Using Stellar Evolution as a Clock to Watch the Dynamical Evolution of a Globular Cluster

We propose a 5 orbit HST program to acquire UV imaging at the center of the massive globular cluster NGC 362, in order to directly measure the rate of diffusion of stars through its core. This measurement will overcome a major observational barrier; that the dynamical time scale for two-body relaxation in the core of a globular cluster is typically much shorter than the age of the population, and so mass segregation in the cluster occured billions of years ago. Our novel technique aims to resolve this dynamical evolution by using the full power of WFC3's exquisite UV sensitivity at <0.3 microns combined with its high spatial resolution. We will uncover ~1000 newly formed stellar remnants - white dwarfs - in the center of the globular cluster and track how their spatial distribution changes as they get "older" on the cooling sequence. Having just experienced a significant episode of mass loss, the youngest white dwarfs with ages <10s of Myr will still be moving slowly like their 0.8 Msun progenitors, whereas the "older" white dwarfs that have been cooling for >100s of Myr will be fully relaxed. To "watch" this dynamical evolution and directly measure the diffusion coefficient, we have selected the very populous globular cluster NGC 362. The cluster is nearby with low reddening, has moderate concentration, and has a theoretical expected relaxation timescale of 60 Myr in its core, perfect to split the young and old white dwarfs that we can observe with Hubble.