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arXiv · 2510.25862

WOCS XCIII: NGC 7789: the Evolution of Li, Stellar Rotation, and Extended Main Sequence Turnoffs

Barbara J. Anthony-Twarog·Samantha W. Brunker·Constantine P. Deliyannis·Evan Rich·Aaron Steinhauer·Qinghui Sun·Bruce A. Twarog
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Abstract

Precision UBVRI photometry of NGC 7789 is combined with Gaia data to map reddening variations across the cluster face. HYDRA spectra, Gaia astrometry, and isochrone fitting constrain the absolute reddening, apparent modulus, and age to E(B-V) = 0.30 +/- 0.02, (m-M)=12.51 +/- 0.06, and 1.46 +/- 0.02 Gyr for [Fe/H] between -0.2 and solar; the spectroscopic [Fe/H] = -0.13 +/- 0.068 (MAD) from 156 single-star members. Corrections for variable reddening reduce the scatter in the unevolved main sequence below the turnoff. A(Li) is derived for only single star members from the G-dwarf Li-Plateau to the tip of the red giant branch. Giants separate into two distinct groups, probable first-ascent giants with detectable Li that declines with evolution toward the red giant tip and stars within the clump and the asymptotic giant branch which only exhibit upper limits. A(Li) structure from the turnoff to the unevolved main sequence, including the Li-Dip, and the presence of an extended color spread among the upper main sequence stars are attributed to the V_ROT distribution, indicating the wall of the Li-Dip as the true hot boundary of the Kraft break. Differences in the color-magnitude diagram topology of NGC 7789 and NGC 752 are explored and attributed to differences in the individual cluster V_ROT distributions. Prior indications that main sequence stars more massive than the Li-Dip evolve redward across the Li-Wall, undergoing rotational spindown and Li depletion like stars within the Li-Dip, are confirmed.

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Barbara J. Anthony-Twarog, Samantha W. Brunker, Constantine P. Deliyannis, Evan Rich, Aaron Steinhauer, Qinghui Sun, Bruce A. Twarog. 2025-10-29. WOCS XCIII: NGC 7789: the Evolution of Li, Stellar Rotation, and Extended Main Sequence Turnoffs. https://arxiv.org/abs/2510.25862

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