Examinando por Autor "Beers, Timothy C."
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Ítem APOGEE Chemical Abundances of the Sagittarius Dwarf Galaxy(Institute of Physics Publishing, 2017-09) Hasselquist, Sten; Shetrone, Matthew; Smith, Verne; Holtzman, Jon; McWilliam, Andrew; Fernández-Trincado J.G.; Beers, Timothy C.; Majewski, Steven R.; Nidever, David L.; Tang, Baitian; Tissera, Patricia B.; Alvar, Emma Fernández; Allende Prieto, Carlos; Almeida, Andres; Anguiano, Borja; Battaglia, Giuseppina; Carigi, Leticia; Delgado Inglada, Gloria; Frinchaboy, Peter; Garcia-Hernández D.A.; Geisler, Doug; Minniti, Dante; Placco, Vinicius M.; Schultheis, Mathias; Sobeck, Jennifer; Villanova, SandroThe Apache Point Observatory Galactic Evolution Experiment provides the opportunity of measuring elemental abundances for C, N, O, Na, Mg, Al, Si, P, K, Ca, V, Cr, Mn, Fe, Co, and Ni in vast numbers of stars. We analyze thechemical-abundance patterns of these elements for 158 red giant stars belonging to the Sagittarius dwarf galaxy (Sgr). This is the largest sample of Sgr stars with detailed chemical abundances, and it is the first time that C, N, P, K, V, Cr, Co, and Ni have been studied at high resolution in this galaxy. We find that the Sgr stars with [Fe/H] ≈ -0.8 are deficient in all elemental abundance ratios (expressed as [X/Fe]) relative to the Milky Way, suggesting that the Sgr stars observed today were formed from gas that was less enriched by Type II SNe than stars formed in the Milky Way. By examining the relative deficiencies of the hydrostatic (O, Na, Mg, and Al) and explosive (Si, P, K, and Mn) elements, our analysis supports the argument that previous generations of Sgr stars were formed with a top-light initial mass function, one lacking the most massive stars that would normally pollute the interstellar medium with the hydrostatic elements. We use a simple chemical-evolution model, flexCE, to further support our claim and conclude that recent stellar generations of Fornax and the Large Magellanic Cloud could also have formed according to a top-light initial mass function. © 2017. The American Astronomical Society. All rights reserved..Ítem APOGEE-2 Discovery of a Large Population of Relatively High-metallicity Globular Cluster Debris(American Astronomical Society, 2021-09-10) Fernández-Trincado, José G.; Beers, Timothy C.; Queiroz, Anna. B. A.; Chiappini, Cristina; Minniti, Dante; Barbuy, Beatriz; Majewski, Steven R.; Ortigoza-Urdaneta, Mario; Moni Bidin, Christian; Robin, Annie C.; Moreno, Edmundo; Chaves-Velasquez, Leonardo; Villanova, Sandro; Lane, Richard R.; Pan, Kaike; Bizyaev, DmitryWe report the discovery of a new, chemically distinct population of relatively high-metallicity ([Fe/H] > -0.7) red giant stars with super-solar [N/Fe] (⪆+0.75) identified within the bulge, disk, and halo of the Milky Way. This sample of stars was observed during the second phase of the Apache Point Observatory Galactic Evolution Experiment (APOGEE-2); the spectra of these stars are part of the seventeenth Data Release (DR 17) of the Sloan Digital Sky Survey. We hypothesize that this newly identified population was formed in a variety of progenitors, and is likely made up of either fully or partially destroyed metal-rich globular clusters, which we refer to as globular cluster debris (GCD), identified by their unusual photospheric nitrogen abundances. It is likely that some of the GCD stars were probable members of the Gaia-Enceladus-Sausage accretion event, along with clusters formed in situ. © 2021. The American Astronomical Society. All rights reserved.Ítem CAPOS: The bulge Cluster APOgee Survey II. The intriguing “Sequoia” globular cluster FSR 1758(EDP Sciences, 2021-08-01) Romero-Colmenares, María; Fernández-Trincado, José G.; Geisler, Doug; Souza, Stefano O.; Villanova, Sandro; Longa-Peña, Penélope; Minniti, Dante; Beers, Timothy C.; Bidin, Cristian Moni; Perez-Villegas, Angeles; Moreno, Edmundo; Garro, Elisa R.; Baeza, Ian; Henao, Lady; Barbuy, Beatriz; Alonso-García, Javier; Cohen, Roger E.; Lane, Richard R.; Muñoz, CesarWe present results from a study of 15 red giant members of the intermediate-metallicity globular cluster (GC) FSR 1758 using high-resolution, near-infrared spectra collected with the Apache Point Observatory Galactic Evolution Experiment II survey (APOGEE-2) that were obtained as part of CAPOS (the bulge Cluster APOgee Survey). Since its very recent discovery as a massive GC in the