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

TL;DR

This study uses Hubble Space Telescope data to analyze the kinematic profiles and estimate the average masses of blue straggler stars in Galactic globular clusters, revealing they are more massive than typical stars and consistent with previous measurements.

Contribution

First to derive radial velocity-dispersion profiles for BSSs in GCs and estimate their average masses using energy equipartition principles and recent simulations.

Findings

BSSs have lower velocity dispersions than main-sequence turnoff stars.

Estimated average BSS mass is approximately 1.22 solar masses.

Results agree with previous individual mass measurements of BSSs.

Abstract

We make use of the Hubble Space Telescope proper-motion catalogs derived by Bellini et al. (2014) to produce the first radial velocity-dispersion profiles sigma(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 BSS population. We show that BSSs typically have lower velocity dispersions than stars with mass equal to the main-sequence turnoff mass, as one would expect for a more massive population of stars. Since GCs are expected to experience some degree of energy equipartition, we use the relation sigma~M^-eta, where eta is related to the degree of energy equipartition, along with our velocity-dispersion profiles to estimate BSS masses. We estimate eta as a function of cluster relaxation from recent Monte Carlo cluster simulations by Bianchini 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 BSSs across 19 GCs. The final error bars include any systematic errors that are random between different clusters, but not any potential biases inherent to our methodology. Our results are in good agreement with the average mass of M_BSS=1.22+/-0.06 M_Sun for the 35 BSSs in Galactic GCs in the literature with properties that have allowed individual mass determination.