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Based on Gaia DR2 data and new CHIRON radial velocities, we have discoveredthat two nearby stellar associations UPK 535 (318.08 $\pm$ 0.29 pc,$25^{+15}_{-10}$ Myr, 174 stars) and Yep 3 (339.54 $\pm$ 0.25 pc ,$45^{+55}_{-20}$ Myr, 297 stars) in the Gum Nebula have recently collided. Weproject stars' current positions, motions, and measurement uncertaintiesbackward and forward through time in a 10,000-trial Monte Carlo simulation. Onaverage, the associations' centres of mass come within 18.89 $\pm$ 0.73 pc ofeach other 0.84 $\pm$ 0.03 Myr ago. A mode of 54 $\pm$ 7 close ($<$1 pc)stellar encounters occur during the collision. We cannot predict specificstar-star close encounters with our current $\sim$7.6 pc distance precision and21.5-per-cent-complete radial velocity sample. Never the less, we find that twostars in UPK 535 and two stars in Yep 3 undergo a nonspecific close encounterin $>$70 per cent of trials and multiple close encounters in $\sim$30 per cent.On average, the closest approach of any two stars is 0.13 $\pm$ 0.06 pc, or27,000 $\pm$ 12,000 au. With impulse-tracing values up to $2.7^{+3.1}_{-1.1}$M$_{\odot}$ pc$^{-2}$ km$^{-1}$ s, such close encounters could perturb stars'Oort cloud comets (if present), cause heavy bombardment events for exoplanets(if present), and reshape solar system architectures. Finally, an expansion ofour simulation suggests other associations in the region are also interacting.Association collisions may be commonplace, at least in the Gum Nebulastraddling the Galactic plane, and may spur solar system evolution more thanpreviously recognized.

Collision of Two Stellar Associations in the Nearby Gum Nebula