Examinando por Autor "Villanova S."
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Ítem Aluminium-enriched metal-poor stars buried in the inner Galaxy(EDP Sciences, 2020-11) Fernández-Trincado J.G.; Beers T.C.; Minniti D.; Tang B.; Villanova S.; Geisler D.; Pérez-Villegas A.; Vieira K.Stars with higher levels of aluminium and nitrogen enrichment are often key pieces in the chemical makeup of multiple populations in almost all globular clusters (GCs). There is also compelling observational evidence that some Galactic components could be partially built from dissipated GCs. The identification of such stars among metal-poor field stars may therefore provide insight into the composite nature of the Milky Way (MW) bulge and inner stellar halo, and could also reveal other chemical peculiarities. Here, based on APOGEE spectra, we report the discovery of 29 mildly metal-poor ([Fe/H] -0.7) stars with stellar atmospheres strongly enriched in aluminium (Al-rich stars: [Al/Fe] +0.5), well above the typical Galactic levels, located within the solar radius toward the bulge region, which lies in highly eccentric orbits (e 0.6). We find many similarities for almost all of the chemical species measured in this work with the chemical patterns of GCs, and therefore we propose that they have likely been dynamically ejected into the bulge and inner halo from GCs formed in situ and/or GCs formed in different progenitors of known merger events experienced by the MW, such as the Gaia-Sausage-Enceladus and/or Sequoia. © 2020 ESO.Ítem Atypical Mg-poor Milky Way Field Stars with Globular Cluster Second-generation-like Chemical Patterns(Institute of Physics Publishing, 2017-09) Fernández-Trincado J.G.; Zamora O.; Garcia-Hernández D.A.; Souto, Diogo; Dell'Agli F.; Schiavon R.P.; Geisler D.; Tang B.; Villanova S.; Hasselquist, Sten; Mennickent R.E.; Cunha, Katia; Shetrone M.; Prieto, Carlos Allende; Vieira K.; Zasowski G.; Sobeck J.; Hayes C.R.; Majewski S.R.; Placco V.M.; Beers T.C.; Schleicher D.R.G.; Robin A.C.; Mészáros, Sz.; Masseron T.; Pérez, Ana E. Garcia; Anders F.; Meza A.; Alves-Brito A.; Carrera R.; Minniti D.; Lane R.R.; Fernández-Alvar E.; Moreno E.; Pichardo B.; Pérez-Villegas A.; Schultheis M.; Roman-Lopes A.; Fuentes C.E.; Nitschelm C.; Harding P.; Bizyaev D.; Pan K.; Oravetz D.; Simmons A.; Ivans, Inese; Blanco-Cuaresma S.; Hernández J.; Alonso-Garcia J.; Valenzuela O.; Chanamé J.We report the peculiar chemical abundance patterns of 11 atypical Milky Way (MW) field red giant stars observed by the Apache Point Observatory Galactic Evolution Experiment (APOGEE). These atypical giants exhibit strong Al and N enhancements accompanied by C and Mg depletions, strikingly similar to those observed in the so-called second-generation (SG) stars of globular clusters (GCs). Remarkably, we find low Mg abundances ([Mg/Fe] < 0.0) together with strong Al and N overabundances in the majority (5/7) of the metal-rich ([Fe/H] -1.0) sample stars, which is at odds with actual observations of SG stars in Galactic GCs of similar metallicities. This chemical pattern is unique and unprecedented among MW stars, posing urgent questions about its origin. These atypical stars could be former SG stars of dissolved GCs formed with intrinsically lower abundances of Mg and enriched Al (subsequently self-polluted by massive AGB stars) or the result of exotic binary systems. We speculate that the stars Mg-deficiency as well as the orbital properties suggest that they could have an extragalactic origin. This discovery should guide future dedicated spectroscopic searches of atypical stellar chemical patterns in our Galaxy, a fundamental step forward to understanding the Galactic formation and evolution. © 2017. The American Astronomical Society. All rights reserved.