WOCS XCV: NGC 7789: The Evolution of Li, Stellar Rotation, and Extended Main-sequence Turnoff

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, respectively, 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 (median absolute deviation) 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 decline with evolution toward the red giant tip, and stars within the clump and the asymptotic giant branch, which only exhibit upper limits. The 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>v</mml:mi> <mml:mi>sin</mml:mi> <mml:mi>i</mml:mi> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>v</mml:mi> <mml:mi>sin</mml:mi> <mml:mi>i</mml:mi> </mml:math> distributions. Prior indications that main-sequence stars more massive than the Li dip evolve redward across the Li wall, undergoing rotational spin-down and Li depletion like stars within the Li dip, are confirmed.