Browsing by Author "Garcia-Hernandez, D. A."

Now showing 1 - 20 of 21
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    A Chemical and Kinematical Analysis of the Intermediate-age Open Cluster IC 166 from APOGEE and Gaia DR2
    (2018) Schiappacasse-Ulloa, J.; Tang, B.; Fernandez-Trincado, J. G.; Zamora, O.; Geisler, D.; Frinchaboy, P.; Schultheis, M.; Dell'Agli, F.; Villanova, S.; Masseron, T.; Meszaros, Sz; Souto, D.; Hasselquist, S.; Cunha, K.; Smith, V. V.; Garcia-Hernandez, D. A.; Vieira, K.; Robin, A. C.; Minniti, D.; Zasowski, G.; Moreno, E.; Perez-Villegas, A.; Lane, R. R.; Ivans, I. I.; Pan, K.; Nitschelm, C.; Santana, F. A.; Carrera, R.; Roman-Lopes, A.
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    An enquiry on the origins of N-rich stars in the inner Galaxy based on APOGEE chemical compositions
    (2021) Kisku, Shobhit; Schiavon, Ricardo P.; Horta, Danny; Mason, Andrew; Mackereth, J. Ted; Hasselquist, Sten; Garcia-Hernandez, D. A.; Bizyaev, Dmitry; Brownstein, Joel R.; Lane, Richard R.; Minniti, Dante; Pan, Kaike; Roman-Lopes, Alexandre
    Recent evidence based on APOGEE data for stars within a few kpc of the Galactic Centre suggests that dissolved globular clusters (GCs) contribute significantly to the stellar mass budget of the inner halo. In this paper, we enquire into the origins of tracers of GC dissolution, N-rich stars, that are located in the inner 4 kpc of the Milky Way. From an analysis of the chemical compositions of these stars, we establish that about 30 per cent of the N-rich stars previously identified in the inner Galaxy may have an accreted origin. This result is confirmed by an analysis of the kinematic properties of our sample. The specific frequency of N-rich stars is quite large in the accreted population, exceeding that of its in situ counterparts by near an order of magnitude, in disagreement with predictions from numerical simulations. We hope that our numbers provide a useful test to models of GC formation and destruction.
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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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    Chemical Abundances of Main-sequence, Turnoff, Subgiant, and Red Giant Stars from APOGEE Spectra. I. Signatures of Diffusion in the Open Cluster M67
    (2018) Souto, Diogo; Cunha, Katia; Smith, Verne V.; Allende Prieto, C.; Garcia-Hernandez, D. A.; Pinsonneault, Marc; Holzer, Parker; Frinchaboy, Peter; Holtzman, Jon; Johnson, J. A.; Jonsson, Henrik; Majewski, Steven R.; Shetrone, Matthew; Sobeck, J
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    Chemical Cartography with APOGEE: Multi-element Abundance Ratios
    (2019) Weinberg, David H.; Holtzman, Jon A.; Hasselquist, Sten; Bird, Jonathan C.; Johnson, Jennifer A.; Shetrone, Matthew; Sobeck, Jennifer; Allende Prieto, Carlos; Bizyaev, Dmitry; Carrera, Ricardo; Cohen, Roger E.; Cunha, Katia; Ebelke, Garrett; Fernandez-Trincado, J. G.; Garcia-Hernandez, D. A.; Hayes, Christian R.; Jonsson, Henrik; Lane, Richard R.; Majewski, Steven R.; Malanushenko, Viktor; Meszaros, Szabolcs; Nidever, David L.; Nitschelm, Christian; Pan, Kaike; Rix, Hans-Walter; Rybizki, Jan; Schiavon, Ricardo P.; Schneider, Donald P.; Wilso, John C.; Zamora, Olga
    We map the trends of elemental abundance ratios across the Galactic disk, spanning R = 3-15 kpc and midplane distance vertical bar Z vertical bar = 0-2 kpc, for 15 elements in a sample of 20,485 stars measured by the SDSS/APOGEE survey (O, Na, Mg, Al, Si, P, S, K, Ca, V, Cr, Mn, Fe, Co, Ni). Adopting Mg rather than Fe as our reference element, and separating stars into two populations based on [Fe/Mg], we find that the median trends of [X/Mg] versus [Mg/H] in each population are nearly independent of location in the Galaxy. The full multi-element cartography can be summarized by combining these nearly universal median sequences with our measured metallicity distribution functions and the relative proportions of the low-[Fe/Mg] (high-alpha) and high-[Fe/Mg] (low-alpha) populations, which depend strongly on R and vertical bar Z vertical bar. We interpret the median sequences with a semi-empirical "two-process" model that describes both the ratio of core collapse and Type Ia supernova (SN Ia) contributions to each element and the metallicity dependence of the supernova yields. These observationally inferred trends can provide strong tests of supernova nucleosynthesis calculations. Our results lead to a relatively simple picture of abundance ratio variations in the Milky Way, in which the trends at any location can be described as the sum of two components with relative contributions that change systematically and smoothly across the Galaxy. Deviations from this picture and future extensions to other elements can provide further insights into the physics of stellar nucleosynthesis and unusual events in the Galaxy's history.
