Parallax-based distances to Galactic H II regions: Nearby spiral structure

¹ Department of Astronomy, School of Physics and Astronomy, Yunnan University, Kunming 650091, China
² School of Physics and Astronomy, Yunnan University, Kunming, 650091, PR China
³ National Astronomical Observatories, CAS, Jia-20 Datun Road, Chaoyang District, Beijing 100101, PR China

^★ Corresponding authors; lghou@nao.cas.cn
hongliliu2012@gmail.com

Received: 23 October 2024
Accepted: 27 February 2025

Abstract

Context. The spiral structure of the Milky Way is not conclusive, even for the disc regions in the solar neighbourhood. 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.

Aims. We aim to re-examine the arm segments traced by young objects and better understand the nearby spiral structure.

Methods. We identified the exciting stars of 459 H II regions and calculated their parallax-based distances according to the Gaia DR3. Together with other H II regions with spectrophotometric or parallax-based distances in the literature, we used the largest ever sample of 572 H II 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.

Results. The structures outlined by different kinds of young objects do not exhibit a significant deviation from each other. The distributions of young objects are in agreement with three arm-like features emerging in the overdensity map of evolved stars. In particular, the Local Arm outlined by young objects follows an arm-like feature delineated by evolved stars and probably spirals outwards towards the direction of ℓ ~ 240° in the third Galactic quadrant.

Conclusions. We conclude that the arm segments traced by young objects and evolved stars are consistent with each other, at least in the solar neighbourhood. In particular, the Local Arm delineated by young objects is reinterpreted as an arm segment with a large pitch angle of 25.2° ± 2.0°, whose inner edge is in good agreement with the recently discovered Radcliffe Wave.