Proper motions of OB stars in the far Carina Arm
J. E. Drew, M. Mongui\'o, N. J. Wright

TL;DR
This study analyzes the proper motions of over 4000 OB stars in the far Carina Arm, revealing complex kinematic patterns, a moving group, and ejection hot spots linked to specific star clusters, enhancing understanding of Galactic structure and star ejection processes.
Contribution
It provides the first detailed proper motion analysis of OB stars in the far Carina Arm, identifying kinematic perturbations, a moving group, and ejection hot spots associated with specific clusters.
Findings
Proper motion pattern shows a saw-tooth modulation with ~1 mas/yr amplitude.
Identified a moving group of over 50 OB stars behind the main arm.
Estimated at least 10 ext{--}20\% of OB stars are runaways, with ejection linked to NGC 3603 and Westerlund 2.
Abstract
In large scale maps of the Galactic disc, the Carina Arm stands out as a clear spiral feature, hosting prominent star clusters and associations rich in massive stars. We study the proper motions of 4199 O and early B most likely in the far Carina Arm, at distances mainly in excess of 4 kpc from the Sun, within the sky region, and (Galactic coordinates). The sample is constructed by extending an existing blue-selected catalogue, and cross-matching with Gaia EDR3 astrometry. The observed pattern of proper motions is modulated into a saw-tooth pattern, with full amplitude approaching 1 mas yr, recurring roughly every 2--3 degrees of longitude (200--300 pc at the median OB-star distance of 5.8 kpc). Kinematic perturbation of underlying circular rotation is most likely present. The data also reveal a moving group…
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