A KINEMATIC STUDY OF THE SCULPTOR DWARF SPHEROIDAL GALAXY

We derive new, accurate radial velocity (V-r) measurements for 23 Scul ptor K-giant stars from high-resolution echelle spectra made with the ESO NTT and 3.6-m telescopes. Comparing our velocities with previous m easurements (Armandroff and 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 (17 km s(-1)). These variations are best explai ned 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 p eriods), the observations suggest a large binary fraction in Sculptor, possibly larger than similar to 20% within the period limits similar to 0.5 to similar to 20 yrs. This result is consistent with the recent discoveries of 2-3 large-amplitude variable stars in Ursa Minor and S extant. Previous kinematic studies of dSph galaxies based on single-ep och measurements are consequently not fully reliable. Radial velocity- variable stars artificially inflate the velocity dispersion and the ma ss-to-light (M/L) ratio estimates. This weakens the case for dark matt er, at least in some of the dSph galaxies. In the case of Sculptor, a velocity dispersion of 6.2 +/- 1.1 km s(-1) and a M/L ratio of 13 +/- 6 (M/L(V))(circle dot) are derived from our sample. These results do n ot provide unambiguous evidence for a dominant dark matter component i n Sculptor, mainly because our sample is likely to contain additional undetected V-r variables of lower amplitude.