Browsing by Author "Tang, Baitian"

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    APOGEE chemical abundances of globular cluster giants in the inner Galaxy
    (2017) Schiavon, Ricardo P.; Johnson, Jennifer A.; Frinchaboy, Peter M.; Zasowski, Gail; Meszaros, Szabolcs; Garcia-Hernandez, D. A.; Cohen, Roger E.; Tang, Baitian; Villanova, Sandro; Geisler, Douglas; Beers, Timothy C.; Fernandez-Trincado, J. G.; Garcia Perez, Ana E.; Lucatello, Sara; Majewski, Steven R.; Martell, Sarah L.; O'Connell, Robert W.; Allende Prieto, Carlos; Bizyaev, Dmitry; Carrera, Ricardo; Lane, Richard R.; Malanushenko, Elena; Malanushenko, Viktor; Munoz, Ricardo R.; Nitschelm, Christian; Oravetz, Daniel; Pan, Kaike; Roman-Lopes, Alexandre; Schultheis, Matthias; Simmons, Audrey
    We report chemical abundances obtained by Sloan Digital Sky Survey (SDSS)-III/Apache Point Observatory Galactic Evolution Experiment for giant stars in five globular clusters located within 2.2 kpc of the Galactic Centre. We detect the presence of multiple stellar populations in four of those clusters (NGC 6553, NGC 6528, Terzan 5 and Palomar 6) and find strong evidence for their presence in NGC 6522. All clusters with a large enough sample present a significant spread in the abundances of N, C, Na and Al, with the usual correlations and anticorrelations between various abundances seen in other globular clusters. Our results provide important quantitative constraints on theoretical models for self-enrichment of globular clusters, by testing their predictions for the dependence of yields of elements such as Na, N, C and Al on metallicity. They also confirm that, under the assumption that field N-rich stars originate from globular cluster destruction, they can be used as tracers of their parental systems in the high-metallicity regime.
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    Central velocity dispersion catalogue of LAMOST-DR7 galaxies
    (2020) Napolitano, Nicola R.; D'Ago, Giuseppe; Tortora, Crescenzo; Zhao, Gang; Luo, A-Li; Tang, Baitian; Zhang, Wei; Zhang, Yong; Li, Rui
    The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) is a major facility to carry out spectroscopic surveys for cosmology and galaxy evolution studies. The seventh data release of the LAMOST ExtraGAlactic Survey (LEGAS) is currently available and including redshifts of 193 361 galaxies. These sources are spread over similar to 11 500 deg(2) of the sky, largely overlapping with other imaging (SDSS and HSC) and spectroscopic (BOSS) surveys. The estimated depth of the galaxy sample, r similar to 17.8, the high signal-to-noise ratio, and the spectral resolution R = 1800, make the LAMOST spectra suitable for galaxy velocity dispersion (VD) measurements, which are invaluable to study the structure and formation of galaxies and to determine their central dark matter content. We present the first estimates of central VD of similar to 86 000 galaxies in LAMOST footprint. We have used a wrap-up procedure to perform the spectral fitting using PPXF, and derive VD measurements. Statistical errors are also assessed by comparing LAMOST VD estimates with the ones of SDSS and BOSS over a common sample of similar to 51 000 galaxies. The two data sets show a good agreement, within the statistical errors, in particular when VD values are corrected to 1 effective radius aperture. We also present a preliminary mass-sigma relation and find consistency with previous analyses based on local galaxy samples. These first results suggest that LAMOST spectra are suitable for galaxy VD measurements to complement the available catalogues of galaxy internal kinematics in the Northern hemisphere. We plan to expand this analysis to next LAMOST data releases.
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    Disentangling the Galactic Halo with APOGEE. I. Chemical and Kinematical Investigation of Distinct Metal-poor Populations
    (2018) Hayes, Christian R.; Majewski, Steven R.; Shetrone, Matthew; Fernandez-Alvar, Emma; Allende Prieto, Carlos; Schuster, William J.; Carigi, Leticia; Cunha, Katia; Smith, Verne V.; Sobeck, Jennifer; Almeida, Andres; Beers, Timothy C.; Carrera, Ricardo; Fernandez-Trincado, J. G.; Garcia-Hernandez, D. A.; Geisler, Doug; Lane, Richard R.; Lucatello, Sara; Matthews, Allison M.; Minniti, Dante; Nitschelm, Christian; Tang, Baitian; Tissera, Patricia B.; Zamora, Olga
    We find two chemically distinct populations separated relatively cleanly in the [Fe/H]-[Mg/Fe] plane, but also distinguished in other chemical planes, among metal-poor stars (primarily with metallicities [Fe/H] < -0.9) observed by the Apache Point Observatory Galactic Evolution Experiment (APOGEE) and analyzed for Data Release 13 (DR13) of the Sloan Digital Sky Survey. These two stellar populations show the most significant differences in their [X/Fe] ratios for the alpha-elements, C+N, Al, and Ni. In addition to these populations having differing chemistry, the low metallicity high-Mg population (which we denote "the HMg population") exhibits a significant net Galactic rotation, whereas the low-Mg population (or "the LMg population") has halo-like kinematics with little to no net rotation. Based on its properties, the origin of the LMg population is likely an accreted population of stars. The HMg population shows chemistry (and to an extent kinematics) similar to the thick disk, and is likely associated with in situ formation. The distinction between the LMg and HMg populations mimics the differences between the populations of low-and high-a halo stars found in previous studies, suggesting that these are samples of the same two populations.
