A comparison of the local spiral structure from Gaia DR2 and VLBI maser
  parallaxes
  Context. The Gaia mission has released the second data set (Gaia DR2), which
contains parallaxes and proper motions for a large number of massive, young
stars. Aims. We investigate the spiral structure in the solar neighborhood
revealed by Gaia DR2 and compare it with that depicted by VLBI maser
parallaxes. Methods. We examined three samples with different constraints on
parallax uncertainty and distance errors and stellar spectral types: (1) all OB
stars with parallax errors of less than 10%; (2) only O-type stars with 0.1 mas
errors imposed and with parallax distance errors of less than 0.2 kpc; and (3)
only O-type stars with 0.05 mas errors imposed and with parallax distance
errors of less than 0.3 kpc. Results. In spite of the significant distance
uncertainties for stars in DR2 beyond 1.4 kpc, the spiral structure in the
solar neighborhood demonstrated by Gaia agrees well with that illustrated by
VLBI maser results. The O-type stars available from DR2 extend the spiral arm
models determined from VLBI maser parallaxes into the fourth Galactic quadrant,
and suggest the existence of a new spur between the Local and Sagittarius arms.
