WIYN Open Cluster Study. LIX. Radial-Velocity Membership of the Evolved Population of the Old Open Cluster NGC 6791

TL;DR

This study provides precise radial velocities for evolved stars in NGC 6791, confirming membership, analyzing blue stragglers, and estimating the cluster's dynamical mass using WIYN data.

Contribution

It offers the first detailed radial velocity membership analysis of NGC 6791's evolved population, including blue stragglers, and derives the cluster's dynamical mass.

Findings

93% of stars have at least one radial velocity measurement.

79% have three measurements over 200 days, enabling secure membership.

The cluster's velocity dispersion is 0.62 km/s, implying a mass of about 4600 solar masses.

Abstract

The open cluster NGC 6791 has been the focus of much recent study due to its intriguing combination of old age and high metallicity ($\sim$8 Gyr, [Fe$/$H]=$+$0.30), as well as its location within the Kepler field. As part of the WIYN Open Cluster Study, we present precise ($\sigma=0.38$ km s$^{-1}$) radial velocities for proper-motion candidate members of NGC 6791 from Platais et al. Our survey, extending down to $g^\prime\sim16.8$, is comprised of the evolved cluster population, including blue stragglers, giants, and horizontal branch stars. Of the 280 proper-motion-selected stars above our magnitude limit, 93% have at least one radial-velocity measurement and 79% have three measurements over the course of at least 200 days, sufficient for secure radial-velocity-determined membership of non-velocity-variable stars. The Platais et al. proper-motion catalog includes twelve anomalous horizontal branch candidates blueward of the red clump, of which we find only four to be cluster members. Three fall slightly blueward of the red clump and the fourth is consistent with being a blue straggler. The cleaned color-magnitude diagram shows a richly populated red giant branch and a blue straggler population. Half of the blue stragglers are in binaries. From our radial-velocity measurement distribution we find the cluster's radial-velocity dispersion to be $\sigma_c=0.62\pm0.10$ km s$^{-1}$. This corresponds to a dynamical mass of $\sim$4600 $M_\odot$.