Context.
The spectra of unevolved metal-poor halo stars uniquely reflect the elemental abundances incorporated during the earliest Galactic epoch. Their heavy-element content is well understood as ...the products of neutron capture on iron-peak elements. However, for the lightest trans-iron elements with atomic number 30 <
Z
< 52, they show striking abundance patterns that defy model predictions. Understanding their sources may illuminate the diverse halo, thick disk, or extragalactic origins of metal-poor stars.
Aims.
The primary goal is the derivation of halo dwarf abundances and their uncertainties for six trans-iron elements from UV spectra, plus optical abundances for four additional trans-Fe elements and two well-understood heavier elements.
Methods.
For five metal-poor dwarfs, we analyzed high-resolution UV spectra from the
Hubble
Space Telescope Imaging Spectrograph, supplemented by archival optical echelle spectra. Two independent analyses adopted different programs, models, and line lists, clarifying systematic errors.
Results.
The results from the separate UV analyses are in good agreement. The largest source of discrepancy is the placement of the UV continuum. Once rectified, the separate results agree to 0.2 dex for moderately unblended, moderately strong lines. Similar agreement is found with previous works, except where new data and line selection become important, notably our exclusion of trans-Fe lines blended by newly identifed Fe
I
lines.
Conclusions.
Improved line lists lead to low As/Ge ratios that no longer require an early arsenic enhancement. All five stars exhibit a high Mo/Ge abundance ratio, independent of Mo/Fe. The trans-Fe elements show an odd-even effect: an odd-
Z
element abundance is depressed relative to those of adjacent even-
Z
elements. Its suggested metallicity dependence is supported by previous studies of Sr-Y-Zr. Some theoretical yields show a metallicity-dependent odd-even effect, but none have predicted a constant Mo/Ge abundance ratio. Our work thus highlights the complexity of predicting the production of light trans-Fe elements in metal-poor stars.
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We use high-precision photometry of red-giant-branch (RGB) stars in 57 Galactic globular clusters (GCs), mostly from the 'Hubble Space Telescope (HST) UV Legacy Survey of Galactic GCs', to identify ...and characterize their multiple stellar populations. For each cluster the pseudo-two-colour diagram (or 'chromosome map') is presented, built with a suitable combination of stellar magnitudes in the F275W, F336W, F438W, and F814W filters that maximizes the separation between multiple populations. In the chromosome map of most GCs (type-I clusters), stars separate in two distinct groups that we identify with the first (1G) and the second generation (2G). This identification is further supported by noticing that 1G stars have primordial (oxygen-rich, sodium-poor) chemical composition, whereas 2G stars are enhanced in sodium and depleted in oxygen. This 1G-2G separation is not possible for a few GCs where the two sequences have apparently merged into an extended, continuous sequence. In some GCs (type-II clusters) the 1G and/or the 2G sequences appear to be split, hence displaying more complex chromosome maps. These clusters exhibit multiple subgiant branches (SGBs) also in purely optical colour-magnitude diagrams, with the fainter SGB joining into a red RGB which is populated by stars with enhanced heavy-element abundance. We measure the RGB width by using appropriate colours and pseudo-colours. When the metallicity dependence is removed, the RGB width correlates with the cluster mass. The fraction of 1G stars ranges from ~8 per cent to ~67 per cent and anticorrelates with the cluster mass, indicating that incidence and complexity of the multiple population phenomenon both increase with cluster mass.
Aims. We seek to constrain the formation of the Galactic bulge by analysing the detailed chemical composition of a large sample of red clump stars in Baade’s window. These stars were selected to ...minimise the contamination by other Galactic components, so they are good tracers of the bulge metallicity distribution in Baade’s window, at least for stars more metal-rich than ~−1.5. Methods. We used an automatic procedure to measure Fe/H differentially with respect to the metal-rich star μLeo in a sample of 219 bulge red clump stars from R = 20 000 resolution spectra obtained with FLAMES/GIRAFFE at the VLT. For a subsample of 162 stars, we also derived Mg/H from spectral synthesis around the Mg i triplet at λ 6319 Å. Results. The Fe and Mg metallicity distributions are both asymmetric with median values of +0.16 and +0.21, respectively. They show only a small proportion of stars at low metallicities, extending down to Fe/H = −1.1 or Mg/H = −0.7. The iron distribution is clearly bimodal, as revealed both by a deconvolution (from observational errors) and a Gaussian decomposition. The decomposition of the observed Fe and Mg metallicity distributions into Gaussian components yields two populations of equal sizes (50% each): a metal-poor component centred on Fe/H = −0.30 and Mg/H = −0.06 with a large dispersion and a narrow metal-rich component centred on Fe/H = +0.32 and Mg/H = +0.35. The metal-poor component shows high Mg/Fe ratios (around 0.3), while stars in the metal-rich component are found to have nearly solar ratios. Kinematical differences between the two components have also been found: the metal-poor component shows kinematics compatible with an old spheroid, while the metal-rich component is consistent with a population supporting a bar. In view of their chemical and kinematical properties, we suggest different formation scenarii for the two populations: a rapid formation time scale as an old spheroid for the metal-poor component (old bulge) and for the metal-rich component, a formation on a longer time scale driven by the evolution of the bar (pseudo-bulge). The observations are described well by a simple model consisting of two components: a simple closed box model to predict the metal-poor population contribution and a local thin disc metallicity distribution, shifted in metallicity, to represent the metal-rich population. The pseudo-bulge is compatible with its being formed from the inner thin disc, assuming high (but plausible) values of the gradients in the early Galactic disc.
