A study of full space motions of outer Galactic disk A and F stars in two deep pencil beams

Abstract

A and F stars can be used as probes of outer Galactic disc kinematics: here we extend the work of Harris et al. by cross-matching their A/F sample with Gaia DR2 to bring in proper motions. These are combined with the already measured radial velocities and spectrophotometric distances to obtain full space motions. We use this sample of 1173 stars, located in two pencil-beam sightlines (ℓ = 178° and ℓ = 118°), to sample the Galactocentric velocity field out to almost RG = 15 kpc. We find there are significant differences in all three (radial, azimuthal, and vertical) kinematic components between the two directions. The rotation curve is roughly flat in the anticentre direction, confirming and extending the result of Kawata et al. thanks to the greater reach of our spectrophotometric distance scale. However at ℓ = 118° the circular velocity rises outwards from RG = 10.5 kpc and there is a more pronounced gradient in radial motion than is seen at ℓ = 178°. Furthermore, the A star radial motion differs from the F stars by ∼10 km s−1. We discuss our findings in the context of perturbers potentially responsible for the trends, such as the central bar, spiral arms, the warp, and external satellites. Our results at ℓ = 178° are broadly consistent with previous work on K giants in the anticentre, but the kinematics at ℓ = 118° in the Perseus region do not yet reconcile easily with bar or spiral arm perturbation.