WIYN OPEN CLUSTER STUDY. LXVI. SPECTROSCOPIC BINARY ORBITS IN THE YOUNG OPEN CLUSTER M35 (NGC 2168)

Abstract

The young (150 Myr) open cluster M35 (NGC 2168) has been one of the core clusters of the WIYN Open Cluster Study since 1997. Over these 17 years we have obtained approximately 8000 radial-velocity (RV) measurements of stars in the M35 field, which we provide here. Our target sample consists of 1355 photometrically selected stars in the field of M35 within the main sequence and binary sequence of the cluster and within and . Using our RV measurements we cleanly separate likely cluster members from field stars. We calculate RV membership probabilities for over 1200 stars in our sample. 418 are probable cluster members, of which 64 are velocity-variable (binary) systems. Here we present 52 orbital solutions for binary members of M35. This sample defines the hard binary population of M35 that dynamically powers the cluster. We also present XMM-Newton X-ray detections within the cluster. We use our large binary sample to search for interacting binaries among the X-ray sources, investigate M35's period?eccentricity distribution, and determine binary frequency. We find a circularization period of 9.9 ? 1.2 days and a binary frequency of 24% ? 3% for main-sequence binaries with days. Determining these properties in a young cluster like M35 is key to defining the initial conditions used in models of cluster dynamical evolution.