bulge region, evoking the name Sequoia, this has been an intriguing object with a highly debated origin, and initially led to the suggestion of a purported progenitor dwarf galaxy of the same name. In this work, we use new spectroscopic and astrometric data to provide additional clues as to the nature of FSR 1758. Our study confirms the GC nature of FSR 1758, and as such we report the existence of the characteristic N-C anticorrelation and Al-N correlation for the first time. We thereby reveal the existence of the multiple-population phenomenon, similar to that observed in virtually all GCs. Furthermore, the presence of a population with strongly enriched aluminum makes it unlikely that FSR 1758 is the remnant nucleus of a dwarf galaxy because Al-enhanced stars are uncommon in dwarf galaxies. We find that FSR 1758 is slightly more metal rich than previously reported in the literature; this source has a mean metallicity [Fe/H] between -1.43 to -1.36, depending on the adopted atmospheric parameters and a scatter within observational error, again pointing to its GC nature. Overall, the α-enrichment ( + 0.3 dex), Fe-peak (Fe, Ni), light (C, N), and odd-Z (Al) elements follow the trend of intermediate-metallicity GCs. Isochrone fitting in the Gaia bands yields an estimated age of ∼11.6 Gyr. We used the exquisite kinematic data, including our CAPOS radial velocities and Gaia eDR3 proper motions, to constrain the N-body density profile of FSR 1758, and found that it is as massive (∼2.9 ± 0.6 × 105 Mpdbl) as NGC 6752. We confirm a retrograde and eccentric orbit for FSR 1758. A new examination of its dynamical properties with the GravPot16 model favors an association with the Gaia-Enceladus-Sausage accretion event. Thus, paradoxically, the cluster that gave rise to the name of the Sequoia dwarf galaxy does not appear to belong to this specific merging event.Ítem Chronography of the milky way's halo system with field blue horizontal-branch stars(Institute of Physics Publishing, 2015-11) Santucci, Rafael M.; Beers, Timothy C.; Placco, Vinicius M.; Carollo, Daniela; Rossi, Silvia; Lee, Young Sun; Denissenkov, Pavel; Tumlinson, Jason; Tissera, Patricia B.In a pioneering effort, Preston et al. reported that the colors of blue horizontal-branch (BHB) stars in the halo of the Galaxy shift with distance, from regions near the Galactic center to about 12 kpc away, and interpreted this as a correlated variation in the ages of halo stars, from older to younger, spanning a range of a few Gyrs. We have applied this approach to a sample of some 4700 spectroscopically confirmed BHB stars selected from the Sloan Digital Sky Survey to produce the first "chronographic map" of the halo of the Galaxy. We demonstrate that the mean de-reddened g - r color, increases outward in the Galaxy from -0.22 to -0.08 (over a color window spanning [-0.3: 0.0]) from regions close to the Galactic center to ∼40 kpc, independent of the metallicity of the stars. Models of the expected shift in the color of the field BHB stars based on modern stellar evolutionary codes confirm that this color gradient can be associated with an age difference of roughly 2-2.5 Gyr, with the oldest stars concentrated in the central ∼15 kpc of the Galaxy. Within this central region, the age difference spans a mean color range of about 0.05 mag (∼0.8 Gyr). Furthermore, we show that chronographic maps can be used to identify individual substructures, such as the Sagittarius Stream, and overdensities in the direction of Virgo and Monoceros, based on the observed contrast in their mean BHB colors with respect to the foreground/background field population. © 2015. The American Astronomical Society. All rights reserved..Ítem Discovery of a nitrogen-enhanced mildly metal-poor binary system: Possible evidence for pollution from an extinct AGB star(Astronomy and Astrophysics, 2019) Fernández-Trincado, José G.; Mennickent, Ronald; Cabezas, Mauricio; Zamora, Olga; Martell, Sarah L.; Beers, Timothy C.; Placco, Vinicius M.; Nataf, David M.; Mészáros, Szabolcsk; Minniti, Dantel; Schleicher, Dominik R. G.; Tang, BaitianWe report the serendipitous discovery of a nitrogen-rich, mildly metal-poor ([Fe/H] = -1.08) giant star in a single-lined spectroscopic binary system found in the SDSS-IV Apache Point Observatory Galactic Evolution Experiment (APOGEE-2) survey, Data Release 14 (DR14). Previous work has assumed that two percent of halo giants with unusual elemental abundances have been