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Accretion disks surrounding stellar mass black holes (BHs) have been suggested as potential locations for the nucleosynthesis of light elements, which are our primary observational discriminant of multiple stellar populations within globular clusters. The population of enriched stars in globular clusters are enhanced in N14, Na23, and sometimes in Al27 and/or in K39. In this study, our aim is to investigate the feasibility of initiating nucleosynthesis for these four elements in BH accretion disks, considering various internal parameters such as the temperature of the gas and timescale of the accretion. To achieve this, we employed a 132-species reaction network. We used the slim disk model, suitable for the Super-Eddington mass accretion rate and for geometrically and optically thick disks. We explored the conditions related to the mass, mass accretion rate, viscosity, and radius of the BH-accretion disk system that would allow for the creation of N14, Na23, Al27, and K39 before the gas is accreted onto the central object. Our findings reveal that there is no region in the parameter space where the formation of Na23 can occur and only a very limited region where the formation of N14, Al27, and K39 is plausible. Specifically, this occurs for BHs with masses lower than 10 solar masses, with a preference toward even lower mass values and extremely low viscosity parameters ($\alpha <10^{-3}$). Such values are highly unlikely based on current observations of stellar mass BHs. However, such low mass BHs could actually exist in the early universe, as so-called primordial BHs. In conclusion, our study suggests that the nucleosynthesis within BH accretion disks of four elements of interest for the multiple stellar populations is improbable, but not impossible, using the slim disk model., Comment: 11 pages, 7 figures, 2 tables, Accepted by A&A

We present CARMA, the Cluster Ages to Reconstruct the Milky Way Assembly project, that aims at determining precise and accurate age measurements for the entire system of known Galactic globular clusters and at using them to trace the most significant merger events experienced by the Milky Way. The strength of CARMA relies on the use of homogeneous photometry, theoretical isochrones, and statistical methods, that will enable to define a systematic-free chronological scale for the complete sample of Milky Way globulars. In this paper we describe the CARMA framework in detail, and present a first application on a sample of six metal-rich globular clusters with the aim of putting the final word on the debated origin of NGC6388 and NGC6441. Our results demonstrate that this pair of clusters is coeval with other four systems having a clear in-situ origin. Moreover, their location in the age-metallicity plane matches the one occupied by in-situ field stars. Such an accurate age comparison enabled by the CARMA methodology rules out the possibility that NGC6388 and NGC6441 have been accreted as part of a past merger event., Comment: 11 pages + Appendix, 13 figures, accepted for publication by A&A

WEAVE, the new wide-field, massively multiplexed spectroscopic survey facility for the William Herschel Telescope, will see first light in late 2022. WEAVE comprises a new 2-degree field-of-view prime-focus corrector system, a nearly 1000-multiplex fibre positioner, 20 individually deployable 'mini' integral field units (IFUs), and a single large IFU. These fibre systems feed a dual-beam spectrograph covering the wavelength range 366$-$959\,nm at $R\sim5000$, or two shorter ranges at $R\sim20\,000$. After summarising the design and implementation of WEAVE and its data systems, we present the organisation, science drivers and design of a five- to seven-year programme of eight individual surveys to: (i) study our Galaxy's origins by completing Gaia's phase-space information, providing metallicities to its limiting magnitude for $\sim$3 million stars and detailed abundances for $\sim1.5$ million brighter field and open-cluster stars; (ii) survey $\sim0.4$ million Galactic-plane OBA stars, young stellar objects and nearby gas to understand the evolution of young stars and their environments; (iii) perform an extensive spectral survey of white dwarfs; (iv) survey $\sim400$ neutral-hydrogen-selected galaxies with the IFUs; (v) study properties and kinematics of stellar populations and ionised gas in $z<0.5$ cluster galaxies; (vi) survey stellar populations and kinematics in $\sim25\,000$ field galaxies at $0.3\lesssim z \lesssim 0.7$; (vii) study the cosmic evolution of accretion and star formation using $>1$ million spectra of LOFAR-selected radio sources; (viii) trace structures using intergalactic/circumgalactic gas at $z>2$. Finally, we describe the WEAVE Operational Rehearsals using the WEAVE Simulator., Comment: 41 pages, 27 figures, accepted for publication by MNRAS; updated version including information on individual grants in a revised Acknowledgements section, corrections to the affiliation list, and an updated references list