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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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    Exploring the Stellar Age Distribution of the Milky Way Bulge Using APOGEE
    (2020) Hasselquist, Sten; Zasowski, Gail; Feuillet, Diane K.; Schultheis, Mathias; Nataf, David M.; Anguiano, Borja; Beaton, Rachael L.; Beers, Timothy C.; Cohen, Roger E.; Cunha, Katia; Fernandez-Trincado, Jose G.; Garcia-Hernandez, D. A.; Geisler, Doug; Holtzman, Jon A.; Johnson, Jennifer; Lane, Richard R.; Majewski, Steven R.; Bidin, Christian Moni; Nitschelm, Christian; Roman-Lopes, Alexandre; Schiavon, Ricardo; Smith, Verne V.; Sobeck, Jennifer
    We present stellar age distributions of the Milky Way bulge region using ages for similar to 6000 high-luminosity (log (g), metal-rich ([Fe/H] >= -0.5) bulge stars observed by the Apache Point Observatory Galactic Evolution Experiment. Ages are derived using The Cannon label-transfer method, trained on a sample of nearby luminous giants with precise parallaxes for which we obtain ages using a Bayesian isochrone-matching technique. We find that the metal-rich bulge is predominantly composed of old stars (>8 Gyr). We find evidence that the planar region of the bulge (vertical bar Z(GC)vertical bar <= 0.25 kpc) is enriched in metallicity, Z, at a faster rate (dZ/dt similar to 0.0034 Gyr(-1)) than regions farther from the plane (dZ/dt similar to 0.0013 Gyr(-1) at vertical bar Z(GC)vertical bar > 1.00 kpc). We identify a nonnegligible fraction of younger stars (age similar to 2-5 Gyr) at metallicities of +0.2 < [Fe/H] < +0.4. These stars are preferentially found in the plane (vertical bar Z(GC)vertical bar <= 0.25 kpc) and at R-cy approximate to 2-3 kpc, with kinematics that are more consistent with rotation than are the kinematics of older stars at the same metallicities. We do not measure a significant age difference between stars found inside and outside the bar. These findings show that the bulge experienced an initial starburst that was more intense close to the plane than far from the plane. Then, star formation continued at supersolar metallicities in a thin disk at 2 kpc less than or similar to R-cy less than or similar to 3 kpc until similar to 2 Gyr ago.
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    How many components? Quantifying the complexity of the metallicity distribution in the Milky Way bulge with APOGEE
    (2020) Rojas-Arriagada, Alvaro; Zasowski, Gail; Schultheis, Mathias; Zoccali, Manuela; Hasselquist, Sten; Chiappini, Cristina; Cohen, Roger E.; Cunha, Katia; Fernandez-Trincado, Jose G.; Fragkoudi, Francesca; Garcia-Hernandez, D. A.; Geisler, Doug; Gran, Felipe; Lian, Jianhui; Majewski, Steven; Minniti, Dante; Monachesi, Antonela; Nitschelm, Christian; Queiroz, Anna B. A.
    We use data of similar to 13 000 stars from the Sloan Digital Sky Survey/Apache Point Observatory Galactic Evolution Experiment survey to study the shape of the bulge metallicity distribution function (MDF) within the region vertical bar l vertical bar <= 11 degrees and vertical bar b vertical bar = 13 degrees, and spatially constrained to R-GC <= 3.5 kpc. We apply Gaussian mixture modelling and non-negative matrix factorization decomposition techniques to identify the optimal number and the properties of MDF components. We find that the shape and spatial variations of the MDF (at [Fe/H] >= -1 dex) are well represented as a smoothly varying contribution of three overlapping components located at [Fe/H] = +0.32, -0.17, and -0.66 dex. The bimodal MDF found in previous studies is in agreement with our trimodal assessment once the limitations in sample size and individual measurement errors are taken into account. The shape of the MDF and its correlations with kinematics reveal different spatial distributions and kinematical structure for the three components co-existing in the bulge region. We confirm the consensus physical interpretation of metal-rich stars as associated with the secularly evolved disc into a boxy/peanut X-shape bar. On the other hand, metal-intermediate stars could be the product of in-situ formation at high redshift in a gas-rich environment characterized by violent and fast star formation. This interpretation would help us to link a present-day structure with those observed in formation in the centre of high-redshift galaxies. Finally, metal-poor stars may correspond to the metal-rich tail of the population sampled at lower metallicity from the study of RR Lyrae stars. Conversely, they could be associated with the metal-poor tail of the early thick disc.
