4 research outputs found
The Gaia-ESO survey: mapping the shape and evolution of the radial abundance gradients with open clusters
The spatial distribution of elemental abundances and their time evolution are
among the major constraints to disentangle the scenarios of formation and
evolution of the Galaxy. We used the sample of open clusters available in the
final release of the Gaia-ESO survey to trace the Galactic radial abundance and
abundance to iron ratio gradients, and their time evolution. We selected member
stars in 62 open clusters, with ages from 0.1 to about 7~Gyr, located in the
Galactic thin disc at Galactocentric radii from about 6 to 21~kpc. We analysed
the shape of the resulting [Fe/H] gradient, the average gradients [El/H] and
[El/Fe] combining elements belonging to four different nucleosynthesis
channels, and their individual abundance and abundance ratio gradients. We also
investigated the time evolution of the gradients dividing open clusters in
three age bins. The[Fe/H] gradient has a slope of -0.054 dex~kpc-1. We saw
different behaviours for elements belonging to different channels. We found
that the youngest clusters in the inner disc have lower metallicity than their
older counterpart and they outline a flatter gradient. We considered some
possible explanations, including the effects of gas inflow and migration. We
suggested that it might be a bias introduced by the standard spectroscopic
analysis producing lower metallicities in low gravity stars. To delineate the
shape of the `true' gradient, we should limit our analysis to stars with low
surface gravity logg>2.5 and xi<1.8 km~s-1. Based on this reduced sample, we
can conclude that the gradient has minimally evolved over the time-frame
outlined by the open clusters, indicating a slow and stationary formation of
the thin disc in the latest Gyr. We found a secondary role of clusters'
migration in shaping the gradient, with a more prominent role of migration for
the oldest clusters.Comment: 25 pages, 14 figures and 4 tables in the main text, 3 figures and 7
tables in the Appendix. Accepted for publication in A&
The Gaia-ESO Survey::Properties of the intermediate age open cluster NGC 4815
Context. NGC 4815 is a populous similar to 500 Myr open cluster at R-gc similar to 7 kpc observed in the first six months of the Gaia-ESO Survey. Located in the inner Galactic disk, NGC 4815 is an important potential tracer of the abundance gradient, where relatively few intermediate age open clusters are found. Aims. The Gaia-ESO Survey data can provide an improved characterization of the cluster properties, such as age, distance, reddening, and abundance profile. Methods. We use the survey derived radial velocities, stellar atmospheric parameters, metallicity, and elemental abundances for stars targeted as potential members of this cluster to carry out an analysis of cluster properties. The radial velocity distribution of stars in the cluster field is used to define the cluster systemic velocity and derive likely cluster membership for stars observed by the Gaia-ESO Survey. We investigate the distributions of Fe and Fe-peak elements, alpha-elements, and the light elements Na and Al and characterize the cluster's internal chemical homogeneity comparing it to the properties of radial velocity non-member stars. Utilizing these cluster properties, the cluster color-magnitude diagram is analyzed and theoretical isochrones are fit to derive cluster reddening, distance, and age. Results. NGC 4815 is found to have a mean metallicity of [Fe/H] = +0.03 +/- 0.05 dex (s.d.). Elemental abundances of cluster members show typically very small internal variation, with internal dispersions of similar to 0.05 dex. The alpha-elements [Ca/Fe] and [Si/Fe] show solar ratios, but [Mg/Fe] is moderately enhanced, while [Ti/Fe] appears slightly deficient. As with many open clusters, the light elements [Na/Fe] and [Al/Fe] are enhanced, [Na/Fe] significantly so, although the role of internal mixing and the assumption of local thermodynamical equilibrium in the analysis remain to be investigated. From isochrone fits to color-magnitude diagrams, we find a cluster age of 0.5 to 0.63 Gyr, a reddening of E(B-V) = 0.59 to 0.65, and a distance modulus (m -M)(0) = 11.95 to 12.20, depending on the choice of theoretical models, leading to a Galactocentric distance of 6.9 kpc
The Gaia-ESO Survey: The analysis of high-resolution UVES spectra of FGK-type stars
The article of record as published may be found at http://dx.doi.org/10.1051/0004-6361/201423937Context. The ongoing Gaia-ESO Public Spectroscopic Survey is using FLAMES at the VLT to obtain high-quality medium-resolution Giraffe spectra for about 10(5) stars and high-resolution UVES spectra for about 5000 stars. With UVES, the Survey has already observed 1447 FGK-type stars.
