WIYN OPEN CLUSTER STUDY. XXXVIII. STELLAR RADIAL VELOCITIES IN THE YOUNG OPEN CLUSTER M35 (NGC 2168) Aaron M. Geller $^{∗}$, Robert D. Mathieu$^{∗}$, Ella K. Braden$^{∗}$, Søren Meibom∗ , † Department of Astronomy, University of Wisconsin - Madison, WI 53706, USA and Imants Platais Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD 21218, USA and Christopher J. Dolan ∗ Department of Astronomy, University of Wisconsin - Madison, WI 53706, USA ABSTRACT We present 5201 radial-velocity measurements of 1144 stars, as part of an ongoing study of the young (150 Myr) open cluster M35 (NGC 2168). We have observed M35 since 1997, using the Hydra Multi-Object Spectrograph on the WIYN 3.5m telescope. Our stellar sample covers main-sequence stars over a magnitude range of 13.0 ≤ V ≤ 16.5 (1.6 - 0.8 M ⊙ ) and extends spatially to a radius of 30 arcminutes (7 pc in projection at a distance of 805 pc or ∼ 4 core radii). Due to its youth, M35 provides a sample of late-type stars with a range of rotation periods. Therefore, we analyze the radial- velocity measurement precision as a function of the projected rotational velocity. For narrow-lined stars ( v sin i ≤ 10 km s − $^{1}$), the radial velocities have a precision of 0.5 km s − $^{1}$, which degrades to 1.0 km s − 1 for stars with v sin i = 50 km s − $^{1}$. The radial-velocity distribution shows a well-defined cluster peak with a central velocity of -8.16 ± 0.05 km s − $^{1}$, permitting a clean separation of the cluster and field stars. For stars with ≥ 3 measurements, we derive radial-velocity membership probabilities and identify radial-velocity variables, finding 360 cluster members, 55 of which show significant radial- velocity variability. Using these cluster members, we construct a color-magnitude diagram for our stellar sample cleaned of field star contamination. We also compare the spatial distribution of the single and binary cluster members, finding no evidence for mass segregation in our stellar sample. Accounting for measurement precision, we place an upper limit on the radial-velocity dispersion of the cluster of 0 . 81 ± 0 . 08 km s − $^{1}$. After correction for undetected binaries, we derive a true radial- velocity dispersion of 0 . 65 ± 0 . 10 km s − $^{1}$. (galaxy:) open clusters and associations: individual (NGC 2168) - (stars:) binaries: spectroscopic open cluster simulation. M35 is a fundamental cluster in the WIYN Open Clus- ter Study (WOCS; Mathieu 2000), and as such has a strong base of astrometric and photometric observations from both the WOCS collaboration and others. The clus- ter is centered at α = 6 $^{h}$09$^{m}$07 . $^{s}$5 and δ = +24 $^{◦}$20$^{′}$28′′ (J2000), towards the Galactic anticenter. Numerous pho- tometric studies have identified the rich main-sequence population (e.g., Kalirai et al. 2003; von Hippel et al. 2002; Sung & Bessell 1999). WOCS CCD photometry places the cluster at a distance of 805 ± 40 pc, with an age of 150 ± 25 Myr, a metallicity of [Fe/H]=-0.18 ± 0.05 and a reddening of E ( B − V )=0.20 ± 0.01 (C. Deliyan- nis, private communication). The most recent published parameters, from Kalirai et al. (2003), place the cluster at a distance of 912 +70 − 65 pc (( m − M ) 0 = 9.80 ± 0.16) with an age of 180 Myr, adopting a E ( B − V )=0.20 and [Fe/H] = -0.21. (See Kalirai et al. (2003) for a thorough review of previous photometry references and their de- rived cluster parameters). We note that these two recent studies used different isochrone families. There have been multiple proper-motion studies of the cluster (Ebbighausen 1942; Cudworth 1971; McNamara & Sekiguchi 1986a), although none deter- mine cluster membership for individual stars fainter than $^{∗}$Visiting Astronomer, Kitt Peak National Observatory, National Optical Astronomy Observatory, which is operated by the Asso- ciation of Universities for Research in Astronomy (AURA) under cooperative agreement with the National Science Foundation. $^{†}$Current address: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA Young open clusters are laboratories for the direct study of the near-primordial characteristics of stellar populations. Their properties, and particularly those of the binary systems, offer unique insights into how stars are born and provide essential guidance for N -body stud- ies of star clusters. Indeed, with sophisticated N -body simulations now able to model real open clusters (e.g., Hurley et al. 2005), knowledge of the correct initial con- ditions are all the more important. In particular, the ini- tial binary population has a vast impact on the dynami- cal evolution of the cluster, and the characteristics of the initial binary population will affect the overall frequency, formation rate and formation mechanisms of anomalous stars, like blue stragglers, as interactions with binaries are thought to be catalysts for the formation of these ex- otic objects (Hurley et al. 2005; Knigge et al. 2009). As a rich open cluster with an age of ∼ 150 Myr, M35 is a prime cluster to define these hitherto poorly known ini- tial conditions for the binary population required for any arXiv:1001.0033v1 [astro-ph.SR] 30 Dec 2009 1. INTRODUCTION