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    H-band discovery of additional second-generation stars in the Galactic bulge globular cluster NGC 6522 as observed by APOGEE and Gaia
    (2019) Fernandez-Trincado, J. G.; Zamora, O.; Souto, Diogo; Cohen, R. E.; Agli, F. Dell; Garcia-Hernandez, D. A.; Masseron, T.; Schiavon, R. P.; Meszaros, Sz; Cunha, K.; Hasselquist, S.; Shetrone, M.; Schiappacasse Ulloa, J.; Tang, B.; Geisler, D.; Schleicher, D. R. G.; Villanova, S.; Mennickent, R. E.; Minniti, D.; Alonso-Garcia, J.; Manchado, A.; Beers, T. C.; Sobeck, J.; Zasowski, G.; Schultheis, M.; Majewski, S. R.; Rojas-Arriagada, A.; Almeida, A.; Santana, F.; Oelkers, R. J.; Longa-Pena, P.; Carrera, R.; Burgasser, A. J.; Lane, R. R.; Roman-Lopes, A.; Ivans, I. I.; Hearty, F. R.
    We present an elemental abundance analysis of high-resolution spectra for five giant stars spatially located within the innermost regions of the bulge globular cluster NGC 6522 and derive Fe, Mg, Al, C, N, O, Si, and Ce abundances based on H-band spectra taken with the multi-object APOGEE-north spectrograph from the SDSS-IV Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey. Of the five cluster candidates, two previously unremarked stars are confirmed to have second-generation (SG) abundance patterns, with the basic pattern of depletion in C and Mg simultaneous with enrichment in N and Al as seen in other SG globular cluster populations at similar metallicity. In agreement with the most recent optical studies, the NGC 6522 stars analyzed exhibit (when available) only mild overabundances of the s-process element Ce, contradicting the idea that NGC 6522 stars are formed from gas enriched by spinstars and indicating that other stellar sources such as massive AGB stars could be the primary polluters of intra-cluster medium. The peculiar abundance signatures of SG stars have been observed in our data, confirming the presence of multiple generations of stars in NGC 6522.
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    Identifying Sagittarius Stream Stars by Their APOGEE Chemical Abundance Signatures
    (2019) Hasselquist, Sten; Carlin, Jeffrey L.; Holtzman, Jon A.; Shetrone, Matthew; Hayes, Christian R.; Cunha, Katia; Smith, Verne; Beaton, Rachael L.; Sobeck, Jennifer; Allende Prieto, Carlos; Majewski, Steven R.; Anguiano, Borja; Bizyaev, Dmitry; Garcia-Hernandez, D. A.; Lane, Richard R.; Pan, Kaike; Nidever, David L.; Fernandez-Trincado, Jose. G.; Wilson, John C.; Zamora, Olga
    The SDSS-IV Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey provides precise chemical abundances of 18 chemical elements for similar to 176,000 red giant stars distributed over much of the Milky Way Galaxy (MW), and includes observations of the core of the Sagittarius dwarf spheroidal galaxy (Sgr). The APOGEE chemical abundance patterns of Sgr have revealed that it is chemically distinct from the MW in most chemical elements. We employ a k-means clustering algorithm to six-dimensional chemical space defined by [(C+N)/Fe], [O/Fe], [Mg/Fe], [Al/Fe], [Mn/Fe], and [Ni/Fe] to identify 62 MW stars in the APOGEE sample that have Sgr-like chemical abundances. Of the 62 stars, 35 have Gaia kinematics and positions consistent with those predicted by N-body simulations of the Sgr stream, and are likely stars that have been stripped from Sgr during the last two pericenter passages (<2 Gyr ago). Another 20 of the 62 stars exhibit chemical abundances indistinguishable from the Sgr stream stars, but are on highly eccentric orbits with median r(apo) similar to 25 kpc. These stars are likely the "accreted" halo population thought to be the result of a separate merger with the MW 8-11 Gyr ago. We also find one hypervelocity star candidate. We conclude that Sgr was enriched to [Fe/H] similar to -0.2 before its most recent pericenter passage. If the "accreted halo" population is from one major accretion event, then this progenitor galaxy was enriched to at least [Fe/H] similar to -0.6, and had a similar star formation history to Sgr before merging.
