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1. Exploring the S-process History in the Galactic Disk: Cerium Abundances and Gradients in Open Clusters from the OCCAM/APOGEE Sample

   https://doi.org/10.3847/1538-4357/ac4254  

   Sales-Silva, J. V. ; Daflon, S. ; Cunha, K. ; Souto, D. ; Smith, V. V. ; Chiappini, C. ; Donor, J. ; Frinchaboy, P. M. ; García-Hernández, D. A. ; Hayes, C. ; et al ( February 2022 , The Astrophysical Journal)

   Abstract 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] andmore »[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.« less
2. The Open Cluster Chemical Abundances and Mapping Survey. VI. Galactic Chemical Gradient Analysis from APOGEE DR17

   https://doi.org/10.3847/1538-3881/ac7ce5  

   Myers, Natalie ; Donor, John ; Spoo, Taylor ; Frinchaboy, Peter M. ; Cunha, Katia ; Price-Whelan, Adrian M. ; Majewski, Steven R. ; Beaton, Rachael L. ; Zasowski, Gail ; O’Connell, Julia ; et al ( August 2022 , The Astronomical Journal)

   The goal of the Open Cluster Chemical Abundances and Mapping (OCCAM) survey is to constrain key Galactic dynamic and chemical evolution parameters by the construction and analysis of a large, comprehensive, uniform data set of infrared spectra for stars in hundreds of open clusters. This sixth contribution from the OCCAM survey presents analysis of SDSS/APOGEE Data Release 17 (DR17) results for a sample of stars in 150 open clusters, 94 of which we designate to be “high-quality” based on the appearance of their color–magnitude diagram. We find the APOGEE DR17-derived [Fe/H] values to be in good agreement with those from previous high-resolution spectroscopic open cluster abundance studies. Using a subset of the high-quality sample, the Galactic abundance gradients were measured for 16 chemical elements, including [Fe/H], for both Galactocentric radius (R_GC) and guiding center radius (R_guide). We find an overall Galactic [Fe/H] versusR_GCgradient of −0.073 ± 0.002 dex kpc⁻¹over the range of 6 >R_GC< 11.5 kpc, and a similar gradient is found for [Fe/H] versusR_guide. Significant Galactic abundance gradients are also noted for O, Mg, S, Ca, Mn, Na, Al, K, and Ce. Our large sample additionally allows us to explore the evolution of the gradients in four agemore »bins for the remaining 15 elements.

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3. Cool stars in the Galactic center as seen by APOGEE: M giants, AGB stars, and supergiant stars and candidates

   https://doi.org/10.1051/0004-6361/202038327  

   Schultheis, M. ; Rojas-Arriagada, A. ; Cunha, K. ; Zoccali, M. ; Chiappini, C. ; Zasowski, G. ; Queiroz, A. B. ; Minniti, D. ; Fritz, T. ; García-Hernández, D. A. ; et al ( October 2020 , Astronomy & Astrophysics)

   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] = −0.5more »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 v gal  >  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.« less
4. Homogeneous analysis of globular clusters from the APOGEE survey with the BACCHUS code – II. The Southern clusters and overview

   https://doi.org/10.1093/mnras/stz3496  

   Mészáros, Szabolcs ; Masseron, Thomas ; García-Hernández, D A ; Allende Prieto, Carlos ; Beers, Timothy C ; Bizyaev, Dmitry ; Chojnowski, Drew ; Cohen, Roger E ; Cunha, Katia ; Dell’Agli, Flavia ; et al ( February 2020 , Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We 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 themore »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.« less
5. Chemical abundances of open clusters from high-resolution infrared spectra – II. NGC 752

   https://doi.org/10.1093/mnras/stz3008  

   Böcek Topcu, G. ; Afşar, M. ; Sneden, C. ; Pilachowski, C. A. ; Denissenkov, P. A. ; VandenBerg, D. A. ; Wright, D. ; Mace, G. N. ; Jaffe, D. T. ; Strickland, E. ; et al ( October 2019 , Monthly Notices of the Royal Astronomical Society)

   We present a detailed near-infrared chemical abundance analysis of 10 red giant members of the Galactic open cluster NGC 752. High-resolution (R ≃ 45000) near-infrared spectral data were gathered with the Immersion Grating Infrared Spectrograph, providing simultaneous coverage of the complete H and K bands. We derived the abundances of H-burning (C, N, O), α (Mg, Si, S, Ca), light odd-Z (Na, Al, P, K), Fe-group (Sc, Ti, Cr, Fe, Co, Ni), and neutron-capture (Ce, Nd, Yb) elements. We report the abundances of S, P, K, Ce, and Yb in NGC 752 for the first time. Our analysis yields solar-metallicity and solar abundance ratios for almost all of the elements heavier than the CNO group in NGC 752. O and N abundances were measured from a number of OH and CN features in the H band, and C abundances were determined mainly from CO molecular lines in the K band. High-excitation $\rm{C\,\small {I}}$ lines present in both near-infrared and optical spectra were also included in the C abundance determinations. Carbon isotopic ratios were derived from the R-branch band heads of first overtone (2−0) and (3−1) 12CO and (2−0) 13CO lines near 23 440 Å and (3−1) 13CO lines at about 23 730 Å. The CNOmore »abundances and 12C/13C ratios are all consistent with our giants having completed ‘first dredge-up’ envelope mixing of CN-cyle products. We independently assessed NGC 752 stellar membership from Gaia astrometry, leading to a new colour–magnitude diagram for this cluster. Applications of Victoria isochrones and MESA models to these data yield an updated NGC 752 cluster age (1.52 Gyr) and evolutionary stage indications for the programme stars. The photometric evidence and spectroscopic light element abundances all suggest that the most, perhaps all of the programme stars are members of the helium-burning red clump in this cluster.

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