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

TL;DR

NGC 7789 is a rich, intermediate-age open cluster with near-solar metallicity and substantial reddening, making it an excellent testbed for Li evolution, rotation, and extended main-sequence turnoffs (eMSTOs). The paper investigates Li evolution, stellar rotation, and CMD topology using precision $UBVRI$ photometry, Gaia astrometry, and HYDRA spectroscopy to derive reddening, metallicity, distance, and age and to assemble a clean single-star member sample. They determine $E(B-V)=0.30\pm0.02$, $(m-M)=12.51\pm0.06$, and age $1.46\pm0.02$ Gyr, and [Fe/H]$=-0.13\pm0.068$ (MAD) from 156 single stars, with differential reddening corrections reducing the unevolved MS scatter. Li abundances reveal a Li-Dip extending from the MSTO to the unevolved MS and two giant groups with distinct Li patterns; the color spread on the upper main sequence is linked to the $V_{ROT}$ distribution, with the Li-Dip wall marking the hot boundary of the Kraft break. The results, contrasted with NGC 752, illuminate how rotation shapes Li depletion and CMD topology in intermediate-age clusters and support the view that rotationally driven processes govern Li evolution beyond the Li-Dip.

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.

Figures (1)

• Figure 1: Standard-errors-of-the-mean (sem) for each index and $V$ as a function of $V$ mag.