We present the results of a spectroscopic survey of the outskirts of 4 globular clusters -- NGC 1261, NGC 4590, NGC 1904, and NGC 1851 -- covering targets within 1 degree from the cluster centres, with 2dF/AAOmega on the Anglo-Australian Telescope (AAT) and FLAMES on the Very Large Telescope (VLT). We extracted chemo-dynamical information for individual stars, from which we estimated the velocity dispersion profile and the rotation of each cluster. The observations are compared to direct $N$-body simulations and appropriate {\sc limepy}/{\sc spes} models for each cluster to interpret the results. In NGC 1851, the detected internal rotation agrees with existing literature, and NGC 1261 shows some rotation signal beyond the truncation radius, likely coming from the escaped stars. We find that the dispersion profiles for both the observations and the simulations for NGC 1261, NGC 1851, and NGC 1904 do not decrease as the {\sc limepy}/{\sc spes} models predict beyond the truncation radius, where the $N$-body simulations show that escaped stars dominate; the dispersion profile of NGC 4590 follows the predictions of the {\sc limepy}/{\sc spes} models, though the data do not effectively extend beyond the truncation radius. The increasing/flat dispersion profiles in the outskirts of NGC 1261, NGC 1851 and NGC 1904, are reproduced by the simulations. Hence, the increasing/flat dispersion profiles of the clusters in question can be explained by the tidal interaction with the Galaxy without introducing dark matter., Comment: 17 pages, 10 figures. Accepted by MNRAS. Comments are welcome

All cosmological models of structure formation predict the existence of a widespread population of dual supermassive black holes in-spiralling inside their common host galaxy, eventually merging and giving rise to intense gravitational waves. These systems can be identified as dual AGNs at kilo parsec separations, but only very few have been confirmed at z>0.5. The appearance of multiple AGNs at small angular separations can also be due to gravitational lensing of single AGNs, which are themselves very important systems for many astrophysical topics. Here we present a novel technique, dubbed the Gaia Multipeak (GMP) method, to obtain large and reliable samples of dual/lensed AGN candidates with sub-arcsec separations by looking for AGNs showing multiple peaks in the light profiles observed by the Gaia satellite. All the GMP-selected sources with high resolution images (26 from the HST archive and 5 from dedicated adaptive-optics assisted imaging at the Large Binocular Telescope) show multiple components with sub-arcsec separation pointing toward a very high reliability of the method. By sampling separations down to ~2 kpc at z>1, this method allows us to probe the physical processes that drive the inspiralling of a pair of SMBHs inside a single galaxy., Comment: Nature Astronomy (2022)

Lithium is one of the few elements produced during the Big Bang Nucleosynthesis in the early universe. Moreover, its fragility makes it useful as a proxy for stellar environmental conditions. As such, the lithium abundance in old systems is at the core of different astrophysical problems. Stars in the lower red giant branch allow studying globular clusters where main sequence stars are too faint to be observed. We use these stars to analyze the initial Li content of the clusters and compare it to cosmological predictions, to measure spreads in Li between different stellar populations, and to study signs of extra depletion in these giants. We use GIRAFFE spectra to measure the lithium and sodium abundances of lower red giant branch stars in 5 globular clusters. These cover an extensive range in metallicity, from [Fe/H]$\sim-0.7$ to [Fe/H]$\sim-2.3$ dex. We find that the lithium abundance in these lower red giant branch stars forms a plateau, with values from $\mathrm{A(Li)_{NLTE}}=0.84$ to $1.03$ dex, showing no clear correlation with metallicity. When using stellar evolutionary models to calculate the primordial abundance of these clusters, we recover values $\mathrm{A(Li)_{NLTE}}=2.1-2.3$ dex, consistent with the constant value observed in warm metal-poor halo stars, the Spite plateau. Additionally, we find no difference in the lithium abundance of first and second population stars in each cluster. We also report the discovery of a Li-rich giant in the cluster NGC3201, with $\mathrm{A(Li)_{NLTE}}=1.63\pm0.18$ dex, where the enrichment mechanism is probably pollution from external sources., Comment: Accepted for publication in A&A

