Radial velocities in the globular cluster ω Centauri

¹ Sterrewacht Leiden, Postbus 9513, 2300 RA Leiden, The Netherlands e-mail: omegacentauri@reijns.com; Richard.Arnold@mcs.vuw.ac.nz; bosch@strw.leidenuniv.nl; vdven@strw.leidenuniv.nl; dezeeuw@strw.leidenuniv.nl
² Department of Astronomy, University of Michigan, Ann Arbor, MI 48109, USA e-mail: seitzer@umich.edu
³ Research School of Astronomy & Astrophysics, Australian National University, Mt. Stromlo Observatory, Cotter Road, Weston ACT 2611, Australia e-mail: kcf@mso.anu.edu.au
⁴ Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatories, Casilla 603, La Serena, Chile e-mail: tingerson@noao.edu

Received: 14 March 2005
Accepted: 7 September 2005

Abstract

We have used the ARGUS multi-object spectrometer at the CTIO 4 m Blanco telescope to obtain 2756 radial velocity measurements for 1966 individual stars in the globular cluster ω Centauri brighter than blue photographic magnitude of about 16.5. Of these, 1589 stars are cluster members. A comparison with two independent radial velocity studies, carried out by Suntzeff & Kraft and by Mayor et al., demonstrates that the median error of our measurements is below 2 km s^-1 for the stars brighter than B-magnitude 15, which constitute the bulk of the sample. The observed velocity dispersion decreases from about 15 km s^-1 in the inner few arcmin to about 6 km s^-1 at a radius of 25'. The cluster shows significant rotation, with a maximum amplitude of about 6 km s^-1 in the radial zone between 6' and 10'. In a companion paper by van de Ven et al., we correct these radial velocities for the perspective rotation caused by the space motion of the cluster, and combine them with the internal proper motions of nearly 8000 cluster members measured by van Leeuwen et al., to construct a detailed dynamical model of ω Centauri and to measure its distance.