Context. Determination of high-precision abundances of late-type stars has been and always will be an important goal of spectroscopic studies, which requires accurate modeling of their stellar ...spectra with non-local thermodynamic equilibrium (NLTE) radiative transfer methods. This entails using up-to-date atomic data of the elements under study, which are still subject to large uncertainties. Aims. We investigate the role of hydrogen collisions in NLTE spectral line synthesis, and introduce a new general empirical recipe to determine inelastic charge transfer (CT) and bound-bound hydrogen collisional rates. This recipe is based on fitting the energy functional dependence of published quantum collisional rate coefficients of several neutral elements (BeI, Na I, Mg I, Al I, Si I and Ca I) using simple polynomial equations. Methods. We perform thorough NLTE abundance calculation tests using our method for four different atoms, Na, Mg, Al and Si, for a broad range of stellar parameters. We then compare the results to calculations computed using the published quantum rates for all the corresponding elements. We also compare to results computed using excitation collisional rates via the commonly used Drawin equation for different fudge factors, SMH, applied. Results. We demonstrate that our proposed method is able to reproduce the NLTE abundance corrections performed with the quantum rates for different spectral types and metallicities for representative Na I and Al I lines to within ≤0.05 dex and ≤0.03 dex, respectively. For Mg I and Si I lines, the method performs better for the cool giants and dwarfs, while larger discrepancies up to 0.2 dex could be obtained for some lines for the subgiants and warm dwarfs. We obtained larger NLTE correction differences between models incorporating Drawin rates relative to the quantum models by up to 0.4 dex. These large discrepancies are potentially due to ignoring either or both CT and ionization collisional processes by hydrogen in our Drawin models. Conclusions. Our general empirical fitting method (EFM) for estimating hydrogen collision rates performs well in its ability to reproduce, within narrow uncertainties, the abundance corrections computed with models incorporating quantum collisional rates. It performs generally best for the cool and warm dwarfs, with slightly larger discrepancies obtained for the giants and subgiants. It could possibly be extended in the future to transitions of the same elements for which quantum calculations do not exist, or, in the absence of published quantum calculations, to other elements as well.
Context.
Open clusters are recognised as excellent tracers of Galactic thin-disc properties. At variance with intermediate-age and old open clusters, for which a significant number of studies is now ...available, clusters younger than ≲150 Myr have been mostly overlooked in terms of their chemical composition until recently (with few exceptions). On the other hand, previous investigations seem to indicate an anomalous behaviour of young clusters, which includes (but is not limited to) slightly sub-solar iron (Fe) abundances and extreme, unexpectedly high barium (Ba) enhancements.
Aims.
In a series of papers, we plan to expand our understanding of this topic and investigate whether these chemical peculiarities are instead related to abundance analysis techniques.
Methods.
We present a new determination of the atmospheric parameters for 23 dwarf stars observed by the
Gaia
-ESO survey in five young open clusters (
τ
< 150 Myr) and one star-forming region (NGC 2264). We exploit a new method based on titanium (Ti) lines to derive the spectroscopic surface gravity, and most importantly, the microturbulence parameter. A combination of Ti and Fe lines is used to obtain effective temperatures. We also infer the abundances of Fe
I
, Fe
II
, Ti
I
, Ti
II
, Na
I
, Mg
I
, Al
I
, Si
I
, Ca
I
, Cr
I
, and Ni
I
.
Results.
Our findings are in fair agreement with
Gaia
-ESO iDR5 results for effective temperatures and surface gravities, but suggest that for very young stars, the microturbulence parameter is over-estimated when Fe lines are employed. This affects the derived chemical composition and causes the metal content of very young clusters to be under-estimated.
Conclusions.
Our clusters display a metallicity Fe/H between +0.04 ± 0.01 and +0.12 ± 0.02; they are not more metal poor than the Sun. Although based on a relatively small sample size, our explorative study suggests that we may not need to call for ad hoc explanations to reconcile the chemical composition of young open clusters with Galactic chemical evolution models.