We describe the preliminary results of a ground-based observing campaign aimed at building a grid of approximately 200 spectro-photometric standard stars (SPSS), with an internal $\simeq 1$\% accuracy (and sub-percent precision), tied to CALSPEC Vega and Sirius systems within $\simeq 1$\%, for the absolute flux calibration of data gathered by {\it Gaia}, the European Space Agency (ESA) astrometric mission. The criteria for the selection and a list of candidates are presented, together with a description of the survey's strategy and the adopted data analysis methods. All candidates were also monitored for constancy (within $\pm 5$ mmag, approximately). The present version of the grid contains about half of the final sample, it has already reached the target accuracy but the precision will substantially improve with future releases. It will be used to calibrate the {\it Gaia} (E)DR3 release of spectra and photometry., Comment: 8 pages, 4 figures, 3 tables, SPIE Astronomical Telescopes + Instrumentation 2020 conference Paper 11449-97

We describe the atomic and molecular data that were used for the abundance analyses of FGK-type stars carried out within the Gaia-ESO Survey. We present an unprecedented effort to create a homogeneous line list, which was used by several abundance analysis groups 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 in electronic form. 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 hyperfine structure or isotopic splitting a concerted effort has been made to collate the necessary data for the individual line components. We also performed a detailed investigation of available data for line broadening due to collisions with neutral hydrogen atoms. Synthetic spectra calculated for the Sun and Arcturus were used to assess the blending properties of the lines. Among a subset of over 1300 lines of 35 elements in the wavelength ranges from 475 nm to 685 nm and from 850 nm 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 available, and to avoid lines of neutral species otherwise. Theoretical broadening data by R.L. Kurucz should be used for Sc II, Ti II, and Y II lines. For ionised rare-earth species the Uns\"old approximation with an enhancement factor of 1.5 for the line width can be used. Desirable improvements in atomic data were identified for a number of species, including Al I, S I, Cr II, Na I, Si I, Ca II, and Ni I., Comment: Accepted for publication in section 12. Atomic, molecular, and nuclear data of Astronomy and Astrophysics; main part 25 pages, 8 tables, 5 figures; appendices 53 pages, 23 tables, 29 figures

Atomic diffusion and mixing processes in stellar interiors influence the structure and the surface composition of stars. Some of these processes cannot yet be modelled from the first principles. This limits their applicability in stellar models used for studies of stellar populations and Galactic evolution. Our main goal is to put constrains on the stellar structure and evolution models using new refined measurements of chemical composition in stars of Galactic open cluster. We use medium-resolution, 19 200 <= R <= 21 500, optical spectra of the stars in the open cluster NGC 2420 obtained within the Gaia-ESO survey. The sample covers all evolutionary stages from the main-sequence to red giant branch. Stellar parameters are derived using a combined Bayesian analysis of spectra, 2MASS photometry, and astrometric data from Gaia DR2. The abundances of Mg, Ca, Fe, and Li are determined from non-local thermodynamic equilibrium (NLTE) synthetic spectra, computed using one-dimensional (1D) and averaged three-dimensional (3D) model atmospheres. We compare our results with a grid of Code d'Evolution Stellaire Adaptatif et Modulaire (CESTAM) stellar evolution models, which include atomic diffusion, turbulent and rotational mixing. We find prominent evolutionary trends in the abundances of Fe, Ca, Mg, and Li with the mass of the stars in the cluster. Fe, Mg, and Ca show a depletion at the cluster turn-off, but the abundances gradually increase and flatten near the base of the RGB. The abundance trend for Li displays a signature of rotational mixing on the main-sequence and abrupt depletion on the subgiant branch, which is caused by advection of Li-poor material to the surface. The analysis of abundances combined with the CESTAM model predictions allows us to place limits on the parameter space of the models and to constrain the zone in the stellar interior where turbulent mixing takes place., Comment: accepted for publication in A&A

