Context. The formation and evolution of the Milky Way bulge is still largely an unanswered question. Some of the most essential observations needed for its modelling are the metallicity distribution ...and the trends of the α elements, as measured in stars. While bulge regions beyond R ≳ 50 pc of the centre have been targeted in several surveys, the central part has escaped a detailed study due to the extreme extinction and crowding. The abundance gradients from the centre are, however, of large diagnostic value. Aims. We aim at investigating the Galactic centre environment by probing M giants in the field by avoiding supergiants and cluster members. Methods. For nine field M-giants in the Galactic centre region, we have obtained high- and low-resolution spectra observed simultaneously with CRIRES and ISAAC on UT1 and UT3 of the VLT. The low-resolution spectra provide a means of determining the effective temperatures, and the high-resolution spectra provide detailed abundances of Fe, Mg, Si, and Ca. Results. We find a metal-rich population at Fe / H = + 0.11 ± 0.15 and a lack of the metal-poor population, which is found further out in the bulge, corroborating earlier studies. Our α/Fe element trends, however, show low values, by following the outer bulge trends. A possible exception of the Ca/Fe trend is found and needs further investigation. Conclusions. The results of the analysed field M-giants in the Galactic centre region exclude a scenario with rapid formation, in which SNIIe played a dominated role in the chemical enrichment of the gas. The high metallicities with low α-enhancement seems to indicate a bar-like population that is, perhaps, related to the nuclear bar.
Context. In light of new observational data related to fluorine abundances in solar neighborhood stars, we present chemical evolution models testing various fluorine nucleosynthesis prescriptions ...with the aim to best fit those new data. Aim. We consider chemical evolution models in the solar neighborhood testing various nucleosynthesis prescriptions for fluorine production with the aim of reproducing the observed abundance ratios F/O versus O/H and F/Fe versus Fe/H. We study in detail the effects of various stellar yields on fluorine production. Methods. We adopted two chemical evolution models: the classical two-infall model, which follows the chemical evolution of halo-thick disk and thin disk phases; and the one-infall model, which is designed only for thin disk evolution. We tested the effects on the predicted fluorine abundance ratios of various nucleosynthesis yield sources, that is, asymptotic giant branch (AGB) stars, Wolf–Rayet (W-R) stars, Type II and Type Ia supernovae, and novae. Results. The fluorine production is dominated by AGB stars but the W-R stars are required to reproduce the trend of the observed data in the solar neighborhood with our chemical evolution models. In particular, the best model both for the two-infall and one-infall cases requires an increase by a factor of 2 of the W-R yields. We also show that the novae, even if their yields are still uncertain, could help to better reproduce the secondary behavior of F in the F/O versus O/H relation. Conclusions. The inclusion of the fluorine production by W-R stars seems to be essential to reproduce the new observed ratio F/O versus O/H in the solar neighborhood. Moreover, the inclusion of novae helps to reproduce the observed fluorine secondary behavior substantially.
We present a detailed study of the composition of 20 M giants in the Galactic center with 15 of them confirmed to be in the nuclear star cluster. As a control sample we have also observed 7 M giants ...in the Milky Way disk with similar stellar parameters. All 27 stars are observed using the NIRSPEC spectrograph on the KECK II telescope in the K-band at a resolving power of R = 23,000. We report the first silicon abundance trends versus Fe/H for stars in the Galactic center. While finding a disk/bulge-like trend at subsolar metallicities, we find that Si/Fe is enhanced at supersolar metallicities. We speculate on possible enrichment scenarios to explain such a trend. However, the sample size is modest and the result needs to be confirmed by additional measurements of silicon and other -elements. We also derive a new distribution of Fe/H and find the most metal-rich stars at Fe/H = +0.5 dex, confirming our earlier conclusions that the Galactic center hosts no stars with extreme chemical compositions.
We report the first high spectral resolution study of 17 M giants kinematically confirmed to lie within a few parsecs of the Galactic center, using spectroscopy from Keck/NIRSPEC and a new line list ...for the infrared K band. We consider their luminosities and kinematics, which classify these stars as members of the older stellar population and the central cluster. We find a median metallicity of and a large spread from approximately −0.3 to (quartiles). We find that the highest metallicities are , with most of the stars being at or below the solar iron abundance. The abundances and the abundance distribution strongly resemble those of the Galactic bulge rather than the disk or halo; in our small sample we find no statistical evidence for a dependence of velocity dispersion on metallicity.
