Probability distribution () for the radial velocity difference between the primary star in a binary and its center of mass. Every line represents a binary with a fixed period and primary mass of 20 M_⊙, but random mass ratios, eccentricities, and phases drawn from flat distributions as well as a random orientation. The periods increase by factors of 10 and are labeled in the figure in units of years. The vertical dashed line marks the intrinsic velocity dispersion within 10 pc of R136 (~6 km   s^-1; Hénault-Brunet et al. 2012a).