Co-moving groups around massive stars in the nuclear stellar disk

Abstract

Over the last sim 30\,Myr, the nuclear stellar disk in the Galactic center has been the most prolific star-forming region of the Milky Way when averaged by volume. Remarkably, the combined mass of the only three clusters present today in the nuclear stellar disk adds up to only sim 10<!PCT!> of the total expected mass of young stars formed in this period. Several causes could explain this apparent absence of clusters and stellar associations. The stellar density in the area is so high that only the most massive clusters would be detectable against the dense background of stars. The extreme tidal forces reigning in the Galactic center could dissolve even the most massive of the clusters in just a few megayears. Close encounters with one of the massive molecular clouds, which are abundant in the nuclear stellar disk, can also rapidly make any massive cluster or stellar association dissolve beyond recognition. However, traces of some dissolving young clusters and associations could still be detectable as co-moving groups. It is our aim to identify so far unknown clusters or groups of young stars in the Galactic center. We focus our search on known, spectroscopically identified massive young stars to see whether their presence can pinpoint such structures. We created an algorithm to detect over-densities in the 5D space spanned by proper motions, positions on the plane of the sky, and line-of-sight distances, using reddening as a proxy for the distances. Since co-moving groups must be young in this environment, proper motions provide a good means to search for young stars in the Galactic center. As such, we combined publicly available data from three different surveys of the Galactic center, covering an area of sim \,160 arcmin$^2$ on the nuclear stellar disk. We find four co-moving groups around massive stars, two of which are very close in position and velocity to the Arches' most likely orbit. These co-moving groups are strong candidates to be clusters or associations of recently formed stars, showing that not all the apparently isolated massive stars are run-away former members of any of the three known clusters in the Galactic center or simply isolated massive stars. Our simulations show that these groups or clusters may dissolve beyond our limits of detection in less than sim 6\,Myr.