Young Stars amid External Radiation and Colliding Clusters

Because stars are not born in isolation, they can be subject to external radiation from neighboring hot stars and to close stellar interactions. The hot stars ζ Pup and γ² Vel powering the diffuse Gum Nebula shine so intensely that their ultraviolet radiation can disperse the protoplanetary disks of stars up to a hundred light years away, hastening disk evolution and possibly impeding planet formation. In the stellar association near cometary globule CG 30 (FUV radiation G₀ = 6.6^+3.2_-2.7), the accretion disk fraction is only 29% ± 14%, low for the association’s young age of ~0.5 Myr. In 7 other clusters we found throughout the Gum Nebula (G₀ = 2.1 - 17.0), 43% of 207 measured projected rotation speeds are >40 km s^-1, high for the clusters’ intermediate ages of 6 — 680 Myr and possibly a symptom of early disk erosion. Based on Gaia DR2 data and our own spectroscopically measured radial velocities, we serendipitously discovered that the Gum Nebula clusters UPK 535 and Theia 120 collided with each other 0.74 ± 0.11 Myr ago. Their closest center-of-mass approach is 17.4 ± 2.4 pc, just slightly larger than the clusters’ half-stars radii of 13.4 pc and 15.3 pc. This event may be the first-ever case study of colliding clusters close enough to Earth for detailed analysis of stellar motions, properties, and disk properties. According to our Monte Carlo simulation, 55 ± 8 stars come within 1 pc of each other during the encounter. The tightest star-star interaction is 0.14 ± 0.06 pc, less than the radial extent of the Sun’s Oort cloud. If these stars possess Oort clouds similar to the Sun’s, the largest impulse on these clouds is 0.51 ± 0.43 M_☉ pc^-1 km^-1 s, large enough to perturb the orbits of Oort cloud comets and plague any present exoplanets with heavy bombardment. Collectively our research on stars and their dramatic interactions elucidates how young stars and their potential planets are affected by their dynamic environments.