As we enter a golden age for studies of internal kinematics and dynamics of Galactic globular clusters (GCs), it is timely to assess the performance of modelling techniques in recovering the mass, mass profile, and other dynamical properties of GCs. Here, we compare different mass-modelling techniques (distribution-function (DF)-based models, Jeans models, and a grid of N-body models) by applying them to mock observations from a star-by-star N-body simulation of the GC M 4 by Heggie. The mocks mimic existing and anticipated data for GCs: surface brightness or number density profiles, local stellar mass functions, line-of-sight velocities, and Hubble Space Telescope- and Gaia-like proper motions. We discuss the successes and limitations of the methods. We find that multimass DF-based models, Jeans, and N-body models provide more accurate mass profiles compared to single-mass DF-based models. We highlight complications in fitting the kinematics in the outskirts due to energetically unbound stars associated with the cluster ("potential escapers", not captured by truncated DF models nor by N-body models of clusters in isolation), which can be avoided with DF-based models including potential escapers, or with Jeans models. We discuss ways to account for mass segregation. For example, three-component DF-based models with freedom in their mass function are a simple alternative to avoid the biases of single-mass models (which systematically underestimate the total mass, half-mass radius, and central density), while more realistic multimass DF-based models with freedom in the remnant content represent a promising avenue to infer the total mass and the mass function of remnants., Comment: 27 pages, 12 figures, MNRAS accepted

We present a family of self-consistent axisymmetric rotating globular cluster models which are fitted to spectroscopic data for NGC 362, NGC 1851, NGC 2808, NGC 4372, NGC 5927 and NGC 6752 to provide constraints on their physical and kinematic properties, including their rotation signals. They are constructed by flattening Modified Plummer profiles, which have the same asymptotic behaviour as classical Plummer models, but can provide better fits to young clusters due to a slower turnover in the density profile. The models are in dynamical equilibrium as they depend solely on the action variables. We employ a fully Bayesian scheme to investigate the uncertainty in our model parameters (including mass-to-light ratios and inclination angles) and evaluate the Bayesian evidence ratio for rotating to non-rotating models. We find convincing levels of rotation only in NGC 2808. In the other clusters, there is just a hint of rotation (in particular, NGC 4372 and NGC 5927), as the data quality does not allow us to draw strong conclusions. Where rotation is present, we find that it is confined to the central regions, within radii of $R \leq 2 r_h$. As part of this work, we have developed a novel q-Gaussian basis expansion of the line-of-sight velocity distributions, from which general models can be constructed via interpolation on the basis coefficients., Comment: submitted to MNRAS

Red giant stars are perhaps the most important type of stars for Galactic and extra-galactic archaeology: they are luminous, occur in all stellar populations, and their surface temperatures allow precise abundance determinations for many different chemical elements. Yet, the full star formation and enrichment history of a galaxy can be traced directly only if two key observables can be determined for large stellar samples - age and chemical composition. While spectroscopy is a powerful method to analyse the detailed abundances of stars, stellar ages are the "missing link in the chain", since they are not a direct observable. However, spectroscopy should be able to estimate stellar masses, which for red giants directly infer ages provided their chemical composition is known. Here we establish a new empirical relation between the shape of the hydrogen line in the observed spectra of red giants and stellar mass determined from asteroseismology. The relation allows to determine stellar masses and ages with the accuracy of 10-15%. The method can be used with confidence for stars in the following range of stellar parameters: 4000 < Teff < 5000 K, 0.5 < log g < 3.5, -2.0 < [Fe/H] < 0.3, and luminosities log L/LSun < 2.5. Our analysis provides observational evidence that the Halpha spectral characteristics of red giant stars are tightly correlated with their mass and therefore their age. We also show that the method samples well all stellar populations with ages above 1 Gyr. Targeting bright giants, the method allows to obtain simultaneous age and chemical abundance information far deeper than would be possible with asteroseismology, extending the possible survey volume to remote regions of the Milky Way and even to neighbouring galaxies like Andromeda or the Magellanic Clouds already with present instrumentation, like VLT and Keck facilities., Comment: 18 pages, accepted for publication in A&A

