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

TL;DR

NGC 188 is analyzed with an extended radial-velocity survey that combines WIYN-Hydra and APOGEE-2 data to nearly double the temporal baseline of previous studies. The study leverages Gaia DR3 to refine membership, yielding a 546-star core sample and identifying 35 new single-lined spectroscopic binaries, resulting in an incompleteness-corrected MS binary fraction of $P<10^4$ days of $33.1\% \pm 3.8\%$. A tidal circularization period of $P_{circ}=14.4^{+0.14}_{-0.11}$ days is derived via an MCMC-based analysis of the $e$–$\log P$ distribution, reaffirming tidal processing on the main sequence. The results illuminate mass-transfer pathways that produce blue stragglers and blue lurkers, with notable systems such as WOCS 3953, 4945, 4230, and 5020 highlighting diverse post-interaction outcomes. Overall, the work advances understanding of binary evolution in old open clusters and the role of mass transfer in shaping the MS, BSS, and BL populations.

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.

Figures (20)

• Figure 1: The two-dimensional proper motions of stars with magnitudes G $\leq$ 16.5 in the field of NGC 188 are displayed. Proper-motion members of NGC 188 are shown in red, while field stars are depicted in gray. Contours in the main panel represent the $1\sigma$, $2\sigma$, and $3\sigma$ levels of the Gaussian distribution for the cluster, along with the $0.5\sigma$ level for the field’s Gaussian distribution. The top and side panels present the one-dimensional projections of the stellar and Gaussian distributions for the cluster and the field, the latter plotted in red and gray, respectively.
• Figure 2: The percentage by G magnitude of proper-motion members that have at least one (dashed line) or three (solid line) RV measurements over a time interval of at least two years. We show the completeness of our sample with only WOCS observations (black) and our full dataset (red).
• Figure 3: RV distributions of stars in the cluster (red) and the field (gray). Distributions were fit to non-velocity variable stars. Stars without orbits were binned using their mean RV measurement, meaning some member binary stars without orbit solutions may appear to not be members in this figure.
• Figure 4: The color-magnitude diagram of member stars of NGC 188. Single stars are plotted as blue circles and binary stars are plotted as red diamonds. Stars have been differentially dereddened using the SFD dustmap along with an additional correction of $\mathrm{ E(G_{BP}-G_{RP})} = 0.0089$ from our isochrone fit. The zero-age main sequence is plotted as a dashed line. Our best fit MIST isochrone of 6.4 Gyr, [Fe/H] = $-0.024$ is shown as a solid line. The BSS region of NGC 188 is shaded in light blue. The upper bound is the terminal-age main sequence.
• Figure 5: NGC 188 SB1 orbit plots. For each binary in Table \ref{['table:orbits']}, we plot RV against orbital phase, showing the data points with black dots and the orbital fit to the data with the solid line; the dotted line marks the $\gamma$-velocity. Beneath each orbit plot, we show the residuals from the fit. Above each plot, we give the binary ID and orbital period.