Ítem Baade's window and APOGEE: Metallicities, ages, and chemical abundances(2017-04) Schultheis M.; Rojas-Arriagada A.; García Pérez A.E.; Jönsson H.; Hayden M.; Nandakumar G.; Cunha K.; Allende Prieto C.; Holtzman J.A.; Beers T.C.; Bizyaev D.; Brinkmann J.; Carrera R.; Cohen R.E.; Geisler D.; Hearty F.R.; Fernandez-Tricado J.G.; Maraston C.; Minnitti D.; Nitschelm C.; Roman-Lopes A.; Schneider D.P.; Tang B.; Villanova S.; Zasowski G.; Majewski S.R.Context. Baade's window (BW) is one of the most observed Galactic bulge fields in terms of chemical abundances. Owing to its low and homogeneous interstellar absorption it is considered the perfect calibration field for Galactic bulge studies. Aims. In the era of large spectroscopic surveys, calibration fields such as BW are necessary for cross calibrating the stellar parameters and individual abundances of the APOGEE survey. Methods. We use the APOGEE BW stars to derive the metallicity distribution function (MDF) and individual abundances for α-and iron-peak elements of the APOGEE ASPCAP pipeline (DR13), as well as the age distribution for stars in BW. Results. We determine the MDF of APOGEE stars in BW and find a remarkable agreement with that of the Gaia-ESO survey (GES). Both exhibit a clear bimodal distribution. We also find that the Mg-metallicity planes of the two surveys agree well, except for the metal-rich part ([Fe/H] > 0.1), where APOGEE finds systematically higher Mg abundances with respect to the GES. The ages based on the [C/N] ratio reveal a bimodal age distribution, with a major old population at ~ 10 Gyr, with a decreasing tail towards younger stars. A comparison of stellar parameters determined by APOGEE and those determined by other sources reveals detectable systematic offsets, in particular for spectroscopic surface gravity estimates. In general, we find a good agreement between individual abundances of O, Na, Mg, Al, Si, K, Ca, Cr, Mn, Co, and Ni from APOGEE with that of literature values. Conclusions. We have shown that in general APOGEE data show a good agreement in terms of MDF and individual chemical abundances with respect to literature works. Using the [C/N] ratio we found a significant fraction of young stars in BW. © ESO, 2017.Ítem Discovery of a Large Population of Nitrogen-enhanced Stars in the Magellanic Clouds(IOP Publishing Ltd, 2020-11) Fernández-Trincado J.G.; Beers T.C.; Minniti D.; Carigi L.; Barbuy B.; Placco V.M.; Bidin C.M.; Villanova S.; Roman-Lopes A.; Nitschelm C.We report the APOGEE-2S+ discovery of a unique collection of nitrogen-enhanced mildly metal-poor giant stars, peaking at [Fe/H] ∼ −0.89 with no carbon enrichment, toward the Small and Large Magellanic Clouds (SMC and LMC), with abundances of light- (C, N), odd-Z (Al, K), and α-elements (O, Mg, Si) that are typically found in Galactic globular clusters (GCs). Here we present 44 stars in the SMC and LMC that exhibit significantly enhanced [N/Fe] abundance ratios, well above ([N/Fe] > +0.6) typical Galactic levels at similar metallicity, and a star that is very nitrogen-enhanced ([N/Fe] > +2.45). Our sample consists of luminous evolved stars on the asymptotic giant branch (AGB), eight of which are classified as bona fide semi-regular (SR) variables, as well as low-luminosity stars similar to those of stars on the tip of the red giant branch of stellar clusters in the SMC and LMC. It seems likely that whatever nucleosynthetic process is responsible for these anomalous SMC and LMC stars it is similar to that which caused the common stellar populations in GCs. We interpret these distinctive C–N patterns as observational evidence of the result of tidally shredded GCs in the SMC and LMC. These findings might explain some previous conflicting results over bulge N-rich stars, and broadly help to understand GC formation and evolution. Furthermore, the discovery of such a large population of N-rich AGB stars in the SMC and LMC suggests that multiple stellar populations might not only be exotic events from the past, but can also form at lower redshift. © 2020. The Author(s). Published by the American Astronomical Society.