1995A&A...300...31Q

We derive new, accurate radial velocity (V_r) measurements for 23 Sculptor K-giant stars from high-resolution echelle spectra made with the ESO NTT and 3.6-m telescopes. Comparing our velocities with previous measurements (Armandroff & Da Costa 1986) for 15 stars, we show that the V_r measurements are reliable, and discover two V_r variable stars with large amplitudes (>17km/s). These variations are best explained by binary orbital motions. As our sample is likely to contain many additional undetected binaries with smaller velocity amplitudes (due to less favorable orbit inclinations, smaller mass ratios, or longer periods), the observations suggest a large binary fraction in Sculptor, possibly larger than ∼20% within the period limits ∼0.5 to ∼20yrs. This result is consistent with the recent discoveries of 2-3 large-amplitude variable stars in Ursa Minor and Sextant. Previous kinematic studies of dSph galaxies based on single-epoch measurements are consequently not fully reliable. Radial velocity-variable stars artificially inflate the velocity dispersion and the mass-to-light (M/L) ratio estimates. This weakens the case for dark matter, at least in some of the dSph galaxies. In the case of Sculptor, a velocity dispersion of 6.2±1.1km/s and a M/L ratio of 13±6(M/L_V)_☉ are derived from our sample. These results do not provide unambiguous evidence for a dominant dark matter component in Sculptor, mainly because our sample is likely to contain additional undetected V_r variables of lower amplitude.