Kinematics and binaries in young stellar aggregates ^*,**

II. NGC 6913 ≡ M 29

¹ Osservatorio Astronomico di Padova, Sede di Asiago, 36012 Asiago (VI), Italy
² Osservatorio Astrofisico del Dipartimento di Astronomia, Universitá di Padova, 36012 Asiago (VI), Italy

Corresponding author: U. Munari, munari@pd.astro.it

Received: 10 July 2003
Accepted: 7 October 2003

Abstract

Between 1996 and 2003 we obtained 226 high resolution spectra of 16 stars in the field of the young open cluster NGC 6913, to constrain its main properties and study its internal kinematics. Twelve of the program stars turned out to be members, one of them probably unbound. Nine are binaries (one eclipsing and another double lined) and for seven of them the observations allowed us to derive the orbital elements. All but two of the nine discovered binaries are cluster members. In spite of the young age (a few Myr), the cluster already shows signs that could be interpreted as evidence of dynamical relaxation and mass segregation. However, they may be also the result of an unconventional formation scenario. The dynamical (virial) mass as estimated from the radial velocity dispersion is larger than the cluster luminous mass, which may be explained by a combination of the optically thick interstellar cloud that occults part of the cluster, the unbound state or undetected very wide binary orbit of some of the members that inflate the velocity dispersion and a high inclination for the axis of possible cluster angular momentum. All the discovered binaries are hard enough to survive average close encounters within the cluster and do not yet show signs of relaxation of the orbital elements to values typical of field binaries.