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 ( \Theta _ { 0 } ) , and the distance to the Galactic center ( R _ { 0 } ) .
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 \sim \Theta _ { 0 } / R dependence , in the Solar neighborhood and other locations in the Galaxy .
These models may explain the \sim 40 % difference between the values for 2 A \mbox { $R _ { 0 } $ } 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 \leq 1 kpc ; Hanson 1987=H87 ) and distant Cepheids ( d = 0.5 - 6 kpc ; Feast & Whitelock 1997=FW97 ) .
However , a consistent picture only emerges if one adopts small values for the Galactic constants : R _ { 0 } = 7.1 \pm 0.4 kpc , and \Theta _ { 0 } = 184 \pm 8 { km s } ^ { -1 } .
These values are consistent with most kinematical methods of determining R _ { 0 } , including the proper motion of Sgr A ^ { * } ( Backer 1996 ) , the direct determination of R _ { 0 } using water masers ( 7.2 \pm 0.7 kpc , Reid 1993 ) , and constraints set by the shape of the Milky Way ’ s dark halo ( Olling & Merrifield 1997b=OM97b ) .