An in-depth analysis of the differentially expanding star cluster Stock 18 (Villafranca O-036) using Gaia DR3 and ground-based data

The Villafranca project is combining Gaia data with ground-based surveys to analyze Galactic stellar groups (clusters, associations, or parts thereof) with OB stars. We want to analyze the poorly studied cluster Stock 18 within the Villafranca project, as it is a very young stellar cluster with a symmetrical and compact region around it, Sh 2-170, so it is likely to provide insights into the structure and dynamics of such objects at an early stage of their evolution. We used Gaia astrometry, photometry, spectrophotometry, and variability data as well as ground-based spectroscopy and imaging to determine the characteristics of Stock 18. We used these data to analyze its core, massive star population, extinction, distance, membership, internal dynamics, density profile, IMF, stellar variability, and Galactic location. Stock 18 is a very young ($ 1.0$ Ma) cluster located at a distance of $2.91 kpc and is dominated by the GLS system, whose primary (Aa) is an O9 V star. We propose that Stock 18 was in a very compact state (sim 0.1 pc) about 1.0 Ma ago and that most massive stars were ejected at that time without significantly affecting the less massive stars as a result of multi-body dynamical interactions. Different age estimates also point toward an age close to 1.0 Ma, indicating that the dynamical interactions took place very shortly after massive star formation. Well-defined expanding stellar clusters have been observed before, but none are as young as this one. If we include all of the stars, the initial mass function is top heavy, but if we discard the ejected ones, it becomes nearly canonical. Therefore, this is another example (in addition to the previous one we found -- the Bermuda cluster) of (a) a very young cluster with an already evolved present day mass function (b) that has significantly contributed to the future population of free-floating compact objects. If confirmed in more clusters, the number of such compact objects may be higher in the Milky Way than previously thought. Stock 18 has a variable extinction with an average value of higher than the canonical one of 3.1. We have discovered a new visual component (Ab) in the GLS system. The cluster is above our Galactic mid-plane, likely as a result of the Galactic warp, and it has a distinct motion with respect to its surrounding old population, which is possibly an influence of the Perseus spiral arm.