We have entered an era of large spectroscopic surveys in which we can measure, through automated pipelines, the atmospheric parameters and chemical abundances for large numbers of stars. Calibrating these survey pipelines using a set of "benchmark stars" in order to evaluate the accuracy and precision of the provided parameters and abundances is of utmost importance. The recent proposed set of Gaia FGK benchmark stars of Heiter et al. (2015) has no recommended stars within the critical metallicity range of $-2.0 <$ [Fe/H] $< -1.0$ dex. In this paper, we aim to add candidate Gaia benchmark stars inside of this metal-poor gap. We began with a sample of 21 metal-poor stars which was reduced to 10 stars by requiring accurate photometry and parallaxes, and high-resolution archival spectra. The procedure used to determine the stellar parameters was similar to Heiter et al. (2015) and Jofre et al. (2014) for consistency. The effective temperature (T$_{\mathrm{eff}}$) of all candidate stars was determined using the Infrared Flux Method utilizing multi-band photometry. The surface gravity (log g) was determined through fitting stellar evolutionary tracks. The [Fe/H] was determined using four different spectroscopic methods fixing the T$_{\mathrm{eff}}$ and log g from the values determined independent of spectroscopy. We discuss, star-by-star, the quality of each parameter including how it compares to literature, how it compares to a spectroscopic run where all parameters are free, and whether Fe I ionisation-excitation balance is achieved. From the 10 stars, we recommend a sample of five new metal-poor benchmark candidate stars which have consistent T$_{\mathrm{eff}}$ , log g, and [Fe/H] determined through several means. These stars can be used for calibration and validation purpose of stellar parameter and abundance pipelines and should be of highest priority for future interferometric studies., Comment: Accepted for publication in A&A, 18 Pages, 8 Figures, 7 Tables

We discuss the properties of H$\alpha$ emission stars across the sample of 22035 spectra from the Gaia-ESO Survey internal data release, observed with the GIRAFFE instrument and largely belonging to stars in young open clusters. Automated fits using two independent Gaussian profiles and a third component that accounts for the nebular emission allow us to discern distinct morphological types of H$\alpha$ line profiles with the introduction of a simplified classification scheme. All in all we find 3765 stars with intrinsic emission and sort their spectra into eight distinct morphological categories: single--component emission, emission blend, sharp emission peaks, double emission, P-Cygni, inverted P-Cygni, self--absorption, and emission in absorption. We have more than one observation for 1430 stars in our sample, thus allowing a quantitative discussion of the degree of variability of H$\alpha$ emission profiles, which is expected for young, active objects. We present a catalogue of stars with properties of their H$\alpha$ emission line profiles, morphological classification, analysis of variability with time and the supplementary information from the SIMBAD, VizieR, and ADS databases. The records in SIMBAD indicate the presence of H$\alpha$ emission for roughly 25% of all stars in our catalogue, while at least 305 of them have already been more thoroughly investigated according to the references in ADS. The most frequently identified morphological categories in our sample of spectra are emission blend (23%), emission in absorption (22%), and self--absorption (16%). Objects with repeated observations demonstrate that our classification into discrete categories is generally stable through time, but categories P-Cygni and self--absorption seem less stable, which is the consequence of discrete classification rules, as well as of the fundamental change in profile shape.

