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We make use of the Hubble Space Telescope proper-motion catalogs derived byBellini et al. (2014) to produce the first radial velocity-dispersion profilessigma(R) for blue straggler stars (BSSs) in Galactic globular clusters (GCs),as well as the first dynamical estimates for the average mass of the entire BSSpopulation. We show that BSSs typically have lower velocity dispersions thanstars with mass equal to the main-sequence turnoff mass, as one would expectfor a more massive population of stars. Since GCs are expected to experiencesome degree of energy equipartition, we use the relation sigma~M^-eta, whereeta is related to the degree of energy equipartition, along with ourvelocity-dispersion profiles to estimate BSS masses. We estimate eta as afunction of cluster relaxation from recent Monte Carlo cluster simulations byBianchini et al. (2016b) and then derive an average mass ratio M_BSS/M_MSTO=1.50+/-0.14 and an average mass M_BSS=1.22+/-0.12 M_Sun from 598 BSSsacross 19 GCs. The final error bars include any systematic errors that arerandom between different clusters, but not any potential biases inherent to ourmethodology. Our results are in good agreement with the average mass ofM_BSS=1.22+/-0.06 M_Sun for the 35 BSSs in Galactic GCs in the literature withproperties that have allowed individual mass determination.

astrophysical journal/the astrophysical journal

Hubble Space Telescope proper motion (HSTPROMO) catalogs of Galactic  globular clusters. IV. Kinematic profiles and average masses of blue  straggler stars