Examinando por Autor "Cunha, K."
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Ítem A Chemical and Kinematical Analysis of the Intermediate-age Open Cluster IC 166 from APOGEE and Gaia DR2(Institute of Physics Publishing, 2018-09) Schiappacasse-Ulloa, J.; Tang, B.; Fernández-Trincado, J.G.; Zamora, O.; Geisler, D.; Frinchaboy, P.; Schultheis, M.; Dell'Agli, F.; Villanova, S.; Masseron, T.; Mészáros, S.; Souto, D.; Hasselquist, S.; Cunha, K.; Smith, V.V.; García-Hernández, D.A.; Vieira, K.; Robin, A.C.; Minniti, D.; Zasowski, G.; Moreno, E.; Pérez-Villegas, A.; Lane, R.R.; Ivans, I.I.; Pan, K.; Nitschelm, C.; Santana, F.A.; Carrera, R.; Roman-Lopes, A.IC 166 is an intermediate-age open cluster (OC) (∼1 Gyr) that lies in the transition zone of the metallicity gradient in the outer disk. Its location, combined with our very limited knowledge of its salient features, make it an interesting object of study. We present the first high-resolution spectroscopic and precise kinematical analysis of IC 166, which lies in the outer disk with R GC ∼ 12.7 kpc. High-resolution H-band spectra were analyzed using observations from the SDSS-IV Apache Point Observatory Galactic Evolution Experiment survey. We made use of the Brussels Automatic Stellar Parameter code to provide chemical abundances based on a line-by-line approach for up to eight chemical elements (Mg, Si, Ca, Ti, Al, K, Mn, and Fe). The α-element (Mg, Si, Ca, and whenever available Ti) abundances, and their trends with Fe abundances have been analyzed for a total of 13 high-likelihood cluster members. No significant abundance scatter was found in any of the chemical species studied. Combining the positional, heliocentric distance, and kinematic information, we derive, for the first time, the probable orbit of IC 166 within a Galactic model including a rotating boxy bar, and found that it is likely that IC 166 formed in the Galactic disk, supporting its nature as an unremarkable Galactic OC with an orbit bound to the Galactic plane. © 2018. The American Astronomical Society.Ítem Abundance analysis of APOGEE spectra for 58 metal-poor stars from the bulge spheroid(Oxford University Press, 2022-12-01) Razera, R.; Barbuy, B.; Moura, T.C.; Ernandes, H.; Pérez Villegas, A.; Souza, S.O.; Chiappini, C.; Queiroz, A.B.A.; Anders, F.; Fernández Trincado, J.G.; Friaça, A.C.S.; Cunha, K.; Smith, V.V.; Santiago, B.X.; Schiavon, R.P.; Valentini, M.; Minniti, D.; Schultheis, M.; Geisler, D.; Sobeck, J.; Placco, V.M.; Zoccali, M.The central part of the Galaxy hosts a multitude of stellar populations, including the spheroidal bulge stars, stars moved to the bulge through secular evolution of the bar, inner halo, inner thick disc, inner thin disc, as well as debris from past accretion events. We identified a sample of 58 candidate stars belonging to the stellar population of the spheroidal bulge, and analyse their abundances. The present calculations of Mg, Ca, and Si lines are in agreement with the ASPCAP abundances, whereas abundances of C, N, O, and Ce are re-examined. We find normal α-element enhancements in oxygen, similar to magnesium, Si, and Ca abundances, which are typical of other bulge stars surveyed in the optical in Baade's Window. The enhancement of [O/Fe] in these stars suggests that they do not belong to accreted debris. No spread in N abundances is found, and none of the sample stars is N-rich, indicating that these stars are not second generation stars originated in globular clusters. Ce instead is enhanced in the sample stars, which points to an s-process origin such as due to enrichment from early generations of massive fast rotating stars, the so-called spinstars. © 2022 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society.Ítem Cool stars in the Galactic center as seen by APOGEE: M giants, AGB stars, and supergiant stars and candidates(EDP Sciences, 2020-10) Schultheis, M.; Rojas-Arriagada, A.; Cunha, K.; Zoccali, M.; Chiappini, C.; Zasowski, G.; Queiroz, A.B.A.; Minniti, D.; Fritz, T.; Garciá-Hernández, D.A.; Nitschelm, C.; Zamora, O.The Galactic center region, including the nuclear disk, has until recently been largely avoided in chemical census studies because of extreme extinction and stellar crowding. Large, near-IR spectroscopic surveys, such as the Apache Point Observatory Galactic Evolution Experiment (APOGEE), allow the measurement of metallicities in the inner region of our Galaxy. Making