A search for spectroscopic binaries in the Galactic globular cluster M4^*

Based on 5973 individual spectra collected at VLT

¹ ESO, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany e-mail: [vsommari;mrejkuba]@eso.org
² Astronomy Department, Padova University, vic. Osservatorio 2, 35122 Padova, Italy e-mail: piotto@unipd.it
³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD, USA e-mail: bedin@stsci.edu
⁴ School of Mathematics and Maxwell Institute for Mathematical Sciences, University of Edinburgh, Edinburgh, EH9 3JZ, UK e-mail: d.c.heggie@ed.ac.uk
⁵ Department of Astronomy, University of Wisconsin Madison, WI, 53706, USA e-mail: mathieu@astro.wisc.edu
⁶ Grupo de Astronomia, Departamento de Fisica, Universidad de Concepcion, Casilla 160-C, Concepcion, Chile e-mail: svillanova@astro-udec.cl

Received: 28 July 2008
Accepted: 2 October 2008

Abstract

Aims. We present a large multi-epoch high resolution spectroscopic investigation searching for binary candidates in the Galactic Globular Cluster (GGC) M4. The aim of our work is the identification of binary candidates, and the determination of the binary fraction and of the binary radial distribution.

Methods. We have obtained 5973 individual spectra of 2469 stars with the multifiber facility FLAMES+GIRAFFE at VLT. Selected stars range from the red giant branch tip to one magnitude below the TO, and cover the entire cluster. Each star has been observed at least twice, with a temporal interval of about three years. For 484 stars we have three epochs. Radial velocities were obtained with the classical cross-correlation technique, using a solar type spectrum as the template.

Results. The average radial velocity of the observed cluster members is 70.29 ± 0.07 (± 0.3)(± 0.1) km s^-1. The search for variations in radial velocities among the stars with multi-epoch observations yielded 57 binary star candidates. Our radial velocity measurement accuracy allowed us to identify at a  level binaries with radial velocity variations larger than ~0.3 km s^-1 for the target stars with V≤15, and larger than ~0.5 km s^-1 for the targets with V≥15. We identified 4 binary star candidates out of 97 observed targets inside the core radius, and 53 candidates out of 2372 observed stars outside the core radius. Accounting for the incompleteness affecting our survey, the lower limit for the total binary fraction is f = 3.0 ± 0.3%. The lower limit for the binary fraction in the cluster core is f = 5.1 ± 2.3%, while outside the core it decreases to f = 3.0 ± 0.4%. Similarly, we found f = 4.5 ± 0.4% and f = 1.8 ± 0.6% for the binary fraction inside and outside the half mass radius.