Nine Seasons of Velocity Measurements in the Draco and Ursa Minor Dwarf Spheroidal Galaxies with the MMT Echelle

Abstract

We have used the Multiple Mirror Telescope echelle spectrograph to measure 112 velocities of 42 stars in the Draco and Ursa Minor dwarf spheroidal galaxies and three velocities of three foreground stars between 1982 April and 1990 September. We used 11 A resolution spectra obtained with the MX multifiber spectrograph at the Steward 90" to find additional giant candidates; 5 UMi and 13 Draco stars were then observed at the MMT and added to the original sample of velocity members. In addition, the MX spectra were used to eliminate 74 stars in the direction of UMI and 59 stars in Draco as likely foreground dwarfs. We detected 7 velocity variables, defined as those stars whose probability of exceeding the measured X^2^ by chance is less than 1.5%. Three of these stars are Carbon (C) stars (UMi K and VA 335 and Draco C1); two have emission lines (Draco CI and UMi M). We show that the C star Draco C4, with a proper motion membership probability of 7%, has a velocity consistent with membership. It is not surprising that these C (most likely CH) stars are binaries because McClure has shown that most Galactic CH stars are in binary systems. Of the remaining 35 stars, only 4 are velocity variables, with measured velocity extrema of 29.1 km s^-1^ (UMi M), 7.2 km s^-1^ (Draco XI-2), 9.0 km s^-1^ (Draco 24), and 8.3 km s^-1^ (Draco 473). The velocity dispersions are 10.1 +/- 1.7 km s^-1^ for UMi, and 9.9 +/- 1.4 km s^-1^ for Draco. These dispersions change to 10.5 +/- 2.0 for UMi, and 8.2 +?- 1.3 for Draco if we eliminate the velocity variables. Our dispersion for UMi differs from that of Hargreaves et al. [MNRAS, 271,693 (1994b)] by 1.3σ of the combined errors. These velocities are combined with the one-component King models of Pryor & Kormendy [AJ, 100,127 (1990)] to give M/L = 73 for UMi, and 77 for Draco.