo, Nov 2011
The chemical composition of red giants in 47 Tucanae I: Fundamental parameters and chemical abundance patterns
Context: The study of chemical abundance patterns in globular clusters is of
key importance to constrain the different candidates for intra-cluster
pollution of light elements. Aims: We aim at deriving accurate abundances for a
large range of elements in the globular cluster 47 Tucanae (NGC 104) to add new
constraints to the pollution scenarios for this particular cluster, expanding
the range of previously derived element abundances. Methods: Using tailored 1D
LTE atmospheric models together with a combination of equivalent width
measurements, LTE, and NLTE synthesis we derive stellar parameters and element
abundances from high-resolution, high signal-to-noise spectra of 13 red giant
stars near the tip of the RGB. Results: We derive abundances of a total 27
elements (O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr,
Mo, Ru, Ba, La, Ce, Pr, Nd, Eu, Dy). Departures from LTE were taken into
account for Na, Al and Ba. We find a mean [Fe/H] = and
in good agreement with previous studies. The
remaining elements show good agreement with the literature, but the inclusion
of NLTE for Al has a significant impact on the behaviour of this key element.
Conclusions: We confirm the presence of an Na-O anti-correlation in 47 Tucanae
found by several other works. Our NLTE analysis of Al shifts the [Al/Fe] to
lower values, indicating that this may be overestimated in earlier works. No
evidence for an intrinsic variation is found in any of the remaining elements.Comment: 22 pages, 16 figures. Accepted for publication in A&
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&
UVES observations of the Canis Major overdensity
We present the first detailed chemical abundances for three giant stars which
are candidate members of the Canis Major overdensity, obtained by using
FLAMES-UVES at VLT. The stars, in the background of the open cluster NGC 2477,
have radial velocities compatible with a membership to this structure. However,
due to Galactic disc contamination, radial velocity by itself is unable to
firmly establish membership. The metallicities span the range -0.5 < [Fe/H] <
+0.1. Assuming that at least one of the three stars is indeed a member of CMa
implies that this structure has undergone a high level of chemical processing,
comparable to that of the Galactic disc. The most metal-rich star of the
sample, EIS 6631, displays several abundance ratios which are remarkably
different from those of Galactic stars: [alpha/Fe] ~-0.2, [Cu/Fe] ~+0.25,
[La/Fe]~+0.6, [Ce/Fe]~+0.8 and [Nd/Fe]~+0.6. These ratios make it likely that
this star was formed in an external galaxy.Comment: 4 pages, 4 figures, accepted for publication in A&A letter
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