evaporated from globular clusters, but other origins for their abundance signatures, including binary mass transfer, must also be explored. We present the results of an abundance reanalysis of the APOGEE-2 high-resolution near-infrared spectrum of 2M12451043+1217401 with the Brussels Automatic Stellar Parameter (BACCHUS) automated spectral analysis code. We manually re-derive the main element families, namely light elements (C, N), elements (O, Mg, Si), the iron-peak element (Fe), s-process element (Ce), and light odd-Z element (Al). Our analysis confirms the N-rich nature of 2M12451043+1217401, which has a [N/Fe] ratio of +0.69, and shows that the abundances of C and Al are slightly discrepant from those of a typical mildly metal-poor red giant branch star, but exhibit Mg, Si, O and s-process abundances (Ce) of typical field stars. We also detect a particularly large variability in the radial velocity of this star over the period of the APOGEE-2 observations; the most likely orbit fit to the radial velocity data has a period of 730.89 ±106.86 days, a velocity semi-amplitude of 9.92 ±0.14 km s-1, and an eccentricity of ∼0.1276 ±0.1174. These data support the hypothesis of a binary companion, which has probably been polluted by a now-extinct asymptotic giant branch star.Ítem Evidence for the Third Stellar Population in the Milky Way's Disk(Institute of Physics Publishing, 2019-12) Carollo, Daniela; Chiba, Masashi; Ishigaki, Miho; Freeman, Ken; Beers, Timothy C.; Lee, Young Sun; Tissera, Patricia; Battistini, Chiara; Primas, FrancescaThe Milky Way is a unique laboratory in which stellar properties can be measured and analyzed in detail. In particular, stars in the older populations encode information on the mechanisms that led to the formation of our Galaxy. In this article, we analyze the kinematics, spatial distribution, and chemistry of a large number of stars in the solar neighborhood, where all of the main Galactic components are well represented. We find that the thick disk comprises two distinct and overlapping stellar populations with different kinematic properties and chemical compositions. The metal-weak thick disk (MWTD) contains two-times less metal content than the canonical thick disk, and exhibits enrichment of light elements typical of the oldest stellar populations of the Galaxy. The rotational velocity of the MWTD around the Galactic center is ∼150 km s-1, corresponding to a rotational lag of 30 km s-1 relative to the canonical thick disk (∼180 km s-1), with a velocity dispersion of 60 km s-1. This stellar population likely originated from the merger of a dwarf galaxy during the early phases of our Galaxy's assembly, or it is a precursor disk, formed in the inner Galaxy and brought into the solar neighborhood by bar instability or spiral-arm formation mechanisms. © 2019. The American Astronomical Society. All rights reserved..Ítem Galactic ArchaeoLogIcaL ExcavatiOns (GALILEO): I. An updated census of APOGEE N-rich giants across the Milky Way(EDP Sciences, 2022-07-01) Fernández Trincado, José G.; Beers, Timothy C.; Barbuy, Beatriz; Minniti, Dante; Chiappini, Cristina; Garro, Elisa R.; Tang, Baitian; Alves Brito, Alan; Villanova, Sandro; Geisler, Doug; Lane, Richard R.; Diaz, Danilo G.We use the 17th data release of the second phase of the Apache Point Observatory Galactic Evolution Experiment (APOGEE-2) to provide a homogenous census of N-rich red giant stars across the Milky Way (MW). We report a total of 149 newly identified N-rich field giants toward the bulge, metal-poor disk, and halo of our Galaxy. They exhibit significant enrichment in their nitrogen abundance ratios ([N/Fe]³a+0.5), along with simultaneous depletions in their [C/Fe] abundance ratios ([C/Fe]< +0.15), and they cover a wide range of metallicities (1.8< [Fe/H]< 0.7). The final sample of candidate N-rich red giant stars with globular-cluster-like (GC-like) abundance patterns from the APOGEE survey includes a grand total of 412 unique objects. These strongly N-enhanced stars are speculated to have been stripped from GCs based on their chemical similarities with these systems. Even though we have not found any strong evidence for binary companions or signatures of pulsating variability yet, we cannot rule out the possibility that some of these objects were members of binary systems in the past and/or are currently part of a variable system. In particular, the fact that we identify such stars among the field stars in