Very strong Sc i lines have recently been found in cool M giants in the Nuclear Star Cluster (NSC) in the Galactic center. Interpreting these as anomalously high scandium abundances in the Galactic ...center would imply a unique enhancement signature and chemical evolution history for NSCs, and a potential test for models of chemical enrichment in these objects. We present high resolution K-band spectra (NIRSPEC/Keck II) of cool M giants situated in the solar neighborhood and compare them with spectra of M giants in the NSC. We clearly identify strong Sc i lines in our solar neighborhood sample as well as in the NSC sample. The strong Sc i lines in M giants are therefore not unique to stars in the NSC and we argue that the strong lines are a property of the line formation process that currently escapes accurate theoretical modeling. We further conclude that for giant stars with effective temperatures below approximately 3800 K these Sc i lines should not be used for deriving the scandium abundances in any astrophysical environment until we better understand how these lines are formed. We also discuss the lines of vanadium, titanium, and yttrium identified in the spectra, which demonstrate a similar striking increase in strength below 3500 K effective temperature.
ABSTRACT
In this work, we study the formation and chemical evolution of the Galactic bulge with particular focus on the abundance pattern (Mg/Fe versus Fe/H), metallicity, and age distribution ...functions. We consider detailed chemical evolution models for the Galactic bulge and inner disc, with the aim of shedding light on the connection between these components and the origin of bulge stars. In particular, we first present a model assuming a fast and intense star formation, with the majority of bulge stars forming on a time-scale less than 1 Gyr. Then we analyse the possibility of two distinct stellar populations in the bulge, as suggested by Gaia-ESO and APOGEE data. These two populations, one metal poor and the other metal rich, can have had two different origins: (i) the metal rich formed after a stop of ∼250 Myr in the star formation rate of the bulge or (ii) the metal-rich population is made of stars formed in the inner disc and brought into the bulge by the early secular evolution of the bar. We also examine the case of multiple starbursts in the bulge with consequent formation of multiple populations, as suggested by studies of microlensed stars. After comparing model results and observations, we suggest that the most likely scenario is that there are two main stellar populations, both made mainly by old stars (>10 Gyr), with the metal-rich and younger one formed from inner thin disc stars, in agreement with kinematical arguments. However, on the basis of dynamical simulations, we cannot completely exclude that the second population formed after a stop in the star formation during the bulge evolution, so that all the stars formed in situ.
ABSTRACT The structure, formation, and evolution of the Milky Way bulge is a matter of debate. Important diagnostics for discriminating between models of bulge formation and evolution include ...-abundance trends with metallicity, and spatial abundance and metallicity gradients. Due to the severe optical extinction in the inner Bulge region, only a few detailed investigations of this region have been performed. Here we aim at investigating the inner 2 degrees of the Bulge (projected galactocentric distance of approximately 300 pc), rarely investigated before, by observing the /Fe element trends versus metallicity, and by trying to derive the metallicity gradient in the b < 2° region. /Fe and metallicities have been determined by spectral synthesis of 2 m spectra of 28 M-giants in the Bulge, lying along the southern minor axis at (l, b) = (0, 0), (0, −1°), and (0, −2°). These were observed with the CRIRES spectrometer at the Very Large Telescope, (VLT) at high spectral resolution. Low-resolution K-band spectra, observed with the ISAAC spectrometer at the VLT, are used to determine the effective temperature of the stars. We present the first connection between the Galactic center (GC) and the Bulge using similar stars, high spectral resolution, and analysis techniques. The /Fe trends in all our three fields show a large similarity among each other and with trends further out in the Bulge. All point to a rapid star formation episode in the Bulge. We find that there is a lack of an /Fe gradient in the Bulge all the way into the center, suggesting a homogeneous Bulge when it comes to the enrichment process and star formation history. We find a large range of metallicities from −1.2 < Fe/H < +0.3, with a lower dispersion in the GC: −0.2 < Fe/H < +0.3. The derived metallicities of the stars in the three fields get, in the mean, progressively higher the closer to the Galactic plane they lie. We could interpret this as a continuation of the metallicity gradient established further out in the Bulge, but due to the low number of stars and possible selection effects, more data of the same sort as presented here is necessary to conclude on the inner metallicity gradient from our data alone. Our results firmly argue for the center being in the context of the Bulge rather than very distinct.
Atomic data for the Gaia -ESO Survey Heiter, U.; Lind, K.; Bergemann, M. ...
Astronomy and astrophysics (Berlin),
2021, Volume:
645
Journal Article
Peer reviewed
Open access
Context.
We describe the atomic and molecular data that were used for the abundance analyses of FGK-type stars carried out within the
Gaia
-ESO Public Spectroscopic Survey in the years 2012 to 2019. ...The
Gaia
-ESO Survey is one among several current and future stellar spectroscopic surveys producing abundances for Milky-Way stars on an industrial scale.
Aims.