One of the ultimate goals of the ESA Ariel space mission is to shed light on the formation pathways and evolution of planetary systems in the Solar neighbourhood. Stellar elemental abundances are the ...cipher key to decode planetary compositional signatures. This makes it imperative to perform a large chemical survey not only of the planets, but their host stars as well. This work is aimed at providing homogeneous chemical abundances for C, N, and O among a sample of 181 stars belonging to Tier 1 of the Ariel mission candidate sample. We applied the spectral synthesis and equivalent width methods to a variety of atomic and molecular indicators ( C i lines at 5052 and 5380.3 O i forbidden line at 6300.3 C _2$ bands at 5128 and 5165 and CN band at 4215 using high-resolution and high signal-to-noise spectra collected with a range of spectrographs. We determined carbon abundances for 180 stars, nitrogen abundances for 105 stars, and oxygen abundances for 89 stars. We analysed the results in the light of the Galactic chemical evolution and in terms of the planetary companion properties. We find that our sample essentially follows standard trends with respect to the metallicity values expected for the C/Fe N/Fe and O/Fe abundance ratios. The proportion between carbon and oxygen abundances (both yields of primary production) is consistent with a constant ratio as O/H increases. Meanwhile, the abundance of nitrogen tends to increase with the increasing of the oxygen abundance, supporting the theoretical assumption of a secondary production of nitrogen. The C/N C/O and N/O abundance ratios are also correlated with Fe/H which might introduce biases in the interpretation of the planetary compositions and formation histories when host stars of different metallicities are compared. Finally, we provide relations that can be used to qualitatively estimate whether the atmospheric composition of planets is enriched (or otherwise) with respect to the host stars.
Context.
The last decade has seen a revolution in our knowledge of the Galaxy thanks to the
Gaia
and asteroseismic space missions and the ground-based spectroscopic surveys.
Aims.
To complete this ...picture, it is necessary to map the ages of its stellar populations. During recent years, the dependence on time of abundance ratios involving slow (s) neutron-capture and
α
elements (called chemical-clocks) has been used to provide estimates of stellar ages, usually in a limited volume close to the Sun. We aim to analyse the relations of chemical clocks in the Galactic disc extending the range to
R
GC
∼ 6–20 kpc.
Methods.
Using the sixth internal data release of the
Gaia
-ESO survey, we calibrated several relations between stellar ages and abundance ratios
s
/
α
using a sample of open clusters, the largest one so far used with this aim (62 clusters). Thanks to their wide galactocentric coverage, we investigated the radial variations of the shape of these relations, confirming their non-universality.
Results.
The multi-variate relations allowed us to infer stellar ages for field stars. We estimated our accuracy (ranging from 0.0 to −0.9 Gyr) and precision (from 0.4 to 2.3 Gyr) in recovering the global ages of open clusters, and the ages of their individual members. We applied the relations with the highest correlation coefficients to the field star population, finding an older population at lower metallicity and higher
α
/Fe in the thin disc, and a younger one at higher Fe/H and low
α
/Fe, as expected.
Conclusion.
We confirm that there is no single age-chemical clock relationship valid for the whole disc, but that there is a dependence on the galactocentric position, which is related to the radial variation of the star formation history combined with the non-monotonic dependence on metallicity of the yields of the
s
-process elements from low- and intermediate-mass stars. Finally, the abundance ratios Ba/
α
are more sensitive to age than those with Y/
α
for young disc stars, and their slopes vary less with galactocentric distance. We remind the reader that the application of such relationships to field stars is only of statistical value.
Ariel stellar characterisation Magrini, L.; Danielski, C.; Bossini, D. ...
Astronomy and astrophysics (Berlin),
07/2022, Letnik:
663
Journal Article
Recenzirano
Odprti dostop
Context.
In 2020 the European Space Agency selected Ariel as the next mission to join the space fleet of observatories to study planets outside our Solar System. Ariel will be devoted to the ...characterisation of 1000 planetary atmospheres in order to understand what exoplanets are made of, how they form, and how they evolve. To achieve the last two goals all planets need to be studied within the context of their own host stars, which in turn must be analysed with the same technique, in a uniform way.
Aims.
We present the spectro-photometric method we developed to infer the atmospheric parameters of the known host stars in the Tier 1 of the Ariel Reference Sample.
Methods.
Our method is based on an iterative approach that combines spectral analysis, the determination of the surface gravity from
Gaia
data, and the determination of stellar masses from isochrone fitting. We validated our approach with the analysis of a control sample, composed of members of three open clusters with well-known ages and metallicities.
Results.
We measured effective temperature
T
eff
, surface gravity log
g
, and the metallicity Fe/H of 187 F-G-K stars within the Ariel Reference Sample. We presented the general properties of the sample, including their kinematics, which allows us to classify them into thin- and thick-disc populations.