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    Quantifying radial migration in the Milky Way: inefficient over short time-scales but essential to the very outer disc beyond ∼15 kpc
    (2022) Lian, Jianhui; Zasowski, Gail; Hasselquist, Sten; Holtzman, Jon A.; Boardman, Nicholas; Cunha, Katia; Fernandez-Trincado, Jose G.; Frinchaboy, Peter M.; Garcia-Hernandez, D. A.; Nitschelm, Christian; Lane, Richard R.; Thomas, Daniel; Zhang, Kai
    Stellar radial migration plays an important role in reshaping a galaxy's structure and the radial distribution of stellar population properties. In this work, we revisit reported observational evidence for radial migration and quantify its strength using the age-[Fe/H] distribution of stars across the Milky Way with APOGEE data. We find a broken age-[Fe/H] relation in the Galactic disc at r > 6 kpc, with a more pronounced break at larger radii. To quantify the strength of radial migration, we assume stars born at each radius have a unique age and metallicity, and then decompose the metallicity distribution function (MDF) of mono-age young populations into different Gaussian components that originated from various birth radii at r(birth) < 13 kpc. We find that, at ages of 2 and 3 Gyr, roughly half the stars were formed within 1 kpc of their present radius, and very few stars (<5 per cent) were formed more than 4 kpc away from their present radius. These results suggest limited short-distance radial migration and inefficient long-distance migration in the Milky Way during the last 3 Gyr. In the very outer disc beyond 15 kpc, the observed age-[Fe/H] distribution is consistent with the prediction of pure radial migration from smaller radii, suggesting a migration origin of the very outer disc. We also estimate intrinsic metallicity gradients at ages of 2 and 3 Gyr of -0.061 and -0.063 dex kpc(-1), respectively.
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    Spatial variations in the Milky Way disc metallicity-age relation
    (2019) Feuillet, Diane K.; Frankel, Neige; Lind, Karin; Frinchaboy, Peter M.; Garcia-Hernandez, D. A.; Lane, Richard R.; Nitschelm, Christian; Roman-Lopes, Alexandre
    Stellar ages are a crucial component to studying the evolution of the Milky Way. Using Gaia DR2 distance estimates, it is now possible to estimate stellar ages for a larger volume of evolved stars through isochrone matching. This work presents [M/H]-age and [alpha/M]-age relations derived for different spatial locations in the Milky Way disc. These relations are derived by hierarchically modelling the star formation history of stars within a given chemical abundance bin. For the first time, we directly observe that significant variation is apparent in the [M/H]-age relation as a function of both Galactocentric radius and distance from the disc midplane. The [M/H]-age relations support claims that radial migration has a significant effect in the plane of the disc. Using the [M/H] bin with the youngest mean age at each radial zone in the plane of the disc, the present-day metallicity gradient is measured to be -0.059 +/- 0.010 dex kpc(-1), in agreement with Cepheids and young field stars. We find a vertically flared distribution of young stars in the outer disc, confirming predictions of models and previous observations. The mean age of the [M/H]-[alpha/M] distribution of the solar neighbourhood suggests that the high-[M/H] stars are not an evolutionary extension of the low-alpha sequence. Our observational results are important constraints to Galactic simulations and models of chemical evolution.
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    Strong chemical tagging with APOGEE: 21 candidate star clusters that have dissolved across the Milky Way disc
    (2020) Price-Jones, Natalie; Bovy, Jo; Webb, Jeremy J.; Prieto, Carlos Allende; Beaton, Rachael; Brownstein, Joel R.; Cohen, Roger E.; Cunha, Katia; Donor, John; Frinchaboy, Peter M.; Garcia-Hernandez, D. A.; Lane, Richard R.; Majewski, Steven R.; Nidever, David L.; Roman-Lopes, Alexandre
    Chemically tagging groups of stars born in the same birth cluster is a major goal of spectroscopic surveys. To investigate the feasibility of such strong chemical tagging, we perform a blind chemical tagging experiment on abundances measured from APOGEE survey spectra. We apply a density-based clustering algorithm to the 8D chemical space defined by [Mg/Fe], [Al/Fe], [Si/Fe], [K/Fe], [Ti/Fe], [Mn/Fe], [Fe/H], and [Ni/Fe], abundances ratios which together span multiple nucleosynthetic channels. In a high-quality sample of 182 538 giant stars, we detect 21 candidate clusterswith more than 15 members. Our candidate clusters are more chemically homogeneous than a population of non-member stars with similar [Mg/Fe] and [Fe/H], even in abundances not used for tagging. Group members are consistent with having the same age and fall along a single stellar-population track in log g versus T-eff space. Each group's members are distributed over multiple kpc, and the spread in their radial and azimuthal actions increases with age. We qualitatively reproduce this increase using N-body simulations of cluster dissolution in Galactic potentials that include transient winding spiral arms. Observing our candidate birth clusters with high-resolution spectroscopy in other wavebands to investigate their chemical homogeneity in other nucleosynthetic groups will be essential to confirming the efficacy of strong chemical tagging. Our initially spatially compact but now widely dispersed candidate clusters will provide novel limits on chemical evolution and orbital diffusion in the Galactic disc, and constraints on star formation in loosely bound groups.
