WIYN Open Cluster Study. XCVII. An Extended Radial-Velocity Survey and Spectroscopic Binary Orbits in the Open Cluster NGC 188

Abstract

We present 35 new spectroscopic-binary orbits from our extended radial-velocity (RV) survey of the old ($6.4 \pm 0.2$ Gyr) open cluster NGC 188. Using data from the WIYN Open Cluster Study (WOCS) and APOGEE-2, this work nearly doubles the temporal baseline of the previous RV study of NGC 188. We obtain orbital solutions within a stellar sample that spans a magnitude range of {$10.8 \leq \mathrm{G} \leq 16.5 \; (0.9\mbox{-}1.2 \; {M_\odot})$}. With revised membership determinations using Gaia DR3 proper-motions and parallaxes, we reassess the cluster binary frequency and period-eccentricity distribution. The incompleteness-corrected binary frequency is $33.1 \% \pm 3.8\%$ for periods less than $10^4$ days, and the tidal-circularization period is $14.4^{+0.14}_{-0.11}$ days. We find evidence that giants are deficient in short-period orbits, and suggest that the missing giants may have undergone mass transfer and in part formed the population of blue straggler stars (BSSs) and blue lurkers. Among the binaries of note, we highlight WOCS 3953 as a blue lurker candidate, WOCS 5020 and WOCS 4945 as very long-period eccentric BSSs, and WOCS 4230, a BSS with a very close WD companion.