The Binary Fraction of Stars in Dwarf Galaxies: the Cases of Draco and Ursa Minor

Measuring the frequency of binary stars in dwarf spheroidal galaxies (dSphs) requires data taken over long time intervals. We combine radial velocity measurements from five literature sources taken over the course of ~30 years to yield the largest multi-epoch kinematic sample for stars in the dSphs Draco and Ursa Minor. With this data set, we are able to implement an improved version of the Bayesian technique described in Spencer et al. (2017b) to evaluate the binary fraction of red giant stars in these dwarf galaxies. Assuming Duquennoy & Mayor (1991) period and mass ratio distributions, the binary fractions in Draco and Ursa Minor are \(0.50^{+0.04}_{-0.06}\) and \(0.78^{+0.09}_{-0.08}\), respectively. We find that a normal mass ratio distribution is preferred over a flat distribution, and that log-normal period distributions centered on long periods (\(\mu_{\log P}>3.5\)) are preferred over distributions centered on short ones. We reanalyzed the binary fractions in Leo II, Carina, Fornax, Sculptor, and Sextans, and find that there is