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1. WIYN open cluster study: The old open cluster, NGC 188, and a re-evaluation of Lithium-richness among red giants

   https://doi.org/10.1093/mnras/stac1251  

   Sun, Qinghui; Deliyannis, Constantine P.; Twarog, Bruce A.; Anthony-Twarog, Barbara J.; Cummings, Jeffrey D.; Steinhauer, Aaron (May 2022, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We present WIYN1/Hydra spectra of 34 red giant candidate members of NGC 188, which, together with WOCS2 and Gaia data yield 23 single members, 6 binary members, 4 single non-members, and 1 binary non-member. We report [Fe/H] for 29 members and derive [Fe/H]NGC188  = +0.064 ± 0.018 dex (σμ) (sky spectra yield A(Fe)⊙  = 7.520 ± 0.015 dex (σμ)). We discuss effects on the derived parameters of varying Yale-Yonsei isochrones to fit the turnoff. We take advantage of the coolest, lowest gravity giants to refine the line list near Li 6707.8 Å. Using synthesis we derive detections of A(Li)3  = 1.17, 1.65, 2.04, and 0.60 dex for stars 4346, 4705, 5027, and 6353, respectively, and 3σ upper limits for the other members. Whereas only two of the detections meet the traditional criterion for ‘Li-richness’ of A(Li) > 1.5 dex, we argue that since the cluster A(Li) vanish as subgiants evolve to the base of the RGB, all four stars are Li-rich in this cluster’s context. An incidence of even a few Li-rich stars in a sample of 29 stars is far higher than what recent large surveys have found in the field. All four stars lie either slightly or substantially away from the cluster fiducial sequence, possibly providing clues about their Li-richness. We discuss a number of possibilities for the origin for the Li in each star, and suggest potentially discriminating future observations. 

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2. Open Cluster Chemical Homogeneity throughout the Milky Way

   https://doi.org/10.3847/1538-4357/abb93e  

   Poovelil, Vijith Jacob; Zasowski, G.; Hasselquist, S.; Seth, A.; Donor, John; Beaton, Rachael L.; Cunha, K.; Frinchaboy, Peter M.; García-Hernández, D. A.; Hawkins, K.; et al (November 2020, The Astrophysical Journal)

   null (Ed.)
3. WIYN Open Cluster Study LXXX: HDI CCD UBVRI Photometry of the Old Open Cluster NGC 7142 and Comparison to M67

   https://doi.org/10.3847/1538-3881/ab8814  

   Sun, Qinghui; Deliyannis, Constantine P.; Twarog, Bruce A.; Anthony-Twarog, Barbara J.; Steinhauer, Aaron (June 2020, The Astronomical Journal)
4. Double degenerate candidates in the open cluster NGC 6633

   https://doi.org/10.1017/S1743921320001040  

   Barnett, Joseph W.; Williams, Kurtis A. (October 2019, Proceedings of the International Astronomical Union)

   null (Ed.)

   Abstract The study of white dwarfs, the end stage of stellar evolution for more than 95% of stars, is critical to bettering our understanding of the late stages of the lives of low mass stars. In particular, the post main sequence evolution of binary star systems is complex, and the identification and analysis of double degenerate systems is a crucial step in constraining models of binary star systems. Binary white dwarfs in open star clusters are particularly useful because cluster parameters such as distance, metal content, and total system age are more tightly constrained than for field double degenerates. Here we use the precision astrometry from the Gaia Data Release 2 catalog to study two other white dwarfs which were identified as candidate double degenerates in the field of the open star cluster NGC 6633. One of the two objects, LAWDS 4, is found to have astrometric properties fully consistent with that of the cluster. In such a case, the object is significantly overluminous for a single white dwarf, strongly indicating binarity. The second candidate binary, LAWDS 7, appears to be inconsistent with cluster membership, though a more thorough analysis is necessary to properly quantify the probability. At present we are proceeding to model the photometric and spectroscopic data for both objects as if they were cluster member double degenerates. Results of this latter analysis are forthcoming. Our results will add crucial data to the study of binary star evolution in open star clusters. 

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5. WIYN Open Cluster Study 89. M48 (NGC 2548) 2: Lithium Abundances in the 420 Myr Open Cluster M48 from Giants through K Dwarfs* †

   https://doi.org/10.3847/1538-4357/acc5e3  

   Sun, Qinghui; Deliyannis, Constantine P.; Steinhauer, Aaron; Anthony-Twarog, Barbara J.; Twarog, Bruce A. (July 2023, The Astrophysical Journal)

   Abstract We consider WIYN/Hydra spectra of 329 photometric candidate members of the 420 Myr old open cluster M48 and report lithium detections or upper limits for 234 members and likely members. The 171 single members define a number of notable Li-mass trends, some delineated even more clearly than in Hyades/Praesepe: the giants are consistent with subgiant Li dilution and prior MS Li depletion due to rotational mixing. A dwarfs (8600–7700 K) have upper limits higher than the presumed initial cluster Li abundance. Two of five late A dwarfs (7700–7200 K) are Li-rich, possibly due to diffusion, planetesimal accretion, and/or engulfment of hydrogen-poor planets. Early F dwarfs already show evidence of Li depletion seen in older clusters. The Li–T_efftrends of the Li Dip (6675–6200 K), Li Plateau (6200–6000 K), and G and K dwarfs (6000–4000 K) are very clearly delineated and are intermediate to those of the 120 Myr old Pleiades and 650 Myr old Hyades/Praesepe, which suggests a sequence of Li depletion with age. The cool side of the Li Dip is especially well defined with little scatter. The Li–T_efftrend is very tight in the Li Plateau and early G dwarfs, but scatter increases gradually for cooler dwarfs. These patterns support and constrain models of the universally dominant Li depletion mechanism for FGK dwarfs, namely rotational mixing due to angular momentum loss; we discuss how diffusion and gravity-wave-driven mixing may also play roles. For late G/K dwarfs, faster rotators show higher Li than slower rotators, and we discuss possible connections between angular momentum loss and Li depletion. 

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