our Galaxy provides strong evidence that the nucleosynthetic process(es) producing the anomalous [N/Fe] abundance ratios occurs over a wide range of metallicities. This may provide evidence either for or against the uniqueness of the progenitor stars to GCs and/or the existence of chemical anomalies associated with likely tidally shredded clusters in massive dwarf galaxies such as Kraken/Koala, Gaia-Enceladus-Sausage, among others, before or during their accretion by the MW. A dynamical analysis reveals that the newly identified N-rich stars exhibit a wide range of dynamical characteristics throughout the MW, indicating that they were produced in a variety of Galactic environments. ©Ítem Homogeneous analysis of globular clusters from the APOGEE survey with the BACCHUS code – II. The Southern clusters and overview(Oxford University Press, 2020-02) Szabolcs, Meszaros; Masseron, Thomas; García-Hernandez, D. A.; Allende Prieto, Carlos; Beers, Timothy C.; Bizyaev, Dmitry; Chojnowski, Drew; Cohen, Roger E.; Cunha, Katia; Dell’Agli, Flavia; Ebelke, Garrett; Fernandez-Trincado, Jose G.; Frinchaboy, Peter; Geisler, Doug; Hasselquist, Sten; Hearty, Fred; Holtzman, Jon; Johnson, Jennifer; Lane, Richard R; Lacerna, Ivan; Longa-Pena, Penelope; Majewski, Steven R.; Martell, Sarah L; Minniti, Dante; Nataf, David; Nidever, David L.; Pan, Kaike; Schiavon, Ricardo P.; Shetrone, Matthew; Smith, Verne V.; . Sobeck, Jennifer S; Stringfellow, Guy S.; Szigeti, Laszlo; Tang, Baitian; Wilson, John C.; Zamora, OlgaWe investigate the Fe, C, N, O, Mg, Al, Si, K, Ca, Ce, and Nd abundances of 2283 red giant stars in 31 globular clusters from high-resolution spectra observed in both the Northern and Southern hemisphere by the SDSS-IV APOGEE-2 survey. This unprecedented homogeneous data set, largest to date, allows us to discuss the intrinsic Fe spread, the shape, and statistics of Al-Mg and N-C anti-correlations as a function of cluster mass, luminosity, age, and metallicity for all 31 clusters. We find that the Fe spread does not depend on these parameters within our uncertainties including cluster metallicity, contradicting earlier observations. We do not confirm the metallicity variations previously observed in M22 and NGC 1851. Some clusters show a bimodal Al distribution, while others exhibit a continuous distribution as has been previously reported in the literature. We confirm more than two populations in ω Cen and NGC 6752, and find new ones in M79. We discuss the scatter of Al by implementing a correction to the standard chemical evolution of Al in the Milky Way. After correction, its dependence on cluster mass is increased suggesting that the extent of Al enrichment as a function of mass was suppressed before the correction. We observe a turnover in the Mg-Al anticorrelation at very low Mg in ω Cen, similar to the pattern previously reported in M15 and M92. ω Cen may also have a weak K-Mg anticorrelation, and if confirmed, it would be only the third cluster known to show such a patternÍtem Is Terzan 5 the remnant of a building block of the Galactic bulge? Evidence from APOGEE(Oxford University Press, 2022-07-01) Taylor, Dominic J.; Mason, Andrew C.; Schiavon, Ricardo P.; Horta, Danny; Nataf, David M.; Geisler, Doug; Kisku, Shobhit; Phillips, Siân G.; Cohen, Roger E.; Fernández Trincado, José G.; Beers, Timothy C.; Bizyaev, Dmitry; García Hernández, Domingo Aníbal; Lane, Richard R.; Longa Peña, Penélope; Minniti, Dante; Muñoz, Cesar; Pan, Kaike; Villanova, SandroIt has been proposed that the globular cluster-like system Terzan 5 is the surviving remnant of a primordial building block of the Milky Way bulge, mainly due to the age/metallicity spread and the distribution of its stars in the α-Fe plane. We employ Sloan Digital Sky Survey data from the Apache Point Observatory Galactic Evolution Experiment to test this hypothesis. Adopting a random sampling technique, we contrast the abundances of 10 elements in Terzan 5 stars with those of their bulge field counterparts with comparable atmospheric parameters, finding that they differ at statistically significant levels. Abundances between the two groups differ by more than 1σ in Ca, Mn, C, O, and Al, and more than 2σ in Si and Mg. Terzan 5 stars have lower [α/Fe] and higher [Mn/Fe] than their bulge counterparts. Given those differences, we conclude that Terzan 5 is not the remnant of a major building block of the bulge. We also estimate the stellar mass of the Terzan 5 progenitor based on predictions by the Evolution and Assembly of GaLaxies and their Environments suite of cosmological numerical simulations, concluding that it may have been as low as ∼3 × 108 M⊙ so that it was likely unable to significantly influence the mean chemistry of the bulge/inner disc, which is significantly more massive (∼1010 M⊙). We briefly discuss existing scenarios for the nature of Terzan 5 and propose an observational test that may help elucidate its origin. © 2022 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society.