Large spectroscopic surveys such as the Gaia-ESO Survey produce huge quantities of data. Automatic tools are necessary to efficiently handle this material. The measurement of equivalent widths in stellar spectra is traditionally done by hand or with semi-automatic procedures that are time-consuming and not very robust with respect to the repeatability of the results. The program DAOSPEC is a tool that provides consistent measurements of equivalent widths in stellar spectra while requiring a minimum of user intervention. However, it is not optimised to deal with large batches of spectra, as some parameters still need to be modified and checked by the user. Exploiting the versatility and portability of BASH, we have built a pipeline called DAOSPEC Option Optimiser (DOOp) automating the procedure of equivalent widths measurement with DAOSPEC. DOOp is organised in different modules that run one after the other to perform specific tasks, taking care of the optimisation of the parameters needed to provide the final equivalent widths, and providing log files to ensure better control over the procedure. In this paper, making use of synthetic and observed spectra, we compare the performance of DOOp with other methods, including DAOSPEC used manually. The measurements made by DOOp are identical to the ones produced by DAOSPEC when used manually, while requiring less user intervention, which is convenient when dealing with a large quantity of spectra. DOOp shows its best performance on high-resolution spectra (R>20 000) and high signal-to-noise ratio (S/N>30), with uncertainties ranging from 6 m{\AA} to 2 m{\AA}. The only subjective parameter that remains is the normalisation, as the user still has to make a choice on the order of the polynomial used for the continuum fitting. As a test, we use the equivalent widths measured by DOOp to re-derive the stellar parameters of four well-studied stars.

We examined the Wilson-Bappu effect, a relationship between the absolute magnitude of the star, $M_V$, and the logarithm of the Ca {\sc ii} emission width, $W_0$, over the largest $M_V$ range to date, +13 to -5, covering M-dwarfs to type Ia supergiants. We used an extensive literature, the latest Hipparcos reduction, data from two globular clusters, and new observations from Apache Point Observatory to compile a sample that allowed us to study the effect of [Fe/H] on the Wilson-Bappu relationship. Our results include reporting the deviations from linearity and demonstrating that the Wilson-Bappu relationship is insensitive to metallicity., Comment: 16 pages, 5 figures, accepted for publication in PASP

In the complex picture of multiple stellar populations in globular clusters (GCs), a special role is played by NGC 5139 Omega Centauri. At variance with the majority of GCs, Omega Cen exhibits significant star-to-star variations in metallicity and in relative neutron-capture element abundance ratios with respect to Fe, along with split evolutionary sequences as revealed from colour-magnitude diagrams. Combining information from photometry and spectroscopy, several studies suggested that an age spread of several Gyr has to be invoked to explain (at least partially) some of the observed features. However, a comprehensive understanding of the formation, evolution and chemical enrichment processes is still not at hand. Relatively metal-rich Omega cen stars display neutron-capture abundance distributions dominated by contributions from the s-process, but it is not clear what roles have been played by the so-called main and weak s-process components in generating these abundances. To gain better insight into this question we derived lead (Pb) abundances for several cluster members, because this element can only be produced by the main s-process. We analysed high-resolution UVES@VLT spectra of a sample of twelve red-giant branch stars, deriving abundances of Pb and also of Y, Zr, La, Ce, Eu, and the C+N+O sum. Spectral synthesis was applied to all features, taking into account isotopic shifts and/or hyperfine structure as needed. We measured for the first time the Pb content in Omega Cen, discovering a clear hint for a Pb production occurring at [Fe/H]>-1.7 dex. Our data suggest that the role of the weak component in the production of s-process elements is negligible. Moreover, evidence gathered from the abundances of other elements indicates that the main component occurring in this GC is peculiar and shifted towards higher mass polluters than the standard one., Comment: A&A, in press

