Refining the Oort Constants: the case for a smaller Milky Way

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The local stellar kinematics of the Milky Way, parameterized by the Oort constants A and B, depend on the local gradient of the rotation curve, its absolute value (Theta₀), and the distance to the Galactic center (R₀). The surface density of interstellar gas in the Milky Way varies non-monotonically with radius, and so contributes significantly to the local gradient of the rotation curve, and the Oort constants. Because of this, the Oort functions A(R) and B(R) differ significantly from the dominant Theta₀/R dependence, in the Solar neighborhood and other locations in the Galaxy. These models may explain the 40% difference between the values for 2 A R₀ derived from radial velocity data originating in the inner and outer Galaxy (Merrifield 1992). Incorporating these local non-linearities explains the significant differences between the Oort constants derived from nearby stars (d <= 1 kpc; Hanson 1987) and distant Cepheids (d=0.5-6 kpc; Feast & Whitelock 1997). However, a consistent picture only emerges if one adopts small values for the Galactic constants: R₀ = 7.1 +/- 0.4 kpc, and Theta₀ = 184 +/- 8 km/s. These values are consistent with most kinematical methods of determining R₀, including the proper motion of Sgr A* (Backer 1996), the direct determination of R₀ using water masers (7.2 +/- 0.7 kpc; Reid 1993), and constraints set by the shape of the Milky Way's dark halo (Olling & Merrifield 1997b).