Examinando por Autor "Nidever, David L."
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Ítem A DECam view of the diffuse dwarf galaxy Crater II – Variable stars(Oxford University Press, 2020-02) Vivas, A. Katherina; Walker, Alistair R.; Martínez-Vazquez, Clara E.; Monelli, Matteo; Bono, Giuseppe; Dorta, Antonio; Nidever, David L.; Fiorentino, Giuliana; Gallart, Carme; Andreuzzi, Gloria; Braga, Vittorio F.; Dall’Ora, Massimo; Olsen, Knut; Stetson, Peter B.Time series observations of a single dithered field centred on the diffuse dwarf satellite galaxy Crater II were obtained with the Dark Energy Camera (DECam) at the 4m Blanco Telescope at Cerro Tololo Inter-American Observatory, Chile, uniformly covering up to two half-light radii. Analysis of the g and i time series results in the identification and characterization of 130 periodic variable stars, including 98 RR Lyrae stars, 7 anomalous Cepheids, and 1 SX Phoenicis star belonging to the Crater II population, and 24 foreground variables of different types. Using the large number of ab-type RR Lyrae stars present in the galaxy, we obtained a distance modulus to Crater II of (m - M)0 = 20.333 ± 0.004 (stat) ± 0.07 (sys). The distribution of the RR Lyrae stars suggests an elliptical shape for Crater II, with an ellipticity of 0.24 and a position angle of 153°. From the RR Lyrae stars, we infer a small metallicity dispersion for the old population of Crater II of only 0.17 dex. There are hints that the most metal-poor stars in that narrow distribution have a wider distribution across the galaxy, while the slightly more metal-rich part of the population is more centrally concentrated. Given the features in the colour-magnitude diagram of Crater II, the anomalous Cepheids in this galaxy must have formed through a binary evolution channel of an old population. © 2019 The Author(s)Ítem APOGEE chemical abundance patterns of the massive milky way satellites(IOP Publishing Ltd, 2021-12) Hasselquist, Sten; Hayes, Christian R; Lian, Jianhui; Weinberg, David H.; Zasowski, Gail; Horta, Danny; Beaton, Rachael; Feuillet, Diane K.; Garro, Elisa R.; Gallart, Carme; Smith, Verne V.; Holtzman, Jon A.; Minniti, Dante; Lacerna, Ivan; Shetrone, Matthew; Jönsson, Henrik; Cioni, Maria-Rosa L.; Fillingham, Sean P.; Cunha, Katia; O'Connell, Robert; Fernández-Trincado, José G.; Munoz, Ricardo R.; Schiavon, Ricardo; Almeida, Andres; Anguiano, Borja; Beers, Timothy C.; Bizyaev, Dmitry; Brownstein, Joel R.; Cohen, Roger E.; Frinchaboy, Peter; García-Hernández, D.A.; Geisler, Doug; Lane, Richard R.; Majewski, Steven R; Nidever, David L.; Nitschelm, Christian; Povick, Joshua; Price-Whelan, Adrian; Roman-Lopes, Alexandre; Rosado, Margarita; Sobeck, Jennifer; Stringfellow, Guy; Valenzuela, Octavio; Villanova, Sandro; Vincenzo, FiorenzoThe SDSS-IV Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey has obtained high resolution spectra for thousands of red giant stars distributed among the massive satellite galaxies of the Milky Way (MW): the Large and Small Magellanic Clouds (LMC/SMC), the Sagittarius Dwarf Galaxy (Sgr), Fornax (Fnx), and the now fully disrupted Gaia Sausage/Enceladus (GSE) system. We present and analyze the APOGEE chemical abundance patterns of each galaxy to draw robust conclusions about their star formation histories, by quantifying the relative abundance trends of multiple elements (C, N, O, Mg, Al, Si, Ca, Fe, Ni, and Ce), as well as by fitting chemical evolution models to the [α/Fe]–[Fe/H] abundance plane for each galaxy. Results show that the chemical signatures of the starburst in the Magellanic Clouds (MCs) observed by Nidever et al. in the α-element abundances extend to C+N, Al, and Ni, with the major burst in the SMC occurring some 3–4 Gyr before the burst in the LMC. We find that Sgr and Fnx also exhibit chemical abundance patterns suggestive of secondary star formation epochs, but these events were weaker and earlier (∼5–7 Gyr ago) than those observed in the MCs. There is no chemical evidence of a second starburst in GSE, but this galaxy shows the strongest initial star formation as compared to the other four galaxies. All dwarf galaxies had greater relative contributions of AGB stars to their enrichment than the MW. Comparing and contrasting these chemical patterns highlight the importance of galaxy environment on its chemical evolution.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 Final Targeting Strategy for the SDSS-IV APOGEE-2S Survey(American Astronomical Society, 2021-12-01) Santana, Felipe A.; Beaton, Rachael L.; Covey, Kevin R.; O'Connell, Julia E.; Longa-Peña, Penélope; Cohen, Roger; Fernández-Trincado, José G.; Hayes, Christian R.; Zasowski, Gail; Sobeck, Jennifer S.; Majewski, Steven R.; Chojnowski, S. D.; De Lee, Nathan; Oelkers, Ryan J.; Stringfellow, Guy S.; Almeida, Andrés; Anguiano, Borja; Donor, John; Frinchaboy, Peter M.; Hasselquist, Sten; Johnson, Jennifer A.; Kollmeier, Juna A.; Nidever, David L.; Price-Whelan, Adrian M.; Rojas-Arriagada, Álvaro; Schultheis, Mathias; Shetrone, Matthew; Simon, Joshua D.; Aerts, Conny; Borissova, Jura; Drout, María R.; Geisler, Doug; Law, C. Y.; Medina, Nicolas; Minniti, Dante; Monachesi, Antonela; Muñoz, Ricardo R.; Poleski, Radosław; Roman-Lopes, Alexandre; Schlaufman, Kevin C.; Stutz, Amelia M.; Teske, Johanna; Tkachenko, Andrew; Van Saders, Jennifer L.; Weinberger, Alycia J.; Zoccali, ManuelaAPOGEE is a high-resolution (R ∼ 22,000), near-infrared, multi-epoch, spectroscopic survey of the Milky Way. The second generation of the APOGEE project, APOGEE-2, includes an expansion of the survey to the Southern Hemisphere called APOGEE-2S. This expansion enabled APOGEE to perform a fully panoramic mapping of all of the main regions of the Milky Way; in particular, by operating in the H band, APOGEE is uniquely able to probe the dust-hidden inner regions of the Milky Way that are best accessed from the Southern Hemisphere. In this paper we present the targeting strategy of APOGEE-2S, with special attention to documenting modifications to the original, previously published plan. The motivation for these changes is explained as well as an assessment of their effectiveness in achieving their intended scientific objective. In anticipation of this being the last paper detailing APOGEE targeting, we present an accounting of all such information complete through the end of the APOGEE-2S project; this includes several main survey programs dedicated to exploration of major stellar populations and regions of the Milky Way, as well as a full list of programs contributing to the APOGEE database through allocations of observing time by the Chilean National Time Allocation Committee and the Carnegie Institution for Science. This work was presented along with a companion article, Beaton et al. (2021), presenting the final target selection strategy adopted for APOGEE-2 in the Northern Hemisphere.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 pattern