An asymmetric Galactic stellar disk traced by OB-type stars from LAMOST DR7

Abstract Using 9943 OB-type stars from LAMOST DR7 in the Solar neighbourhood, we fit the vertical stellar density profile with the model including a single exponential distribution at different positions $(R,\Phi)$. The distributions of the scale heights and scale length show that the young disk traced by the OB-type stars is not axisymmetric. The scale length decreases versus the azimuthal angle $\Phi$, i.e. from $1.14^{+0.30}_{-0.19}$ kpc with $\Phi=-3^\circ$ to $0.77^{+0.08}_{-0.06}$ kpc with $\Phi=9^\circ$. Meanwhile we find signal of non-symmetry in the distribution of the scale height of the north and south of the disk plane. The scale height in the north side shows signal of flaring of the disk, while that of the south disk keeps almost constant around $h_s=130$ pc. The distribution of the displace of the disk plane $Z_0$ also shows variance versus the azimuthal angle $\Phi$, which shows significant differences with the warp model constrained by the Cepheid stars. We also test different values for the position of the Sun, the distance between the Sun and the Galactic center affect the scale heights and the displace of the disk significantly, but that does not change our conclusion that the disk is not axisymmetric.