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    The 16th Data Release of the Sloan Digital Sky Surveys: First Release from the APOGEE-2 Southern Survey and Full Release of eBOSS Spectra
    (2020) Ahumada, Romina; Allende Prieto, Carlos; Almeida, Andres; Anders, Friedrich; Anderson, Scott F.; Andrews, Brett H.; Anguiano, Borja; Arcodia, Riccardo; Armengaud, Eric; Aubert, Marie; Avila, Santiago; Avila-Reese, Vladimir; Badenes, Carles; Balland, Christophe; Barger, Kat; Barrera-Ballesteros, Jorge K.; Basu, Sarbani; Bautista, Julian; Beaton, Rachael L.; Beers, Timothy C.; Benavides, B. Izamar T.; Bender, Chad F.; Bernardi, Mariangela; Bershady, Matthew; Beutler, Florian; Bidin, Christian Moni; Bird, Jonathan; Bizyaev, Dmitry; Blanc, Guillermo A.; Blanton, Michael R.; Boquien, Mederic; Borissova, Jura; Bovy, Jo; Brandt, W. N.; Brinkmann, Jonathan; Brownstein, Joel R.; Bundy, Kevin; Bureau, Martin; Burgasser, Adam; Burtin, Etienne; Cano-Diaz, Mariana; Capasso, Raffaella; Cappellari, Michele; Carrera, Ricardo; Chabanier, Solene; Chaplin, William; Chapman, Michael; Cherinka, Brian; Chiappini, Cristina; Choi, Peter Doohyun; Chojnowski, S. Drew; Chung, Haeun; Clerc, Nicolas; Coffey, Damien; Comerford, Julia M.; Comparat, Johan; da Costa, Luiz; Cousinou, Marie-Claude; Covey, Kevin; Crane, Jeffrey D.; Cunha, Katia; Ilha, Gabriele da Silva; Dai, Yu Sophia; Damsted, Sanna B.; Darling, Jeremy; Davidson, James W., Jr.; Davies, Roger; Dawson, Kyle; De, Nikhil; de la Macorra, Axel; Lee, Nathan De; de Andrade Queiroz, Anna Barbara; Machado, Alice Deconto; de la Torre, Sylvain; Dell'Agli, Flavia; des Bourboux, Helion du Mas; Diamond-Stanic, Aleksandar M.; Dillon, Sean; Donor, John; Drory, Niv; Duckworth, Chris; Dwelly, Tom; Ebelke, Garrett; Eftekharzadeh, Sarah; Eigenbrot, Arthur Davis; Elsworth, Yvonne P.; Eracleous, Mike; Erfanianfar, Ghazaleh; Escoffier, Stephanie; Fan, Xiaohui; Farr, Emily; Fernandez-Trincado, Jose G.; Feuillet, Diane; Finoguenov, Alexis; Fofie, Patricia; Fraser-McKelvie, Amelia; Frinchaboy, Peter M.; Fromenteau, Sebastien; Fu, Hai; Galbany, Lluis; Garcia, Rafael A.; Garcia-Hernandez, D. A.; Oehmichen, Luis Alberto Garma; Ge, Junqiang; Maia, Marcio Antonio Geimba; Geisler, Doug; Gelfand, Joseph; Goddy, Julian; Gonzalez-Perez, Violeta; Grabowski, Kathleen; Green, Paul; Grier, Catherine J.; Guo, Hong; Guy, Julien; Harding, Paul; Hasselquist, Sten; Hawken, Adam James; Hayes, Christian R.; Hearty, Fred; Hekker, S.; Hogg, David W.; Holtzman, Jon A.; Horta, Danny; Hou, Jiamin; Hsieh, Bau-Ching; Huber, Daniel; Hunt, Jason A. S.; Chitham, J. Ider; Imig, Julie; Jaber, Mariana; Angel, Camilo Eduardo Jimenez; Johnson, Jennifer A.; Jones, Amy M.; Jonsson, Henrik; Jullo, Eric; Kim, Yerim; Kinemuchi, Karen; Iv, Charles C. Kirkpatrick; Kite, George W.; Klaene, Mark; Kneib, Jean-Paul; Kollmeier, Juna A.; Kong, Hui; Kounkel, Marina; Krishnarao, Dhanesh; Lacerna, Ivan; Lan, Ting-Wen; Lane, Richard R.; Law, David R.; Le Goff, Jean-Marc; Leung, Henry W.; Lewis, Hannah; Li, Cheng; Lian, Jianhui; Lin, Lihwai; Long, Dan; Longa-Pena, Penelope; Lundgren, Britt; Lyke, Brad W.; Mackereth, J. Ted; MacLeod, Chelsea L.; Majewski, Steven R.; Manchado, Arturo; Maraston, Claudia; Martini, Paul; Masseron, Thomas; Masters, Karen L.; Mathur, Savita; McDermid, Richard M.; Merloni, Andrea; Merrifield, Michael; Meszaros, Szabolcs; Miglio, Andrea; Minniti, Dante; Minsley, Rebecca; Miyaji, Takamitsu; Mohammad, Faizan Gohar; Mosser, Benoit; Mueller, Eva-Maria; Muna, Demitri; Munoz-Gutierrez, Andrea; Myers, Adam