Examinando por Autor "Roman-Lopes A."
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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 Rubin Observatory LSST Stars Milky Way and Local Volume Star Clusters Roadmap(Institute of Physics, 2023-07) Usher, Christopher; Dage, Kristen C.; Girardi, Léo; Barmby, Pauline; Bonatto, Charles J.; Chies-Santos, Ana L.; Clarkson, William I.; Camus, Matias Gómez; Hartmann, Eduardo A.; Ferguson, Annette M. N.; Pieres, Adriano; Prisinzano, Loredana; Rhode, Katherine L.; Rich, R. Michael; Ripepi, Vincenzo; Santiago, Basilio; Stassun, Keivan G.; Street R.A.; Szabó, Róbert; Venuti, Laura; Zaggia, Simone; Canossa, Marco; Floriano, Pedro; Lopes, Pedro; Miranda, Nicole L.; Oliveira, Raphael A. P.; Reina-Campos, Marta; Roman-Lopes A.; Sobeck, JenniferThe Vera C. Rubin Observatory will undertake the Legacy Survey of Space and Time, providing an unprecedented, volume-limited catalog of star clusters in the Southern Sky, including Galactic and extragalactic star clusters. The Star Clusters subgroup of the Stars, Milky Way and Local Volume Working Group has identified key areas where Rubin Observatory will enable significant progress in star cluster research. This roadmap represents our science cases and preparation for studies of all kinds of star clusters from the Milky Way out to distances of tens of megaparsecs. © 2023. The Author(s). Published by IOP Publishing Ltd on behalf of the Astronomical Society of the Pacific (ASP). All rights reserved.Ítem The Milky Way bar and bulge revealed by APOGEE and Gaia EDR3(EDP Sciences, 2021-12-01) Queiroz A.B.A.; Chiappini C.; Perez-Villegas A.; Khalatyan A.; Anders F.; Barbuy B.; Santiago B.X.; Steinmetz M.; Cunha K.; Schultheis M.; Majewski S.R.; Minchev I.; Minniti D.; Beaton R.L.; Cohen R.E.; Da Costa L.N.; Fernández-Trincado J.G.; Garcia-Hernández D.A.; Geisler D.; Hasselquist S.; Lane R.R.; Nitschelm C.; Rojas-Arriagada A.; Roman-Lopes A.; Smith V.; Zasowski G.We investigate the inner regions of the Milky Way using data from APOGEE and Gaia EDR3. Our inner Galactic sample has more than 26 500 stars within |XGal|< 5 kpc, |YGal|< 3.5 kpc, |ZGal|< 1 kpc, and we also carry out the analysis for a foreground-cleaned subsample of 8000 stars that is more representative of the bulge-bar populations. These samples allow us to build chemo-dynamical maps of the stellar populations with vastly improved detail. The inner Galaxy shows an apparent chemical bimodality in key abundance ratios [α/Fe], [C/N], and [Mn/O], which probe different enrichment timescales, suggesting a star formation gap (quenching) between the high- and low-α populations. Using a joint analysis of the distributions of kinematics, metallicities, mean orbital radius, and chemical abundances, we can characterize the different populations coexisting in the innermost regions of the Galaxy for the first time. The chemo-kinematic data dissected on an eccentricity-|Z|max plane reveal the chemical and kinematic signatures of the bar, the thin inner disc, and an inner thick disc, and a broad metallicity population with large velocity dispersion indicative of a pressure-supported component. The interplay between these different populations is mapped onto the different metallicity distributions seen in the eccentricity-|Z|max diagram consistently with the mean orbital radius and Vφ distributions. A clear metallicity gradient as a function of |Z|max is also found, which is consistent with the spatial overlapping of different populations. Additionally, we find and chemically and kinematically characterize a group of counter-rotating stars that could be the result of a gas-rich merger event or just the result of clumpy star formation during the earliest phases of the early disc that migrated into the bulge. Finally, based on 6D information, we assign stars a probability value of being on a bar orbit and find that most of the stars with large bar orbit probabilities come from the innermost 3 kpc, with a broad dispersion of metallicity. Even stars with a high probability of belonging to the bar show chemical bimodality in the [α/Fe] versus [Fe/H] diagram. This suggests bar trapping to be an efficient mechanism, explaining why stars on bar orbits do not show a significant, distinct chemical abundance ratio signature.