use of the latest APOGEE data release (DR16), we are able for the first time to study cool Asymptotic Giant branch (AGB) stars and supergiants in this region. The stellar parameters of five known AGB stars and one supergiant star (VR 5-7) show that their location is well above the tip of the red giant branch. We studied metallicities of 157 M giants situated within 150 pc of the Galactic center from observations obtained by the APOGEE survey with reliable stellar parameters from the APOGEE pipeline making use of the cool star grid down to 3200 K. Distances, interstellar extinction values, and radial velocities were checked to confirm that these stars are indeed situated in the Galactic center region. We detect a clear bimodal structure in the metallicity distribution function, with a dominant metal-rich peak of [Fe/H] ∼+0.3 dex and a metal-poor peak around {Fe/H] = -0.5 dex, which is 0.2 dex poorer than Baade's Window. The α-elements Mg, Si, Ca, and O show a similar trend to the Galactic bulge. The metal-poor component is enhanced in the α-elements, suggesting that this population could be associated with the classical bulge and a fast formation scenario. We find a clear signature of a rotating nuclear stellar disk and a significant fraction of high-velocity stars with vgal > 300 km s-1; the metal-rich stars show a much higher rotation velocity (∼200 km s-1) with respect to the metal-poor stars (∼140 km s-1). The chemical abundances as well as the metallicity distribution function suggest that the nuclear stellar disk and the nuclear star cluster show distinct chemical signatures and might be formed differently. © M. Schultheis et al. 2020.Ítem Disentangling the Galactic Halo with APOGEE. I. Chemical and Kinematical Investigation of Distinct Metal-poor Populations(Institute of Physics Publishing, 2018) Hayes, C.R.; Majewski, S.R.; Shetrone, M.; Fernández-Alvar, E.; Prieto, C.A.; Schuster, W.J.; Carigi, L.; Cunha, K.; Smith, V.V.; Sobeck, J.; Almeida, A.; Beers, T.C.; Carrera, R.; Fernández-Trincado, J.G.; García-Hernández, D.A.; Geisler, D.; Lane, R.R.; Lucatello, S.; Matthews, A.M.; Minniti, D.; Nitschelm, C.; Tang, B.; Tissera, P.B.; Zamora, O.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 α-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-α halo stars found in previous studies, suggesting that these are samples of the same two populations.Ítem Exploring the S-process History in the Galactic Disk: Cerium Abundances and Gradients in Open Clusters from the OCCAM/APOGEE Sample(Astrophysical Journal, 2022-02) Sales-Silva, J.; Daflon, S.; Cunha, K.; Souto, D.; Smith, V.; Chiappini, C.; Donor, J.; Frinchaboy, P.; García-Hernández, D.; Hayes, C.; Majewski, S.; Masseron, T.; Schiavon, R.; Weinberg, D.; Beaton, R.; Fernández-Trincado, J.; Jönsson, H.; Lane, R.; Minniti, D.; Manchado, A.; Moni, B.; Nitschelm, C.; O'Connell, J.; Villanova, S.The APOGEE Open Cluster Chemical Abundances and Mapping survey is used to probe the chemical evolution of the s-process element cerium in the Galactic disk. Cerium abundances were derived from measurements of Ce ii lines in the APOGEE spectra using the Brussels Automatic Code for Characterizing High Accuracy Spectra in 218 stars belonging to 42 open clusters. Our results indicate that, in general, for ages < 4 Gyr, younger open clusters have higher [Ce/Fe] and [Ce/α-element] ratios than older clusters. In addition, metallicity segregates open clusters in the [Ce/X]-age plane (where X can be H, Fe, or the α-elements O, Mg, Si, or Ca). These metallicity-dependent relations result in [Ce/Fe] and [Ce/α] ratios with ages that are not universal clocks. Radial gradients of [Ce/H] and [Ce/Fe] ratios in open clusters, binned by age, were derived for the first time, with d[Ce/H]/d R GC being negative, while d[Ce/Fe]/d R GC is positive. [Ce/H] and [Ce/Fe] gradients are approximately constant over time, with the [Ce/Fe] gradient becoming slightly steeper, changing by ∼+0.009 dex kpc-1 Gyr-1. Both the [Ce/H] and [Ce/Fe] gradients are shifted to lower values of [Ce/H] and [Ce/Fe] for older open clusters. The chemical pattern of Ce in open clusters across the Galactic disk is discussed within the context of s-process yields from asymptotic giant branch (AGB) stars, gigayear time delays in Ce enrichment of the interstellar medium, and the strong dependence of Ce nucleosynthesis on the metallicity of its AGB stellar sources.