Conclusions.
A homogeneous determination of the parameters of the host stars is fundamental in the study of the stars themselves and their planetary systems. Our analysis systematically improves agreement with theoretical models and decreases uncertainties in the mass estimate (from 0.21 ± 0.30 to 0.10 ± 0.02
M
⊙
), providing useful data for the Ariel consortium and the astronomical community at large.
Context.
The spatial distribution of elemental abundances and their time evolution are among the major constraints to disentangling the scenarios of formation and evolution of the Galaxy.
Aims.
In ...this paper 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.
Methods.
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 (
R
GC
) 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.
Results.
The Fe/H gradient has a slope of −0.054 dex kpc
−1
. It can be better approximated with a two-slope shape, steeper for
R
GC
≤ 11.2 kpc and flatter in the outer regions. We saw different behaviours for elements belonging to different channels. For the time evolution of the gradient, we found that the youngest clusters (age < 1 Gyr) in the inner disc have lower metallicity than their older counterparts and that they outline a flatter gradient. We considered some possible explanations, including the effects of gas inflow and migration. We suggest that the most likely one may be related to a bias introduced by the standard spectroscopic analysis producing lower metallicities in the analysis of low-gravity stars.
Conclusions.
To delineate the shape of the ‘true’ gradient, we should most likely limit our analysis to stars with low surface gravity log
g
> 2.5 and microturbulent parameter
ξ
< 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 over the last 3 Gyr. We found a secondary role of cluster migration in shaping the gradient, with a more prominent role of migration for the oldest clusters.
Context.
It has recently been suggested that all giant stars with masses below 2
M
⊙
suffer an episode of surface lithium enrichment between the tip of the red giant branch (RGB) and the red clump ...(RC).
Aims.
We test if the above result can be confirmed in a sample of RC and RGB stars that are members of open clusters.
Methods.
We discuss Li abundances in six open clusters with ages between 1.5 and 4.9 Gyr (turn-off masses between 1.1 and 1.7
M
⊙
). We compare these observations with the predictions of different models that include rotation-induced mixing, thermohaline instability, mixing induced by the first He flash, and energy losses by neutrino magnetic moment.
Results.
In six clusters, we find close to 35% of RC stars have Li abundances that are similar or higher than those of upper RGB stars. This can be a sign of fresh Li production. Because of the extra-mixing episode connected to the luminosity bump, the expectation has been for RC stars to have systematically lower surface Li abundances. However, we cannot confirm that this possible Li production is ubiquitous. For about 65% of RC giants, we can only determine upper limits in abundances that could be hiding very low Li content.
Conclusions.
Our results indicate the possibility that Li is being produced in the RC, at levels that would not typically permit the classification of these the stars as Li rich. The determination of their carbon isotopic ratio would help to confirm that the RC giants have suffered extra mixing followed by subsequent Li enrichment. The Li abundances of the RC stars can be qualitatively explained by the models including an additional mixing episode close to the He flash.
Context.
Young open clusters (ages of less than 200 Myr) have been observed to exhibit several peculiarities in their chemical compositions. These anomalies include a slightly sub-solar iron content, ...super-solar abundances of some atomic species (e.g. ionised chromium), and atypical enhancements of Ba/Fe, with values up to ~0.7 dex. Regarding the behaviour of the other
s
-process elements like yttrium, zirconium, lanthanum, and cerium, there is general disagreement in the literature: some authors claim that they follow the same trend as barium, while others find solar abundances at all ages.
Aims.
In this work we expand upon our previous analysis of a sample of five young open clusters (IC 2391, IC 2602, IC 4665, NGC 2516, and NGC 2547) and one star-forming region (NGC 2264), with the aim of determining abundances of different neutron-capture elements, mainly Cu
I
, Sr
I
, Sr
II
, Y
II
, Zr
II
, Ba
II
, La
II
, and Ce
II
. For NGC 2264 and NGC 2547 we present the measurements of these elements for the first time.
Methods.
We analysed high-resolution, high signal-to-noise spectra of 23 solar-type stars observed within the
Gaia
-ESO survey. After a careful selection, we derived abundances of isolated and clean lines via spectral synthesis computations and in a strictly differential way with respect to the Sun.
Results.