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Context. The isotope abundances provide powerful diagnostics of the chemical enrichment in our Galaxy. The star HD 140283 is one of the best-studied very metal-poor dwarf stars. It is very old, and ...the chemical abundance in this star is a good witness of the chemical composition of the matter in the early Galaxy. Aims. The aim of this work is to measure the precise abundances of carbon, nitrogen, oxygen, and mainly the 12C/13C isotopic ratio in this very old metal-poor star in order to have a good reference for the computations of the chemical evolution of the Galaxy. Methods. We used very high spectral resolution data, with extremely high signal-to-noise ratios obtained with the spectrographs ESPaDOnS at the CFHT, ESPRESSO at the VLT, and HARPS at the ESO 3.6 m telescope. Results. For the first time, we were able to measure the 12C/13C ratio in a very old metal-poor dwarf that was born at the very beginning of the Galaxy: 12C/13C = 33−6+12. We also obtained a precise determination of the abundance of the CNO elements in this star. These abundances give information about the early composition of the cloud from which HD 140283 was formed. They suggest that the effect of super-asymptotic giant branch stars or fast-rotating massive stars was significant in the early Galaxy.
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We combine high-resolution spectroscopic data from APOGEE-2 survey Data Release 16 (DR16) with broad-band photometric data from several sources as well as parallaxes from
Gaia
Data Release 2 (DR2). ...Using the Bayesian isochrone-fitting code
StarHorse
, we derived the distances, extinctions, and astrophysical parameters for around 388 815 APOGEE stars. We achieve typical distance uncertainties of ∼6% for APOGEE giants, ∼2% for APOGEE dwarfs, and extinction uncertainties of ∼0.07 mag, when all photometric information is available, and ∼0.17 mag if optical photometry is missing.
StarHorse
uncertainties vary with the input spectroscopic catalogue, available photometry, and parallax uncertainties. To illustrate the impact of our results, we show that thanks to
Gaia
DR2 and the now larger sky coverage of APOGEE-2 (including APOGEE-South), we obtain an extended map of the Galactic plane. We thereby provide an unprecedented coverage of the disc close to the Galactic mid-plane (|
Z
Gal
| < 1 kpc) from the Galactic centre out to
R
Gal
∼ 20 kpc. The improvements in statistics as well as distance and extinction uncertainties unveil the presence of the bar in stellar density and the striking chemical duality in the innermost regions of the disc, which now clearly extend to the inner bulge. We complement this paper with distances and extinctions for stars in other public released spectroscopic surveys: 324 999 in GALAH DR2, 4 928 715 in LAMOST DR5, 408 894 in RAVE DR6, and 6095 in GES DR3.
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In this paper we describe a new UV-initiative Hubble Space Telescope project (GO-13297) that will complement the existing F606W and F814W database of the Advanced Camera for Surveys Globular Cluster ...(GC) Treasury by imaging most of its clusters through UV/blue WFC3/UVIS filters F275W, F336W, and F438W. This "magic trio" of filters has shown an uncanny ability to disentangle and characterize multiple population (MP) patterns in GCs in a way that is exquisitely sensitive to C, N, and O abundance variations. Combination of these passbands with those in the optical also gives the best leverage for measuring helium enrichment. The dozen clusters that had previously been observed in these bands exhibit a bewildering variety of MP patterns, and the new survey will map the full variance of the phenomenon. The ubiquity of multiple stellar generations in GCs has made the formation of these cornerstone objects more intriguing than ever; GC formation and the origin of their MPs have now become one and the same problem. In this paper we will describe the database and our data reduction strategy, as well as the uses we intend to make of the final photometry, astrometry, and PMs. We will also present preliminary color-magnitude diagrams from the data so far collected. These diagrams also draw on data from GO-12605 and GO-12311, which served as a pilot project for the present GO-13297.
