Spiral arms are important sites of star formation, yet in spite of this knowledge, the spatial scale in which it occurs still needs to be fully quantified. As tracers of the position and motion of the spiral arms, young stellar clusters and associations constitute suitable probes towards solving this question. On the one hand, they stay close to their birth environment; on the other hand, it is easier to determine accurate distances for them rather than individual stars.

In this work, we identify stellar groups within the Galactic spiral arms, that are consistent both in velocity and age, in order to estimate their size and thus the spatial scale of star formation. This requires a reliable determination of their membership, which is possible thanks to the combination of recent photometric, kinematic and spectroscopic data with modern computing tools, allowing us to better define their members and use them as probes.

To that end, the first applied tool is an SED fitting code, It includes an observed SED based on Gaia DR3 parallax together with the photometry from several optical and near-IR surveys (including Gaia DR3). It also includes a model SED based on stellar atmosphere and evolutionary models, whilst the reddening is constrained through a 3D extinction map. The fitting process utilises a Bayesian inference and maximum-likelihood test to derive stellar physical parameters such as effective temperature, luminosity, initial mass and distance. The second tool is a clustering algorithm that have then been applied to identify cluster members with coherent kinematics, exploiting proper motions from Gaia DR3 to assess strong membership.

We selected the association Cas OB5 as our first target to apply these methods. Recent investigations have placed its members at distances between 2.5 and 3 kpc, thereby intercepting the Perseus arm. We redefine the membership of Cas OB5 and its surrounding open clusters, especially their brightest and most massive stars, and analyse them.

The moderate extinction (A_V = 2-3 mag) of Cas OB5 facilitates its analysis and lays out the foundations to explore more distant and extinguished regions. It is particularly true for the Perseus arm, for which few tracers are known at $l > 140°, and $l$ between 70° and 90° where the Perseus arm is thought to be located as far as about 4 kpc away. These techniques, complemented with robust spectroscopic analysis, allow us to identify obscured star clusters in this area and determine their size and membership.