D.; Nadathur, Seshadri; Nair, Preethi; Nandra, Kirpal; do Nascimento, Janaina Correa; Nevin, Rebecca Jean; Newman, Jeffrey A.; Nidever, David L.; Nitschelm, Christian; Noterdaeme, Pasquier; O'Connell, Julia E.; Olmstead, Matthew D.; Oravetz, Daniel; Oravetz, Audrey; Osorio, Yeisson; Pace, Zachary J.; Padilla, Nelson; Palanque-Delabrouille, Nathalie; Palicio, Pedro A.; Pan, Hsi-An; Pan, Kaike; Parker, James; Paviot, Romain; Peirani, Sebastien; Ramirez, Karla Pena; Penny, Samantha; Percival, Will J.; Perez-Fournon, Ismael; Perez-Rafols, Ignasi; Petitjean, Patrick; Pieri, Matthew M.; Pinsonneault, Marc; Poovelil, Vijith Jacob; Povick, Joshua Tyler; Prakash, Abhishek; Price-Whelan, Adrian M.; Raddick, M. Jordan; Raichoor, Anand; Ray, Amy; Rembold, Sandro Barboza; Rezaie, Mehdi; Riffel, Rogemar A.; Riffel, Rogerio; Rix, Hans-Walter; Robin, Annie C.; Roman-Lopes, A.; Roman-Zuniga, Carlos; Rose, Benjamin; Ross, Ashley J.; Rossi, Graziano; Rowlands, Kate; Rubin, Kate H. R.; Salvato, Mara; Sanchez, Ariel G.; Sanchez-Menguiano, Laura; Sanchez-Gallego, Jose R.; Sayres, Conor; Schaefer, Adam; Schiavon, Ricardo P.; Schimoia, Jaderson S.; Schlafly, Edward; Schlegel, David; Schneider, Donald P.; Schultheis, Mathias; Schwope, Axel; Seo, Hee-Jong; Serenelli, Aldo; Shafieloo, Arman; Shamsi, Shoaib Jamal; Shao, Zhengyi; Shen, Shiyin; Shetrone, Matthew; Shirley, Raphael; Aguirre, Victor Silva; Simon, Joshua D.; Skrutskie, M. F.; Slosar, Anze; Smethurst, Rebecca; Sobeck, Jennifer; Sodi, Bernardo Cervantes; Souto, Diogo; Stark, David, V; Stassun, Keivan G.; Steinmetz, Matthias; Stello, Dennis; Stermer, Julianna; Storchi-Bergmann, Thaisa; Streblyanska, Alina; Stringfellow, Guy S.; Stutz, Amelia; Suarez, Genaro; Sun, Jing; Taghizadeh-Popp, Manuchehr; Talbot, Michael S.; Tayar, Jamie; Thakar, Aniruddha R.; Theriault, Riley; Thomas, Daniel; Thomas, Zak C.; Tinker, Jeremy; Tojeiro, Rita; Toledo, Hector Hernandez; Tremonti, Christy A.; Troup, Nicholas W.; Tuttle, Sarah; Unda-Sanzana, Eduardo; Valentini, Marica; Vargas-Gonzalez, Jaime; Vargas-Magana, Mariana; Vazquez-Mata, Jose Antonio; Vivek, M.; Wake, David; Wang, Yuting; Weaver, Benjamin Alan; Weijmans, Anne-Marie; Wild, Vivienne; Wilson, John C.; Wilson, Robert F.; Wolthuis, Nathan; Wood-Vasey, W. M.; Yan, Renbin; Yang, Meng; Yeche, Christophe; Zamora, Olga; Zarrouk, Pauline; Zasowski, Gail; Zhang, Kai; Zhao, Cheng; Zhao, Gongbo; Zheng, Zheng; Zheng, Zheng; Zhu, Guangtun; Zou, Hu
    This paper documents the 16th data release (DR16) from the Sloan Digital Sky Surveys (SDSS), the fourth and penultimate from the fourth phase (SDSS-IV). This is the first release of data from the Southern Hemisphere survey of the Apache Point Observatory Galactic Evolution Experiment 2 (APOGEE-2); new data from APOGEE-2 North are also included. DR16 is also notable as the final data release for the main cosmological program of the Extended Baryon Oscillation Spectroscopic Survey (eBOSS), and all raw and reduced spectra from that project are released here. DR16 also includes all the data from the Time Domain Spectroscopic Survey and new data from the SPectroscopic IDentification of ERosita Survey programs, both of which were co-observed on eBOSS plates. DR16 has no new data from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey (or the MaNGA Stellar Library "MaStar"). We also preview future SDSS-V operations (due to start in 2020), and summarize plans for the final SDSS-IV data release (DR17).