Aims. These UVES spectra are analyzed in parallel by several state-of-the-art methodologies. Our aim is to present how these analyses were implemented, to discuss their results, and to describe how a final recommended parameter scale is defined. We also discuss the precision (method-to-method dispersion) and accuracy (biases with respect to the reference values) of the final parameters. These results are part of the Gaia-ESO second internal release and will be part of its first public release of advanced data products.
Methods. The final parameter scale is tied to the scale defined by the Gaia benchmark stars, a set of stars with fundamental atmospheric parameters. In addition, a set of open and globular clusters is used to evaluate the physical soundness of the results. Each of the implemented methodologies is judged against the benchmark stars to define weights in three different regions of the parameter space. The final recommended results are the weighted medians of those from the individual methods.
Results. The recommended results successfully reproduce the atmospheric parameters of the benchmark stars and the expected T(eff)-log g relation of the calibrating clusters. Atmospheric parameters and abundances have been determined for 1301 FGK-type stars observed with UVES. The median of the method-to-method dispersion of the atmospheric parameters is 55K for T(eff), 0.13 dex for log g and 0.07 dex for [Fe/H]. Systematic biases are estimated to be between 50â100 K for T(eff), 0.10â0.25 dex for log g and 0.05â0.10 dex for [Fe/H]. Abundances for 24 elements were derived: C, N, O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Mo, Ba, Nd, and Eu. The typical method-to-method dispersion of the abundances varies between 0.10 and 0.20 dex.
Conclusions. The Gaia-ESO sample of high-resolution spectra of FGK-type stars will be among the largest of its kind analyzed in a homogeneous way. The extensive list of elemental abundances derived in these stars will enable significant advances in the areas of stellar evolution and Milky Way formation and evolution.Support by the Swedish National Space Board (SNSB) through several grants
The Gaia -ESO Survey: The N/O abundance ratio in the Milky Way?
The abundance ratio N/O is a useful tool to study the interplay of galactic
processes, e.g. star formation efficiency, time-scale of infall and outflow
loading factor We aim to trace log(N/O) versus [Fe/H] in the Milky Way and to
compare it with a set of chemical evolution models to understand the role of
infall, outflow and star formation efficiency in the building-up of the
Galactic disc. We use the abundances from idr2-3, idr4, idr5 data releases of
the Gaia-ESO Survey both for Galactic field and open cluster stars.We determine
membership and average composition of open clusters and we separate thin and
thick disc field stars.We consider the effect of mixing in the abundance of N
in giant stars. We compute a grid of chemical evolution models, suited to
reproduce the main features of our Galaxy, exploring the effects of the star
formation efficiency, the infall time-scale and the differential outflow. With
our samples, we map the metallicity range -0.6<[Fe/H]<0.3 with a corresponding
-1.2<log(N/O)<-0.2, where the secondary production of N dominates. Thanks to
the wide range of Galactocentric distances covered by our samples, we can
distinguish the behaviour of log(N/O) in different parts of the Galaxy. Our
spatially resolved results allow us to distinguish differences in the evolution
of N/O with Galactocentric radius. Comparing the data with our models, we can
characterise the radial regions of our Galaxy. A shorter infall time-scale is
needed in the inner regions, while the outer regions need a longer infall
time-scale, coupled with a higher star formation efficiency. We compare our
results with nebular abundances obtained in MaNGA galaxies, finding in our
Galaxy a much wider range of log(N/O) than in integrated observations of
external galaxies of similar stellar mass, but similar to the ranges found in
studies of individual H ii regions