Mass segregation in Palomar 14

The mass function slope is shown as it changes with radius of Palomar 14 (measured from the cluster center). A low value of this slope means that we detect more massive stars at this radius than further out, where the slope is larger. Such a signature is called mass segregation, and is usually a consequence of dynamical evolution of a star cluster.

Matthias Frank, Eva Grebel (both in Heidelberg), and I have recently published another paper on one of our Milky Way’s outer-halo globular clusters. Using archival data from the Hubble Space Telescope and Matthias’ sophisticated photometry tools, we measured the masses of stars within one of the most controversial globular clusters known to us: Palomar 14. 

With a distance of about 70 kpc (or 230,000 light years) from the Galactic Center, Palomar 14 is one of the most distant star clusters of our Galaxy. It is also one of the least massive clusters, and, rather surprisingly, it is the most extended stellar system without dark matter in the nearby Universe. As such, it has been the subject of numerous observational and theoretical studies since its discovery half a century ago by Sidney van den Bergh. According to Sidney, it was nothing more than a faint smudge on one of the photographic plates of the Palomar Observatory Sky Survey. This unusual fuzziness is due to its large extent: compared to regular globular clusters of the same mass, which show a characteristic radius of 3 pc (10 light years), Palomar 14 has a radius of 46 pc (150 light years)!

This was also the motivation for our investigation: in such a loosely bound system you don’t expect the stars to interact much – they stay together as a system, but they don’t exchange energy through fly-bys like all these NASA satellites do with planets when they swing by an inner planet to gain velocity in order to reach a more distant planet/comet/whatever. However, consequences of this energy-exchange process are clearly visible in Palomar 14: the most massive stars tend to sit in the center of the cluster, whereas the least massive ones preferentially orbit further out (summarized in the plot above). This means that either Palomar 14 must have been more than a factor of 10 denser in the past, or it was born with this very unusual configuration. Both scenarios give us a headache, since we haven’t been able to reproduce them in simulations. Even more so, since its distant companion, Palomar 4, has exactly the same issues!


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