Abstract
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.
Highlights
The Milky Way has been thought to be a spiral galaxy since as far back as the 1850s (Alexander 1852), but it is extremely difficult to observe its spiral structure due to our edge-on view from its interior and limited by copious dust extinction
We investigate the spiral structure in the solar neighborhood revealed by Gaia DR2 and compare it with that depicted by Very Long Baseline Interferometry (VLBI) maser parallaxes
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
Summary
The Milky Way has been thought to be a spiral galaxy since as far back as the 1850s (Alexander 1852), but it is extremely difficult to observe its spiral structure due to our edge-on view from its interior and limited by copious dust extinction. It was not until the 1950s that researchers started to use high-mass stars (OB stars) and H ii regions to trace spiral arm segments in the solar neighborhood (Morgan et al 1952, 1953). We note that even when Gaia reaches its target accuracy, direct measurement of spiral structure will be limited by dust extinction in the plane of the inner Galaxy to stars typically within a few kiloparsec
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