We find that our clusters have solar Cu/Fe within the uncertainties, while we confirm that Ba/Fe is super-solar, with values ranging from +0.22 to +0.64 dex. Our analysis also points to a mild enhancement of Y, with Y/Fe ratios covering values between 0 and +0.3 dex. For the other
s
-process elements we find that X/Fe ratios are solar at all ages.
Conclusions.
It is not possible to reconcile the anomalous behaviour of Ba and Y at young ages with standard stellar yields and Galactic chemical evolution model predictions. We explore different possible scenarios related to the behaviour of spectral lines, from the dependence on the different ionisation stages and the sensitivity to the presence of magnetic fields (through the Landé factor) to the first ionisation potential effect. We also investigate the possibility that they may arise from alterations of the structure of the stellar photosphere due to the increased levels of stellar activity that affect the spectral line formation, and consequently the derived abundances. These effects seem to be stronger in stars at ages of less than ~ 100 Myr. However, we are still unable to explain these enhancements, and the Ba puzzle remains unsolved. With the present study we suggest that other elements, for example Sr, Zr, La, and Ce, might be more reliable tracer of the
s
-process at young ages, and we strongly encourage further critical observations.
Context.
A renewed interest in the origin of
r
-process elements has been stimulated by the multi-messenger observation of the gravitational event GW170817, with the detection of both gravitational ...waves and electromagnetic waves corresponding to the merger of two neutron stars. Such a phenomenon has been proposed as one of the main sources of the
r
-process. However, the origin of the
r
-process elements at different metallicities is still under debate.
Aims.
We aim at investigate the origin of the
r
-process elements in the Galactic thin-disc population.
Methods.
From the sixth internal data release of the
Gaia
-ESO, we have collected a large sample of Milky Way (MW) thin- and thick-disc stars for which abundances of Eu, O, and Mg are available. The sample consists of members of 62 open clusters (OCs), located at a Galactocentric radius between ∼5 kpc and ∼20 kpc in the disc, in the metallicity range − 0.5, 0.4, and covering an age interval from 0.1 to 7 Gy, and about 1300 Milky Way disc field stars in the metallicity range − 1.5, 0.5. We compare the observations with the results of a chemical evolution model, in which we varied the nucleosynthesis sources for the three elements considered.
Results.
Our main result is that Eu in the thin disc is predominantly produced by sources with short lifetimes, such as magneto-rotationally driven SNe. There is no strong evidence for additional sources at delayed times.
Conclusions.
Our findings do not imply that there cannot be a contribution from mergers of neutron stars in other environments, as in the halo or in dwarf spheroidal galaxies, but such a contribution is not needed to explain Eu abundances at thin-disc metallicities.
Context
. Fundamental atomic transition parameters, such as oscillator strengths and rest wavelengths, play a key role in modelling and understanding the chemical composition of stars in the ...universe. Despite the significant work under way to produce these parameters for many astrophysically important ions, uncertainties in these parameters remain large and can limit the accuracy of chemical abundance determinations.
Aims
. The Belgian repository of fundamental atomic data and stellar spectra (BRASS) aims to provide a large systematic and homogeneous quality assessment of the atomic data available for quantitative spectroscopy. BRASS shall compare synthetic spectra against extremely high-quality observed spectra, at a resolution of ∼85 000 and signal-noise ratios of ∼1000, for approximately 20 bright BAFGK spectral-type stars, in order to critically evaluate the atomic data available for over a thousand potentially useful spectral lines.
Methods
. A large-scale homogeneous selection of atomic lines is performed by synthesising theoretical spectra of literature atomic lines for FGK-type stars including the Sun, resulting in a selection of 1091 theoretically deep and unblended lines in the wavelength range 4200–6800 Å, which may be suitable for quality assessment. Astrophysical log(
g
f
) values are determined for the 1091 transitions using two commonly employed methods. The agreement of these log(
g
f
) values are used to select well-behaved lines for quality assessment.
Results
. We found 845 atomic lines to be suitable for quality assessment, of which 408 were found to be robust against systematic differences between analysis methods. Around 53% of the quality-assessed lines were found to have at least one literature log(
g
f
) value in agreement with our derived values, though the remaining values can disagree by as much as 0.5 dex. Only ∼38% of Fe
I
lines were found to have sufficiently accurate log(
g
f
) values, increasing to ∼70–75% for the remaining Fe-group lines.
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