We derive the proper motions , membership probabilities , and velocity dispersions of stars in the regions of the young ( \sim 2 - 4 Myr-old ) open clusters NGC 2244 ( the central cluster in the Monoceros R2 association ) and NGC 6530 ( the dominant cluster in the Sgr OB1 association ) from photographic plate material obtained at Shanghai Astronomical Observatory , with time baselines of 34 and 87 years , respectively .
Both clusters show clear evidence of mass segregation , but they do not exhibit any significant velocity-mass ( or , equivalently , a velocity-luminosity ) dependence .
This provides strong support for the suggestion that the observed mass segregation is – at least partially – due to the way in which star formation has proceeded in these complex star-forming regions ( “ primordial ” mass segregation ) .
Based on arguments related to the clusters ’ published initial mass functions , in conjunction with our new measurements of their internal velocity dispersions ( \sim 35 and 8 km s ^ { -1 } for NGC 2244 and NGC 6530 , respectively ) , we provide strong arguments in favor of the dissolution of NGC 2244 on very short time-scales , while we speculate that NGC 6530 may be more stable against the effects of internal two-body relaxation .
However , this latter object may well be destroyed by the strong tidal field prevalent at its location in the Galactic plane in the direction of the Galactic Center .