The Galactic spiral structure as revealed by O- and early B-type stars
  We investigate the morphology and kinematics of the Galactic spiral structure
based on a new sample of O- and early B-type stars. We select 6,858 highly
confident OB star candidates from the combined data of the VST Photometric
H$\alpha$ Survey Data Release 2 (VPHAS+ DR2) and the Gaia Data Release 2 (Gaia
DR2). Together with the O-B2 stars from the literature, we build a sample
consisting of 14,880 O- and early B-type stars, all with Gaia parallax
uncertainties smaller than 20 per cent. The new sample, hitherto the largest
one of O- and early B-type stars with robust distance and proper motion
estimates, covers the Galactic plane of distances up to $\sim$ 6 kpc from the
Sun. The sample allows us to examine the morphology of the Scutum, Sagittarius,
Local and Perseus Arms in great detail. The spiral structure of the Milky Way
as traced by O- and early B-type stars shows flocculent patterns. Accurate
structure parameters, as well as the means and dispersions of the vertical
velocity distributions of the individual Spiral Arms are presented.
