Modified Moments and the Oort Constants

The standard epicyclc approach to estimating the slope of the Galactic circular-speed curve at the solar neighbourhood is subject to severe systematic errors. A new method, involving 'modified moments', is introduced which suppresses these systematic effects by assigning low weight to stars in the wings of the velocity distributions. Application of this method to the McCormick K and M dwarfs more than confirms the implication of classical theory that the circular-speed curve must have a significantly negative slope at the solar radius unless the Galactic potential is triaxial. Since direct estimates of the circular speed suggest that it is constant or rising beyond the Sun, we conclude with Kuijken & Tremaine that the Galactic disc is probably significantly elliptical at the Sun because the heavy halo is triaxial.