From observations of almost 500 RGB stars in 29 Galactic open and globular clusters, we have investigated the behaviour of the infrared Ca II triplet (8498, 8542 and 8662 \AA) in the age range 13$\leq$Age/Gyr$\leq$0.25 and the metallicity range $-2.2\leq$ [Fe/H] $\leq$+0.47. These are the widest ranges of ages and metallicities in which the behaviour of the Ca II triplet lines has been investigated in a homogeneous way. We report the first empirical study of the variation of the CaII triplet lines strength, for given metallicities, with respect to luminosity. We find that the sequence defined by each cluster in the Luminosity-$\Sigma$Ca plane is not exactly linear. However, when only stars in a small magnitude interval are observed, the sequences can be considered as linear. We have studied the the Ca II triplet lines on three metallicities scales. While a linear correlation between the reduced equivalent width ($W'_V$ or $W'_I$) versus metallicity is found in the \citet{cg97} and \citet{ki03} scales, a second order term needs to be added when the \citet{zw84} scale is adopted. We investigate the role of age from the wide range of ages covered by our sample. We find that age has a weak influence on the final relationship. Finally, the relationship derived here is used to estimate the metallicities of three poorly studied open clusters: Berkeley 39, Trumpler 5 and Collinder 110. For the latter, the metallicity derived here is the first spectroscopic estimate available., Comment: 52 pages, 16 figures, accepted for publication in Astronomical Journal

We present a study of the central parts of the Sagittarius dwarf spheroidal galaxy (Sgr). We found a clear overdensity of Sgr's stars around M~54 (hereafter NS). NS is well represented by a King model and it has the characteristics of a typical dwarf elliptical nucleus. Whether this means that M~54 has spiraled into the potential well of NS or that M~54 is the real nucleus and NS has formed into its potential wells, remains an open question to be addressed., Comment: 4 pages, 2 figures. To be published in ``Satellites and Tidal Streams'', proceedings of the conference held in La Palma 26-30 May 2003, Spain, eds, F. Prada,D. Martinez-Delgado, T. Mahoney

We report on the recovery of a Blue Horizontal Branch (BHB) population belonging to the Sagittarius Dwarf Spheroidal Galaxy (Sgr). The sequence is clearly identified in the (V, V-I) Color Magnitude Diagram (CMD) obtained for about 500,000 stars in the region of the globular cluster M~54. The BHB morphology is similar to the analogous sequence in M~54, but it is unambiguously associated with Sgr since {\it(i)} it is detected well outside the main body of the cluster, up to more than 5 tidal radii from the cluster center and {\it(ii)} the BHB stars follow the radial distribution of the other stellar populations of Sgr. This finding finally demonstrates that the Sgr galaxy hosts a significant (of the order of $\sim$10%) old and metal-poor stellar population ([Fe/H]\ltsima -1.3; age \gtsima 10 Gyr), similar to that of its oldest clusters (M~54, Ter~8). We also show that the Sgr BHB sequence found here is the counterpart of the analogous feature observed by Newberg et al. (2002) in the Sgr Stream, in a field more than 80$\degr$ away from the center of the galaxy., Comment: 12 pages, 4 figures. Accepted for publication in ApJ Letters

In this contribution I am going to present some preliminary results of a high-resolution spectroscopic campaign focussed on the most metal rich red giant stars in Omega Cen. This study is part of a long term project we started a few years ago, which is aimed at studying the properties of the different stellar populations in Omega Cen. The final goal of the whole project is the global understanding of both the star formation and the chemical evolution history of this complex stellar system., Comment: 12 pages, 5 figures, to appear in "Omega Centauri: a unique window into astrophysics", proceedings of the Workshop held in Cambridge in August 2001

We have obtained an accurate estimate of the absolute I magnitude of the Tip of the Red Giant Branch (M_I(TRGB)) for the globular cluster Omega Cen, based {\em (a)} on the largest photometric database ever assembled for a globular, by Pancino et al. (2000), and {\em (b)} on a direct distance estimate for \omen, recently obtained by Thompson et al.(2001) from a detached eclipsing binary. The derived value M_I(TRGB) = -4.04\pm 0.12 provides, at present, the most accurate empirical zero-point for the calibration of the \mtip -- [Fe/H] relation, at [Fe/H] ~ -1.7., Comment: 6 pages, 2 .ps figures, latex, hrd_pasp.sty macro included. To appear in "Observed HR diagrams and stellar evolution: the interplay between observational constraints and theory", (Coimbra, June, 2001) ASP Conf. Ser., 2001, T. Lejeune and J. Fernandes, eds