Absolute motions of globular clusters

II. HST astrometry and VLT radial velocities in NGC 6397

¹ Dip. di Astronomia, Univ. degli studi di Padova, vic. Osservatorio 2, 35122 Padova, Italy e-mail: milone-villanova-piotto@pd.astro.it
² European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany e-mail: lbedin@eso.org
³ Andes Fellow, Departamento de Astronómia, Universidad de Chile, Casilla 36-d, Santiago, Chile e-mail: gcarraro@das.uchile.cl
⁴ Dept.  of Physics and Astronomy, Mail Stop 108, Rice University, 6100 Main Street, Houston, TX 77005, USA e-mail: jay@eeyore.rice.edu
⁵ Dept. of Astronomy, Univ. of Washington, Box 351580, Seattle, WA 98195-1580, USA e-mail: king@astro.washington.edu
⁶ INAF - Osservatorio Astronomico di Trieste, via Tiepolo 11, 34131 Trieste, Italy e-mail: zaggia@ts.astro.it

Received: 3 February 2006
Accepted: 27 April 2006

Abstract

In this paper we present a new, accurate determination of the three components of the absolute space velocity of the Galactic globular cluster NGC 6397 (, ). We used three HST/WFPC2 fields with multi-epoch observations to obtain astrometric measurements of objects in three different fields in this cluster. The identification of 33 background galaxies with sharp nuclei allowed us to determine an absolute reference point and measure the absolute proper motion of the cluster. The third component was obtained from radial velocities measured from spectra from the multi-fiber spectrograph FLAMES at UT2-VLT. We find (, )_J2000.0 = (, ) mas yr^-1 and (±0.10) km s^-1. Assuming a Galactic potential, we calculate the cluster orbit for various assumed distances and briefly discuss the implications.