Ítem The bulge globular cluster Terzan 6 as seen from multi-conjugate adaptive optics and HST(EDP Sciences, 2025-03) Loriga M.; Pallanca C.; Ferraro F.R.; Dalessandro E.; Lanzoni B.; Cadelano M.; Origlia L.; Fanelli C.; Geisler D.; Villanova S.This work consists of the first detailed photometric study of Terzan 6, one of the least known globular clusters in the Galactic bulge. Through the analysis of high angular resolution and multiwavelength data obtained from adaptive optics corrected and space observations, we built deep, optical and near-infrared color-magnitude diagrams reaching ≈4 magnitudes below the main-sequence turnoff. Taking advantage of four different epochs of observations, we measured precise relative proper motions for a large sample of stars, from which cluster members have been solidly distinguished from Galactic field interlopers. A noncanonical reddening law (with RV = 2.85) and high-resolution differential reddening map, with color excess variations up to δE(B-V) ≈0.8 mag, have been derived in the direction of the system. According to these findings, new values for the extinction and distance modulus have been obtained: E(B-V) = 2.36 ± 0.05 and (m-M)0 = 14.46 ± 0.10 (corresponding to d = 7.8 ± 0.3 kpc), respectively. We also provide the first determinations of the cluster center and projected density profile from resolved star counts. The center is offset by more than 7″ to the east from the literature value, and the structural parameters obtained from the King model fitting to the density profile indicate that Terzan 6 is in an advanced stage of its dynamical evolution, with a large value for the concentration parameter (c = 1.94-0.26+0.24) and a small core radius (rc = 2.6-0.7+0.09 arcsec). We also determined the absolute age of the system, finding t = 13 ± 1 Gyr, in agreement with the old ages found for the globular clusters in the Galactic bulge. From the redetermination of the absolute magnitude of the red giant branch bump and the recent estimate of the cluster global metallicity, we find that Terzan 6 nicely matches the tight relation between these two parameters drawn by the Galactic globular cluster population. © 2025 The Authors.Ítem The Gaia -ESO survey: The inner disk intermediate-age open cluster NGC 6802(EDP Sciences, 2017-05) Tang B.; Geisler D.; Friel E.; Villanova S.; Smiljanic R.; Casey A.R.; Randich S.; Magrini L.; San Roman I.; Muñoz C.; Cohen R.E.; Mauro F.; Bragaglia A.; Donati P.; Tautvaišiene G.; Drazdauskas A.; Ženoviene R.; Snaith O.; Sousa S.; Adibekyan V.; Costado M.T.; Blanco-Cuaresma S.; Jiménez-Esteban F.; Carraro G.; Zwitter T.; François P.; Jofrè P.; Sordo R.; Gilmore G.; Flaccomio E.; Koposov S.; Korn A.J.; Lanzafame A.C.; Pancino E.; Bayo A.; Damiani F.; Franciosini E.; Hourihane A.; Lardo C.; Lewis J.; Monaco L.; Morbidelli L.; Prisinzano L.; Sacco G.; Worley C.C.; Zaggia S.Milky Way open clusters are very diverse in terms of age, chemical composition, and kinematic properties. Intermediate-age and old open clusters are less common, and it is even harder to find them inside the solar Galactocentric radius, due to the high mortality rate and strong extinction inside this region. NGC 6802 is one of the inner disk open clusters (IOCs) observed by the Gaia-ESO survey (GES). This cluster is an important target for calibrating the abundances derived in the survey due to the kinematic and chemical homogeneity of the members in open clusters. Using the measurements from Gaia-ESO internal data release 4 (iDR4), we identify 95 main-sequence dwarfs as cluster members from the GIRAFFE target list, and eight giants as cluster members from the UVES target list. The dwarf cluster members have a median radial velocity of 13.6 ± 1.9 km s-1, while the giant cluster members have a median radial velocity of 12.0 ± 0.9 km s-1 and a median [Fe/H] of 0.10 ± 0.02 dex. The color-magnitude diagram of these cluster members suggests an age of 0.9 ± 0.1 Gyr, with (m-M)0 = 11.4 and E(B-V) = 0.86. We perform the first detailed chemical abundance analysis of NGC 6802, including 27 elemental species. To gain a more general picture about IOCs, the measurements of NGC 6802 are compared with those of other IOCs previously studied by GES, that is, NGC 4815, Trumpler 20, NGC 6705, and Berkeley 81. NGC 6802 shows similar C, N, Na, and Al abundances as other IOCs. These elements are compared with nucleosynthetic models as a function of cluster turn-off mass. The α, iron-peak, and neutron-capture elements are also explored in a self-consistent way. © ESO, 2017.