Local stellar kinematics and Oort constants from the LAMOST A-type stars

We estimate the solar peculiar velocities and Oort constants using a sample of 5627 A-type stars with $d\lt 0.6\, \rm kpc$ and $|z|\lt 0.1\, \rm kpc$ , selected from the LAMOST (Large Sky Area Multi-Object Fiber Spectroscopic Telescope) surveys. The radial and tangential velocities of these A-type stars are fitted by using a non-axisymmetric model. The best-fitting result yields the solar peculiar velocities $(U_\odot,V_\odot,W_\odot)=(11.69\pm 0.68, 10.16\pm 0.51, 7.67\pm 0.10)\, \rm km\, s^{-1}$ and Oort constants $A=16.31\pm 0.89\, \rm km\, s^{-1}\, kpc^{-1}$ , $B=-11.99\pm 0.79\, \rm km\, s^{-1}\, kpc^{-1}$ , $C=-3.10\pm 0.48\, \rm km\, s^{-1}\, kpc^{-1}$ , and $K=-1.25\pm 1.04\, \rm km\, s^{-1}\, kpc^{-1}$ . $|K+C|\gt 4\, \rm km\, s^{-1}\, kpc^{-1}$ means that there is a radial velocity gradient in the extended local disc, implying that the local disc is in a non-asymmetric potential. Using the derived Oort constants, we derive the local angular velocity $\Omega \, {\approx }\, A-B=28.30\pm 1.19\, \rm km\, s^{-1}\, kpc^{-1}$ . By using A-type star sample of different volumes, we further try to evaluate the impacts of the ridge pattern in the R–V ϕ plane on constraining the solar motions and Oort constants. As the volume becomes larger towards the anticentre direction, the values of A and B become larger (implying a steeper slope of the local rotation curve) and the value of V ⊙ becomes smaller probably caused by the ridge structure and its signal increasing with distance.