Colliding star clusters

If it’s gigantic enough, a cold cloud of molecules can collapse and fragment under its own gravity to give birth to a litter of thousands of new stars: an open cluster. The stars’ mutual gravity is strong enough to hold the cluster together as it orbits its galactic host, but the attraction is too weak to keep cluster members from eventually straying, either on their own or as result of a dynamically disruptive event.

Galileo Galilei and other early telescope-wielding astronomers identified open clusters as improbable congregations of similar stars. Now clusters can be automatically cataloged by algorithms that trawl through astrometric data. In April 2021 Wilton Dias of the Federal University of Itajubá in Brazil and his collaborators published an updated catalog of 1743 open clusters based on an analysis of data gathered by the European Space Agency’s Gaia spacecraft. Andrés Piatti of the Interdisciplinary Institute of Basic Science in Mendoza, Argentina, and Khyati Malhan of Stockholm University in Sweden have now used that catalog set to look for pairs of clusters that are close together.

Close clusters are rare. And when they do occur, they tend to be of similar ages, which suggests that they formed from the same giant molecular cloud. But the members of one pair that Piatti and Malhan found, IC 4665 (shown here) and Collinder 350, have ages that differ by more than 500 million years. What’s more, the clusters’ stellar populations overlap: IC 4665 and Collinder 350 appear to be merging.

The disparate ages suggest that IC 4665 and Collinder 350 could have formed in different parts of the Milky Way. To see whether that was the case, Piatti and Malhan tracked the members of the two clusters back in time. They first noted the stars’ current positions in the six-dimensional phase space made up of the stars’ 3D positions and 3D velocities. For the source of gravity acting on the stars, they used two different continuous models of the distribution of gravitational mass in the galaxy. Both models include an extended disk, a central bulge, and a dark-matter halo. They differ in total mass.

Piatti and Malhan integrated the stars’ equations of motion backward for 80 Myr. Although the two mass models gave slightly different answers, the result was the same: 60 Myr ago, IC 4665 and Collinder 350 were around 500 parsecs (1500 light-years) apart. That distance is more than twice as large as the diameters of the biggest molecular clouds. The two clusters had different parents.

Whether IC 4665 and Collinder 350 will coalesce or separate after passing through each other remains to be determined. The question pertains to how an observed spread in the ages and chemical composition of a cluster are accounted for. If coalescence is a possibility, distinct episodes of star formation need not be invoked. (A. E. Piatti, K. Malhan, Mon. Not. R. Astron. Soc.: Lett., 2021, doi:10.1093/mnrasl/slab130 .)