We present an unprecedented effort to create a homogeneous common line list, which was used by several abundance analysis groups using different radiative transfer codes to calculate synthetic spectra and equivalent widths. The atomic data are accompanied by quality indicators and detailed references to the sources. The atomic and molecular data are made publicly available at the CDS.
Methods.
In general, experimental transition probabilities were preferred but theoretical values were also used. Astrophysical
gf
-values were avoided due to the model-dependence of such a procedure. For elements whose lines are significantly affected by a hyperfine structure or isotopic splitting, a concerted effort has been made to collate the necessary data for the individual line components. Synthetic stellar spectra calculated for the Sun and Arcturus were used to assess the blending properties of the lines. We also performed adetailed investigation of available data for line broadening due to collisions with neutral hydrogen atoms.
Results.
Among a subset of over 1300 lines of 35 elements in the wavelength ranges from 475 to 685 nm and from 850 to 895 nm, we identified about 200 lines of 24 species which have accurate
gf
-values and are free of blends in the spectra of the Sun and Arcturus. For the broadening due to collisions with neutral hydrogen, we recommend data based on Anstee-Barklem-O’Mara theory, where possible. We recommend avoiding lines of neutral species for which these are not available. Theoretical broadening data by R.L. Kurucz should be used for Sc
II
, Ti
II
, and Y
II
lines; additionally, for ionised rare-earth species, the Unsöld approximation with an enhancement factor of 1.5 for the line width can be used.
Conclusions.
The line list has proven to be a useful tool for abundance determinations based on the spectra obtained within the
Gaia
-ESO Survey, as well as other spectroscopic projects. Accuracies below 0.2 dex are regularly achieved, where part of the uncertainties are due to differences in the employed analysis methods. Desirable improvements in atomic data were identified for a number of species, most importantly Al
I
, S
I
, and Cr
II
, but also Na
I
, Si
I
, Ca
II
, and Ni
I
.
ABSTRACT
We investigate the evolution of the abundance of fluorine in the Milky Way thick and thin discs by means of detailed chemical evolution models compared with recent observational data. The ...chemical evolution models adopted here have already been shown to fit the observed abundance patterns of CNO and α-elements as well as the metallicity distribution functions for the Galactic thick and thin disc stars. We apply them here to the study of the origin and evolution of fluorine, which is still a matter of debate. First, we study the importance of the various sites proposed for the production of fluorine. Then, we apply the reference models to follow the evolution of the two different Galactic components. We conclude that rotating massive stars are important producers of F and they can set a plateau in F abundance below Fe/H = −0.5 dex, though its existence for Fe/H<−1 has yet to be confirmed by extensive observations of halo stars. In order to reproduce the F abundance increase in the discs at late times, instead, a contribution from lower mass stars – single asymptotic giant branch stars and/or novae – is required. The dichotomy between the thick and thin discs is more evident in the F/O versus O/H plot than in the F/Fe versus Fe/H one, and we confirm that the thick disc has evolved much faster than the thin disc, in agreement with findings from the abundance patterns of other chemical elements.
Context. With the existing and upcoming large multifibre low-resolution spectrographs, the question arises of how precise stellar parameters such as Teff and Fe/H can be obtained from low-resolution ...K-band spectra with respect to traditional photometric temperature measurements. Until now, most of the effective temperatures in Galactic bulge studies come directly from photometric techniques. Uncertainties in interstellar reddening and in the assumed extinction law could lead to large systematic errors (>200 K). Aims. We obtain and calibrate the relation between Teff and the 12CO first overtone bands for M giants in the Galactic bulge covering a wide range in metallicity. Methods. We used low-resolution spectra for 20 M giants with well-studied parameters from photometric measurements covering the temperature range 3200 <Teff< 4500 K and a metallicity range from 0.5 dex down to −1.2 dex and study the behaviour of Teff and Fe/H on the spectral indices. Results. We find a tight relation between Teff and the 12CO(2−0) band with a dispersion of 95 K and between Teff and the 12CO(3−1) with a dispersion of 120 K. We do not find any dependence of these relations on the metallicity of the star, which makes them attractive for Galactic bulge studies. This relation is also not sensitive to the spectral resolution, which allows this relation to be applied in a more general way. We also find a correlation between the combination of the Na i, Ca i, and the 12CO band with the metallicity of the star. However, this relation is only valid for subsolar metallicities. Conclusions. We show that low-resolution spectra provide a powerful tool for obtaining effective temperatures of M giants. We show that this relation does not depend on the metallicity of the star within the investigated range and is also applicable to different spectral resolutions making this relation in general useful for deriving effective temperatures in high-extinction regions where photometric temperatures are not reliable.
PDF
