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Fig. 1.

image

Locations of the O–B2 stars (red dots, left) and O–B0 stars (red dots, right) projected onto the Galactic plane, together with their 1σ distance uncertainties. The Sun (Sun symbol) is at (0, 8.15) kpc. Only the stars with formal parallax uncertainties better than 10% are shown. The solid and dashed curved lines denote the arm center and widths (enclose 90% of the masers) fitted by Reid et al. (2019) from their parallax data of VLBI masers. The Perseus Arm (black), the Local Arm (blue), the Sagittarius-Carina Arm (magenta), the Scutum-Centaurus Arm (dark cyan), and two spur-like structures (white) are indicated with different colors. Straight dashed lines (gray) denote the Galactic longitudes.

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