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    The age-chemical abundance structure of the Galactic disc - II. α-dichotomy and thick disc formation
    (2020) Lian, Jianhui; Thomas, Daniel; Maraston, Claudia; Beers, Timothy C.; Moni Bidin, Christian; Fernandez-Trincado, Jose G.; Garcia-Hernandez, D. A.; Lane, Richard R.; Munoz, Ricardo R.; Nitschelm, Christian; Roman-Lopes, Alexandre; Zamora, Olga
    We extend our previous work on the age-chemical abundance structure of the Galactic outer disc to the inner disc (4 < r < 8 kpc) based on the SDSS/APOGEE survey. Different from the outer disc, the inner disc stars exhibit a clear bimodal distribution in the [Mg/Fe]-[Fe/H] plane. While a number of scenarios have been proposed in the literature, it remains challenging to recover this bimodal distribution with theoretical models. To this end, we present a chemical evolution model embedding a complex multiphase inner disc formation scenario that matches the observed bimodal [Mg/Fe]-[Fe/H] distribution. In this scenario, the formation of the inner disc is dominated by two main starburst episodes 6 Gyr apart with secular, low-level star formation activity in between. In our model, the first starburst occurs at early cosmic times (t similar to 1 Gyr) and the second one 6 Gyr later at a cosmic time of t similar to 7 Gyr. Both these starburst episodes are associated with gas accretion events in our model, and are quenched rapidly. The first starburst leads to the formation of the high-a sequence, and the second starburst leads to the formation of the metal-poor low-a sequence. The metal-rich low-alpha stars, instead, form during the secular evolution phase between the two bursts. Our model shows that the alpha-dichotomy originates from the rapid suppression of star formation after the first starburst. The two starburst episodes are likely to be responsible for the formation of the geometric thick disc (z > 1 kpc), with the old inner thick disc and the young outer thick disc forming during the first and the second starbursts, respectively.
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    The contribution of N-rich stars to the Galactic stellar halo using APOGEE red giants
    (2021) Horta, Danny; Mackereth, J. Ted; Schiavon, Ricardo P.; Hasselquist, Sten; Bovy, Jo; Prieto, Carlos Allende; Beers, Timothy C.; Cunha, Katia; Garcia-Hernandez, D. A.; Kisku, Shobhit S.; Lane, Richard R.; Majewski, Steven R.; Mason, Andrew C.; Nataf, David M.; Roman-Lopes, Alexandre; Schultheis, Mathias
    The contribution of dissolved globular clusters (GCs) to the stellar content of the Galactic halo is a key constraint on models for GC formation and destruction, and the mass assembly history of the Milky Way. Earlier results from APOGEE pointed to a large contribution of destroyed GCs to the stellar content of the inner halo, by as much as 25 per cent, which is an order of magnitude larger than previous estimates for more distant regions of the halo. We set out to measure the ratio between nitrogen-rich (N-rich) and normal halo field stars, as a function of distance, by performing density modelling of halo field populations in APOGEE DR16. Our results show that at 1.5 kpc from the Galactic Centre, N-rich stars contribute a much higher 16.81(-7.0)(+10.0) per cent fraction to the total stellar halo mass budget than the 2.7(-0.8)(-1.0) per cent ratio contributed at 10 kpc. Under the assumption that N-rich stars are former GC members that now reside in the stellar halo field, and assuming the ratio between first and second population GC stars being 1:2, we estimate a total contribution from disrupted GC stars of the order of 27.5(-11.5)(+15.4) per cent at r = 1.5 kpc and 4.2(-1:3)(+1.5) per cent at r = 10 kpc. Furthermore, since our methodology requires fitting a density model to the stellar halo, we integrate such density within a spherical shell from 1.5 to 15 kpc in radius, and find a total stellar mass arising from dissolved and/or evaporated GCs of M-GC(total) = 9.6-2.6(+4.0) x 10(7) M-circle dot.
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    The Milky Way bar and bulge revealed by APOGEE and Gaia EDR3
    (2021) 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.; Fernandez-Trincado, J. G.; Garcia-Hernandez, 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 |X-Gal|< 5 kpc, |Y-Gal|< 3.5 kpc, |Z(Gal)|< 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 [alpha/Fe], [C/N], and [Mn/O], which probe different enrichment timescales, suggesting a star formation gap (quenching) between the high- and low-alpha 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 V-phi 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 [alpha/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.
