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Abstract
Establishing the rotation curve of the Milky Way is one of the fundamental contributions needed to understand the Galaxy and its mass distribution. We have undertaken a systematic spectroscopic survey of open star clusters which can serve as tracers of Galactic disk dynamics. We report on our initial sample of 67 clusters for which the Hydra multi-fiber spectrographs on the WIYN and Blanco telescopes have delivered ∼1-2 km s-1 radial velocities (RVs) of many dozens of stars in the fields of each cluster, which are used to derive cluster membership and bulk cluster kinematics when combined with Tycho-2 proper motions. The clusters selected for study have a broad spatial distribution in order to be sensitive to the disk velocity field in all Galactic quadrants and across a Galactocentric radius range as much as 3.0 kpc from the solar circle. Through analysis of the cluster sample, we find (1) the rotation velocity of the Local Standard of Rest (LSR) is [special characters omitted] km s-1, (2 ) the local rotation curve is declining with radius having a slope of -9.1 km s-1 kpc -1, (3) we find (using R0 = 8.5 kpc) the following Galactic parameters: A = 17.0 km s-1 kpc-1 and B = -8.9 km s-1 kpc-1, which using a flat rotation curve and our determined values for the rotation velocity of the LSR yields a Galaxy mass within 1.5 R0 of M = 1.4 ± 0.2 × 10 11 [special characters omitted] and a M/L of 9 [special characters omitted]. We also explore the distribution of the local velocity field and find evidence for non-circular motion due to the spiral arms. Additionally, a number of outer disk (Rgc > 12 kpc) open clusters, including Be29 and Sa1, are studied that have potentially critical leverage on radial, age and metallicity gradients in the outer Galactic disk. We find that the measured kinematics of Sa1 and Be29 are consistent with being associated with the Galactic anticenter stellar structure (GASS; or Monoceros stream), which points to a possible "accretion" origin for these and possibly other outer disk open clusters, if one believes that GASS represents an accreting dwarf galaxy system.
