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Post Info TOPIC: NGC 6819


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Posts: 131433
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RE: NGC 6819
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Title: The Puzzling Li-rich Red Giant Associated with NGC 6819
Author: Joleen K. Carlberg, Verne V. Smith, Katia Cunha, Steven R. Majewski, Szabolcs Meszaros, Matthew Shetrone, Carlos Allende Prieto, Dmitry Bizyaev, Keivan G. Stassun, Scott W. Fleming, Gail Zasowski, Fred Hearty, David L. Nidever, Donald P. Schneider, Jon A. Holtzman, Peter M. Frinchaboy

A Li-rich red giant star (2M19411367+4003382) recently discovered in the direction of NGC 6819 belongs to the rare subset of Li-rich stars that have not yet evolved to the luminosity bump, an evolutionary stage where models predict Li can be replenished. The currently favoured model to explain Li enhancement in first-ascent red giants like 2M19411367+4003382 requires deep mixing into the stellar interior. Testing this model requires a measurement of 12C/13C, which is possible to obtain from APOGEE spectra. However, the Li-rich star also has abnormal asteroseismic properties that call into question its membership in the cluster, even though its radial velocity and location on colour-magnitude diagrams are consistent with membership. To address these puzzles, we have measured a wide array of abundances in the Li-rich star and three comparison stars using spectra taken as part of the APOGEE survey to determine the degree of stellar mixing, address the question of membership, and measure the surface gravity. We confirm that the Li-rich star is a red giant with the same overall chemistry as the other cluster giants. However, its log g is significantly lower, consistent with the asteroseismology results and suggestive of a very low mass if the star is indeed a cluster member. Regardless of the cluster membership, the 12C/13C and C/N ratios of the Li-rich star are consistent with standard first dredge-up, indicating that Li dilution has already occurred, and inconsistent with internal Li enrichment scenarios that require deep mixing.

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Posts: 131433
Date:
Melotte 223
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NGC 6819 (also Fox Head cluster, Melotte 223, Collinder 403) is a magnitude +7.3 open cluster located 7,200 light-years away in the constellation Cygnus.
NGC 6819 contains roughly two dozen stars of magnitude 10 to 12, with around 150 member stars ranging from magnitude 11 to 15. 

NGC 6819 was discovered by Caroline Herschel at Datchet, Berkshire, on the 12th May 1784.

Right ascension19h 41m 18s,  Declination+40° 11' 00"

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Posts: 131433
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NGC 6819
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Title: A Lithium-Rich Red Giant Below the Clump in the Kepler Cluster, NGC 6819
Authors: Barbara J. Anthony-Twarog, Constantine P. Deliyannis, Evan Rich, Bruce A. Twarog

WIYN/HYDRA spectra in the Li 6708 Angstrom region have been obtained for 332 probable members of the old open cluster, NGC 6819. Preliminary analysis shows a pattern of Li depletion from the top of the turnoff to the base of the giant branch. Starting one magnitude below the level of the clump, all brighter giants have A(Li) below 1.0, with most having upper limits below 0.5. Star W007017, located BELOW the first-ascent red giant bump is Li-rich with A(Li) = 2.3. As a highly probable single-star astrometric and radial-velocity cluster member, its discrepant asteroseismic membership could be a by-product of the processes that triggered Li-enhancement. Its colour-magnitude diagram location is consistent with only one proposed enhanced mixing process among first-ascent red giants.

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