ROCKETS I: Investigating the impact of the rocket effect on two nearby young open clusters encased in infrared bubbles

Abstract

We introduce ROCKETS (Rocket-driven Cluster Kinematics & Triggered Star-formation), a Gaia-based programme investigating kinematic fingerprints of stellar feedback or rocket effect in young star-forming regions. We analyse two young clusters with O-type stars enclosed by mid-infrared bubbles, Collinder 69 and IC 1396 as a methodological demonstration on two nearby benchmark regions. Co-moving candidates are selected within an astrometric ellipse defined by the median and median absolute deviation of Class I/II YSOs with Gaia data. For each star, we compute a relative proper-motion angle (RPMA) relative to the ionising source. In both regions the RPMA distribution shows a strong excess at RPMA <15°, corresponding to sources moving away from the respective ionising sources. We quantify this using the rocket effect index (REI), finding REI≃ 1.1 for Collinder 69 and REI≃ 1.7 for IC 1396. When considering only Class I/II YSOs, the REI of both regions increases to REI ≳ 10. Velocity-structure analysis of the sample shows that the RPMA <15° subset has stronger positive pairwise expansion than the full co-moving population, providing additional support for the rocket effect. The outward-moving stars are not uniformly distributed; instead, their position angles are strongly anisotropic and preferentially aligned towards the bright-rimmed clouds. Gaia colour--magnitude diagrams show no strong age offset between sources moving outward and literature cluster members. The observed kinematic signatures support feedback-driven acceleration of gas and triggered star formation preferentially towards higher density regions of expanding bubbles due to the rocket effect.