Rotation in Globular Cluster stars^*

Turn-off and subgiant stars in NGC 104, NGC 6397 and NGC 6752

¹ Dipartimento di Astronomia, Università di Padova, Vicolo dell'Osservatorio, 2, 35122 Padova, Italy
² INAF – Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, 35122 Padova, Italy e-mail: gratton@pd.astro.it

Corresponding author: S. Lucatello, lucatello@pd.astro.it

Received: 16 July 2001
Accepted: 21 May 2003

Abstract

We present a derivation of upper limits to the rotation of Main Sequence stars in three globular clusters using spectra obtained during time allocated to the ESO Large Programs 165-L0263 and 167.D-0173, with UVES at VLT2 (Kueyen). The stars analyzed in this work do not show any evidence of high values of rotational velocities as far as the outer layers are concerned, in particular robust estimates for the upper limits of the values of the mean projected rotational velocities are placed, about  km s^-1 and about  km s^-1  for, respectively, the program turn-off and subgiant stars. On the basis of statistical considerations, assuming that inclination of the rotational axis i is randomly oriented, and that all stars within the same group rotate at the same rate, we obtain estimates for the values of the true rotational velocities. These values are upper limits to the true rotational velocities if there is some star-to-star scatter in rotational or macro-turbulent velocities. The mean values of these upper limits for  for the stars of the same spectral type averaged over the three clusters are found to be respectively,  km s^-1 and  km s^-1. Thus, the explanation for the large rotational velocities found for the Horizontal Branch stars must be looked for either in the rotation of the core not detectable in the outer layers (Sills & Pinsennault [CITE]) or in the acquisition of angular momentum during their evolution.