Parent arm of the Local (Orion) Arm

Ascertain whether the Local Arm (Orion Arm) is a spur of the Carina–Sagittarius arm or of the Perseus arm within the Milky Way’s grand-design spiral structure.

Background

The Local Arm (Orion Arm) is widely regarded as a spur rather than a main component of the Galaxy’s grand-design spiral structure. Numerous analyses using masers, H II regions, OB stars, and Cepheids have constrained its pitch angle, yet model-dependent fits yield varying interpretations of the spiral pattern, complicating assignment to a specific parent arm.

This review consolidates direct, model-independent measurements of the Local Arm geometry and emphasizes that, despite improved mapping, ambiguity persists regarding whether the Local Arm branches from the Carina–Sagittarius arm or from the Perseus arm.

References

It should be noted that, according to modern estimates, the Orion arm is not part of the global galactic spiral structure grand design, but is a branch (spur) from some arm of this structure. However, at present, it is not yet clear from which one specifically is: from the Carina--Sagittarius arm or Perseus.

— Place of the Radcliffe Wave in the Local System  (2510.25570 - Bobylev et al., 29 Oct 2025) in Section: Orion Arm and Grand Design Structure