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    Timing the Evolution of the Galactic Disk with NGC 6791: An Open Cluster with Peculiar High-alpha Chemistry as Seen by APOGEE
    (IOP PUBLISHING LTD, 2017) Linden, Sean T.; Pryal, Matthew; Hayes, Christian R.; Troup, Nicholas W.; Majewski, Steven R.; Andrews, Brett H.; Beers, Timothy C.; Carrera, Ricardo; Cunha, Katia; Fernandez Trincado, J. G.; Frinchaboy, Peter; Geisler, Doug; Lane, Richard R.; Nitschelm, Christian; Pan, Kaike; Allende Prieto, Carlos; Roman Lopes, Alexandre; Smith, Verne V.; Sobeck, Jennifer; Tang, Baitian; Villanova, Sandro; Zasowski, Gail
    We utilize elemental-abundance information for Galactic red giant stars in five open clusters (NGC 7789, NGC 6819, M67, NGC 188, and NGC 6791) from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) DR13 data set to age-date the chemical evolution of the high- and low-alpha element sequences of the Milky Way (MW). Key to this time-stamping is the cluster NGC 6791, whose stellar members have mean abundances that place it in the high-alpha, high-[Fe/H] region of the [alpha/Fe]-[Fe/H] plane. Based on the cluster's age (similar to 8 Gyr), Galactocentric radius, and height above the Galactic plane, as well as comparable chemistry reported for APOGEE stars in Baade's Window, we suggest that the two most likely origins for NGC 6791 are as an original part of the thick disk, or as a former member of the Galactic bulge. Moreover, because NGC 6791 lies at the high-metallicity end ([Fe/H] similar to 0.4) of the high-alpha sequence, the age of NGC 6791 places a limit on the youngest age of stars in the high-metallicity, high-alpha sequence for the cluster's parent population (i.e., either the bulge or the disk). In a similar way, we can also use the age and chemistry of NGC 188 to set a limit of similar to 7 Gyr on the oldest age of the low-alpha sequence of the MW. Therefore, NGC 6791 and NGC 188 are potentially a pair of star clusters that bracket both the timing and the duration of an important transition point in the chemical history of the MW.
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    Two groups of red giants with distinct chemical abundances in the bulge globular cluster NGC 6553 through the eyes of APOGEE
    (OXFORD UNIV PRESS, 2017) Tang, Baitian; Cohen, Roger E.; Geisler, Doug; Schiavon, Ricardo P.; Majewski, Steven R.; Villanova, Sandro; Carrera, Ricardo; Zamora, Olga; Garcia Hernandez, D. A.; Shetrone, Matthew; Frinchaboy, Peter; Meza, Andres; Fernandez Trincado, J. G.; Munoz, Ricardo R.; Lin, Chien Cheng; Lane, Richard R.; Nitschelm, Christian; Pan, Kaike; Bizyaev, Dmitry; Oravetz, Daniel; Simmons, Audrey
    Multiple populations revealed in globular clusters (GCs) are important windows to the formation and evolution of these stellar systems. The metal-rich GCs in the Galactic bulge are an indispensable part of this picture, but the high optical extinction in this region has prevented extensive research. In this work, we use the high-resolution near-infrared (NIR) spectroscopic data from Apache Point Observatory Galactic Evolution Experiment (APOGEE) to study the chemical abundances of NGC 6553, which is one of the most metal-rich bulge GCs. We identify 10 red giants as cluster members using their positions, radial velocities, iron abundances, and NIR photometry. Our sample stars show a mean radial velocity of -0.14 +/- 5.47 km s(-1), land a mean [Fe/H] of -0.15 +/- 0.05. We clearly separate two populations of stars in C and N in this GC for the first time. NGC 6553 is the most metal-rich GC where the multiple stellar population phenomenon is found until now. Substantial chemical variations are also found in Na, O, and Al. However, the two populations show similar Si, Ca, and iron-peak element abundances. Therefore, we infer that the CNO, NeNa, and MgAl cycles have been activated, but the MgAl cycle is too weak to show its effect on Mg. Type Ia and Type II supernovae do not seem to have significantly polluted the second generation stars. Comparing with other GC studies, NGC 6553 shows similar chemical variations as other relatively metal-rich GCs. We also confront current GC formation theories with our results, and suggest possible avenues for improvement in the models.