Star Clusters

Dense star clusters can have hundreds of thousands of stars in the space that typically holds just a few stars in the Solar neighborhood. Dynamical interactions in these clusters can drive binaries to ever-closer separations, exchange binary members, or eject stars from the cluster entirely. The Galaxy's globular clusters have a number of puzzling properties, including unexplained spreads in their chemical abundances. They are also home to exotic objects like millisecond pulsars, neutron stars that have accreted mass and angular momentum from a close companion until they rotate hundreds of times per second. Millisecond pulsars slowly radiate their immense stores of rotational energy over many billions of years.

The Galaxy's population of globular star clusters is dynamic. Clusters slowly evaporate as their most fastest moving members escape, with tight binaries serving as a store of energy. Clusters can also disrupt if they pass too close to the Galaxy's center (where the tidal field is strong), spilling all of their contents into the field. A few percent of stars in the inner Galactic bulge have odd chemical abundances, which are normally only seen in globular clusters. An excess of gamma rays has also been observed around the Galactic center. The spectrum of these excess gamma rays is well-matched by a population of millisecond pulsars.

The excess gamma rays from the Galactic center are very exciting as possible products of dark matter annihilation, but this interpretation requires ruling out astrophysical sources. Millisecond pulsars from disrupted clusters are a well-motivated background, and match the spectrum and normalization of the excess. The disrupted cluster hypothesis is also consistent with field stars with globular cluster characteristics. If the gamma rays are indeed from millisecond pulsars from disrupted clusters, they provide an important constraint on the history of the Galaxy's cluster population.