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    The Milky Way's bulge star formation history as constrained from its bimodal chemical abundance distribution
    (2020) Lian, Jianhui; Zasowski, Gail; Hasselquist, Sten; Nataf, David M.; Thomas, Daniel; Bidin, Christian Moni; Fernandez-Trincado, Jose G.; Garcia-Hernandez, D. A.; Lane, Richard R.; Majewski, Steven R.; Roman-Lopes, Alexandre; Schultheis, Mathias
    We conduct a quantitative analysis of the star formation history (SFH) of the Milky Way's (MW) bulge by exploiting the constraining power of its stellar [Fe/H] and [Mg/Fe] distribution functions. Using Apache Point Observatory Galactic Evolution Experiment survey data, we confirm the previously established bimodal [Mg/Fe]-[Fe/H] distribution within 3 kpc of the inner Galaxy. To fit the chemical bimodal distribution, we use a simple but flexible star formation framework, which assumes two distinct stages of gas accretion and star formation, and systematically evaluate a wide multidimensional parameter space. We find that the data favour a three-phase SFH that consists of an initial starburst, followed by a rapid star formation quenching episode, and a lengthy, quiescent secular evolution phase. The metal-poor, high-alpha bulge stars ([Fe/H] < 0.0 and [Mg/Fe] > 0.15) are formed rapidly (<2Gyr) during the early starburst. The density gap between the high- and low-alpha sequences is due to the quenching process. The metal-rich, low-a population ([Fe/H] > 0.0 and [Mg/Fe] < 0.15) then accumulates gradually through inefficient star formation during the secular phase. This is qualitatively consistent with the early SFH of the inner disc. Given this scenario, a notable fraction of young stars (age <5Gyr) is expected to persist in the bulge. Combined with extragalactic observations, these results suggest that a rapid star formation quenching process is responsible for bimodal distributions in both the MW's stellar populations and in the general galaxy population and thus plays a critical role in galaxy evolution.
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    The Open Cluster Chemical Abundances and Mapping Survey. IV. Abundances for 128 Open Clusters Using SDSS/APOGEE DR16
    (2020) Donor, John; Frinchaboy, Peter M.; Cunha, Katia; O'Connell, Julia E.; Prieto, Carlos Allende; Almeida, Andres; Anders, Friedrich; Beaton, Rachael; Bizyaev, Dmitry; Brownstein, Joel R.; Carrera, Ricardo; Chiappini, Cristina; Cohen, Roger; Garcia-Hernandez, D. A.; Geisler, Doug; Hasselquist, Sten; Jonsson, Henrik; Lane, Richard R.; Majewski, Steven R.; Minniti, Dante; Bidin, Christian Moni; Pan, Kaike; Roman-Lopes, Alexandre; Sobeck, Jennifer S.; Zasowski, Gail
    The Open Cluster Chemical Abundances and Mapping (OCCAM) survey aims to constrain key Galactic dynamical and chemical evolution parameters by the construction of a large, comprehensive, uniform, infrared-based spectroscopic data set of hundreds of open clusters. This fourth contribution from the OCCAM survey presents analysis using Sloan Digital Sky Survey/APOGEE DR16 of a sample of 128 open clusters, 71 of which we designate to be "high quality" based on the appearance of their color-magnitude diagram. We find the APOGEE DR16 derived [Fe/H] abundances to be in good agreement with previous high-resolution spectroscopic open cluster abundance studies. Using the high-quality sample, we measure Galactic abundance gradients in 16 elements, and find evolution of some of the [X/Fe] gradients as a function of age. We find an overall Galactic [Fe/H] versus R-GC gradient of -0.068 0.001 dex kpc(-1) over the range of 6 R-GC < 13.9 kpc; however, we note that this result is sensitive to the distance catalog used, varying as much as 15%. We formally derive the location of a break in the [Fe/H] abundance gradient as a free parameter in the gradient fit for the first time. We also measure significant Galactic gradients in O, Mg, S, Ca, Mn, Cr, Cu, Na, Al, and K, some of which are measured for the first time. Our large sample allows us to examine four well-populated age bins in order to explore the time evolution of gradients for a large number of elements and comment on possible implications for Galactic chemical evolution and radial migration.
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    The Origin of the 300 km s(-1) Stream near Segue 1
    (2018) Fu, Sal Wanying; Simon, Joshua D.; Shetrone, Matthew; Bovy, Jo; Beers, Timothy C.; Fernandez-Trincado, J. G.; Placco, Vinicius M.; Zamora, Olga; Allende Prieto, Carlos; Garcia-Hernandez, D. A.; Harding, Paul; Ivans, Inese; Lane, Richard; Nitschelm, Christian; Roman-Lopes, Alexandre; Sobeck, Jennifer