Abstract
Bulge globular clusters (GCs) with metallicities Fe/H ≲ −1.0 and blue horizontal branches are candidates to harbor the oldest populations in the Galaxy. Based on the analysis of
HST
...proper-motion-cleaned color–magnitude diagrams in filters F435W and F625W, we determine physical parameters for the old bulge GCs NGC 6522 and NGC 6626 (M28), both with well-defined blue horizontal branches. We compare these results with similar data for the inner halo cluster NGC 6362. These clusters have similar metallicities (−1.3 ≤ Fe/H ≤ −1.0) obtained from high-resolution spectroscopy. We derive ages, distance moduli, and reddening values by means of statistical comparisons between observed and synthetic fiducial lines employing likelihood statistics and the Markov chain Monte Carlo method. The synthetic fiducial lines were generated using
α
-enhanced BaSTI and Dartmouth stellar evolutionary models, adopting both canonical (
Y
∼ 0.25) and enhanced (
Y
∼ 0.30–0.33) helium abundances. RR Lyrae stars were employed to determine the HB magnitude level, providing an independent indicator to constrain the apparent distance modulus and the helium enhancement. The shape of the observed fiducial line could be compatible with some helium enhancement for NGC 6522 and NGC 6626, but the average magnitudes of RR Lyrae stars tend to rule out this hypothesis. Assuming canonical helium abundances, BaSTI and Dartmouth models indicate that all three clusters are coeval, with ages between ∼12.5 and 13.0 Gyr. The present study also reveals that NGC 6522 has at least two stellar populations, since its CMD shows a significantly wide subgiant branch compatible with 14% ± 2% and 86% ± 5% for first and second generations, respectively.
Context. Globular clusters trace the formation and evolution of the Milky Way and surrounding galaxies, and outline their chemical enrichment history. To accomplish these tasks it is important to ...have large samples of clusters with homogeneous data and analysis to derive kinematics, chemical abundances, ages and locations. Aims. We obtain homogeneous metallicities and alpha-element enhancement for 51 Galactic bulge, disc, and halo globular clusters that are among the most distant and/or highly reddened in the Galaxy's globular cluster system. We also provide membership selection based on stellar radial velocities and atmospheric parameters. The implications of our results are discussed. Methods. We observed R~ 2000 spectra in the wavelength interval 456-586 nm for over 800 red giant stars in 51 Galactic globular clusters. We applied full spectrum fitting with the code ETOILE together with libraries of observed and synthetic spectra. We compared the mean abundances of all clusters with previous work and with field stars. We used the relation between mean metallicity and horizontal branch morphology defined by all clusters to select outliers for discussion. Results. Fe/H, Mg/Fe, and alpha/Fe were derived in a consistent way for almost one-third of all Galactic globular clusters. We find our metallicities are comparable to those derived from high-resolution data to within sigma= 0.08 dex over the interval -2.5< Fe/H < 0.0. Furthermore, a comparison of previous metallicity scales with our values yields sigma< 0.16 dex. We also find that the distribution of Mg/Fe and alpha/Fe with Fe/H for the 51 clusters follows the general trend exhibited by field stars. It is the first time that the following clusters have been included in a large sample of homogeneous stellar spectroscopic observations and metallicity derivation: BH 176, Djorg 2, Pal 10, NGC 6426, Lynga 7, and Terzan 8. In particular, only photometric metallicities were available previously for the first three clusters, and the available metallicity for NGC 6426 was based on integrated spectroscopy and photometry. Two other clusters, HP 1 and NGC 6558, are confirmed as candidates for the oldest globular clusters in the Milky Way. Conclusions. Stellar spectroscopy in the visible at R~ 2000 for a large sample of globular clusters is a robust and efficient way to trace the chemical evolution of the host galaxy and to detect interesting objects for follow-up at higher resolution and with forthcoming giant telescopes. The technique used here can also be applied to globular cluster systems in nearby galaxies with current instruments and to distant galaxies with the advent of ELTs.
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Aims. The abundance patterns of the neutron-capture elements in metal-poor stars provide a unique record of the nucleosynthesis products of the earlier massive primitive objects. Methods. We measured ...new abundances of so-called light neutron-capture of first peak elements using local thermodynamic equilibrium (LTE) 1D analysis; this analysis resulted in a sample of 11 very metal-poor stars, from Fe/H = –2.5 to Fe/H = –3.4, and one carbon-rich star, CS 22949-037 with Fe/H = –4.0. The abundances were compared to those observed in two classical metal-poor stars: the typical r-rich star CS 31082-001 (Eu/Fe > +1.0) and the r-poor star HD 122563 (Eu/Fe < 0.0), which are known to present a strong enrichment of the first peak neutron-capture elements relative to the second peak. Results. Within the first peak, the abundances are well correlated in analogy to the well-known correlation inside the abundances of the second-peak elements. In contrast, there is no correlation between any first peak element with any second peak element. We show that the scatter of the ratio of the first peak abundance over second peak abundance increases when the mean abundance of the second peak elements decreases from r-rich to r-poor stars. We found two new r-poor stars that are very similar to HD 122563. A third r-poor star, CS 22897-008, is even more extreme; this star shows the most extreme example of first peak elements enrichment to date. On the contrary, another r-poor star (BD–18 5550) has a pattern of first peak elements that is similar to the typical r-rich stars CS 31082-001, however this star has some Mo enrichment. Conclusions. The distribution of the neutron-capture elements in our very metal-poor stars can be understood as the combination of at least two mechanisms: one that enriches the forming stars cloud homogeneously through the main r-process and leads to an element pattern similar to the r-rich stars, such as CS 31082-001; and another that forms mainly lighter, first peak elements.
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