摘要：Context.The origin and dynamical evolution of star clusters is an important topic in stellar astrophysics. Several models have been proposed in order to understand the formation of bound and unbound clusters and their evolution, and they can be tested by examining the kinematical and dynamical properties of clusters over a wide range of ages and masses.Aims.We use theGaia-ESO Survey products to study four open clusters (IC 2602, IC 2391, IC 4665, and NGC 2547) that lie in the age range between 20 and 50 Myr.Methods.We employ the gravity indexγand the equivalent width of the lithium line at 6708 Å together with effective temperatureTeffand the metallicity of the stars in order to discard observed contaminant stars. Then we derive the cluster radial velocity dispersionsσc, the total cluster massMtot, and the half mass radiusrhm. Using theGaia-DR1 TGAS catalogue, we independently derive the intrinsic velocity dispersion of the clusters from the astrometric parameters of cluster members.Results.The intrinsic radial velocity dispersions derived by the spectroscopic data are higher than those derived from the TGAS data, possibly due to the different masses of the considered stars. UsingMtotandrhmwe derive the virial velocity dispersionσvirand we find that three out of four clusters are supervirial. This result is in agreement with the hypothesis that these clusters are dispersing, as predicted by the “residual gas expulsion” scenario. However, recent simulations show that the virial ratio of young star clustersmay be overestimated if it is determined using the global velocity dispersion, since the clusters are not fully relaxed.
关键词：Key wordsenstars: pre-main sequencestars: kinematics and dynamicsopen clusters and associations: generalstars: formationtechniques: spectroscopictechniques: radial velocities
