THE MASS-TO-LIGHT RATIOS OF THE DRACO AND URSA-MINOR DWARF SPHEROIDALGALAXIES .2. THE BINARY POPULATION AND ITS EFFECT ON THE MEASURED VELOCITY DISPERSIONS OF DWARF SPHEROIDALS

We use a large set of radial velocities in the Ursa Minor and Draco dw arf spheroidal galaxies to search for binary stars and to infer the bi nary frequency. Of the 118 stars in our sample with multiple observati ons, six are velocity variables with chi(2) probabilities below 0.001. We use Monte Carlo simulations that mimic our observations to determi ne the efficiency with which our observations find binary stars. Our b est, though significantly uncertain, estimate of the binary frequency for stars near the turnoff in Draco and UMi is 0.2-0.3 per decade of p eriod in the vicinity of periods of one year, which is 3-5x that found for the solar neighborhood. This frequency is high enough that binary stars might significantly affect the measured velocity dispersions of some dwarf spheroidal galaxies according to some previous numerical e xperiments. However, in the course of performing our own experiments, we discovered that this previous work had inadvertently overestimated binary orbital velocities. Our first set of simulations of the effects of binaries is based on the observed scatter in the individual veloci ty measurements for the multiply-observed Draco and Ursa Minor stars. This scatter is small compared to measured velocity dispersions and, s o, the effect of binaries on the dispersions is slight. This result is supported by our second set of experiments, which are based on a mode l binary population normalized by the observed binary frequency in Dra co and Ursa Minor. We conclude that binary stars have had no significa nt effect on the measured velocity dispersion and inferred mass-to-lig ht ratio of any dwarf spheroidal galaxy. (C) 1996 American Astronomica l Society.