Parallax-based distances to Galactic HII regions

In particular, the arm-like structures uncovered from the overdensity maps of evolved stars are inconsistent with the commonly adopted spiral arm models based on young objects. We aim to reexamine the arm segments traced by young objects and better understand the nearby spiral structure.

We identified the exciting stars of 459 HII regions and calculated their parallax-based distances according to the Gaia DR3. Together with other HII regions with spectrophotometric or parallax-based distances in the literature, we used the largest ever sample of 572 HII regions with accurate distances to reveal the features shown in their distributions projected onto the Galactic disc. We then compared the results to the features traced by other young objects (high-mass star-forming region masers, O-type stars, and young open clusters) and evolved stars.