Ítem The enigmatic globular cluster UKS 1 obscured by the bulge: H -band discovery of nitrogen-enhanced stars(EDP Sciences, 2020-11-01) Fernández-Trincado, José G.; Minniti, Dante; Beers, Timothy C.; Villanova, Sandro; Geisler, Doug; Souza, Stefano O.; Smith, Leigh C.; Placco, Vinicius M.; Vieira, Katherine; Pérez-Villegas, Angeles; Barbuy, Beatriz; Alves-Brito, Alan; Moni Bidin, Christian; Alonso-García, Javier; Tang, Baitian; Palma, TaliThe presence of nitrogen-enriched stars in globular clusters provides key evidence for multiple stellar populations (MPs), as has been demonstrated with globular cluster spectroscopic data towards the bulge, disk, and halo. In this work, we employ the VVV Infrared Astrometric Catalogue (VIRAC) and the DR16 SDSS-IV release of the APOGEE survey to provide the first detailed spectroscopic study of the bulge globular cluster UKS 1. Based on these data, a sample of six selected cluster members was studied. We find the mean metallicity of UKS 1 to be [Fe/H] = -0.98 ± 0.11, considerably more metal-poor than previously reported, and a negligible metallicity scatter, typical of that observed by APOGEE in other Galactic globular clusters. In addition, we find a mean radial velocity of 66.1 ± 12.9 km s-1, which is in good agreement with literature values, within 1σ. By selecting stars in the VIRAC catalogue towards UKS 1, we also measure a mean proper motion of (μαcos(δ), μδ) = (-2.77 ± 0.23, -2.43 ± 0.16) mas yr-1. We find strong evidence for the presence of MPs in UKS 1, since four out of the six giants analysed in this work have strong enrichment in nitrogen ([N/Fe] - +0.95) accompanied by lower carbon abundances ([C/Fe] -0.2). Overall, the light- (C, N), α- (O, Mg, Si, Ca, Ti), Fe-peak (Fe, Ni), Odd-Z (Al, K), and the s-process (Ce, Nd, Yb) elemental abundances of our member candidates are consistent with those observed in globular clusters at similar metallicity. Furthermore, the overall star-to-star abundance scatter of elements exhibiting the multiple-population phenomenon in UKS 1 is typical of that found in other global clusters (GCs), and larger than the typical errors of some [X/Fe] abundances. Results from statistical isochrone fits in the VVV colour-magnitude diagrams indicate an age of 13.10-1.29+0.93 Gyr, suggesting that UKS 1 is a fossil relic in the Galactic bulge.Ítem The Origin of the Milky Way's Halo Age Distribution(Institute of Physics Publishing, 2018-05) Carollo, Daniela; Tissera, Patricia B.; Beers, Timothy C.; Gudin, Dmitrii; Gibson, Brad K.; Freeman, Ken C.; Monachesi, AntonelaWe present an analysis of the radial age gradients for the stellar halos of five Milky Way (MW) mass-sized systems simulated as part of the Aquarius Project. The halos show a diversity of age trends, reflecting their different assembly histories. Four of the simulated halos possess clear negative age gradients, ranging from approximately -7 to -19 Myr kpc-1, shallower than those determined by recent observational studies of the Milky Way's stellar halo. However, when restricting the analysis to the accreted component alone, all of the stellar halos exhibit a steeper negative age gradient with values ranging from -8 to -32 Myr kpc-1, closer to those observed in the Galaxy. Two of the accretion-dominated simulated halos show a large concentration of old stars in the center, in agreement with the Ancient Chronographic Sphere reported observationally. The stellar halo that best reproduces the current observed characteristics of the age distributions of the Galaxy is that formed principally by the accretion of small satellite galaxies. Our findings suggest that the hierarchical clustering scenario can reproduce the MW's halo age distribution if the stellar halo was assembled from accretion and the disruption of satellite galaxies with dynamical masses less than ∼109.5 M, and a minimal in situ contribution. © 2018. The American Astronomical Society..