The segmented spiral structure of the Solar neighbourhood traced by young clustered populations

Abstract

The Milky Way spiral pattern remains poorly constrained, and the youngest tracers in the solar neighbourhood do not always follow a few smooth, continuous logarithmic arms. We analyse young open clusters, both independently and combined with young stellar object (YSO)-based groups, to test whether they define continuous spiral-arm ridges or shorter, partially connected structures. In the (theta_G, ln R_G) plane, we identify local overdensities using a density-supported Bayesian Gaussian Mixture Model (BGMM), introducing published Perseus, Local, Sagittarius, and Scutum arm tracks only afterwards as reference curves. We then apply a Minimum Spanning Tree (MST) analysis in the heliocentric (X,Y) plane to examine spatial connectivity. The open-cluster sample already resolves into several local components, while the addition of YSO-based groups highlights branches and intermediate regions without producing continuous arms. The MST likewise shows distinct local branches at small pruning scales that progressively merge as the linking scale increases. The Local--Sagittarius region provides the clearest agreement between both diagnostics: the BGMM identifies an intermediate component between the reference arms, while the MST connects neighbouring branches through the same region. Overall, young structures near the Sun appear as fragmented, spiral-like segments with partial links rather than smooth continuations of a few grand-design arms. Determining whether this morphology reflects spiral-arm formation mechanisms or subsequent evolution will require additional age, vertical, and kinematic information.