Capture of interstellar objects I: the capture cross-section

Abstract

We study the capture of interstellar objects (ISOs) by a planet-star binary with mass ratio $q\ll1$, semi-major axis $a_p$, orbital speed $v_c$, and eccentricity $e_p$. Very close (slingshot) and wide encounters with the planet are amenable to analytic treatment, while numerically obtained capture cross-sections $σ$ closely follow the analytical results even in the intermediate regime. Wide interactions can only generate energy changes $ΔE\lesssim qv_c^2$, when $σ\propto v_\infty^{-2} |\lnΔE|^{2/3}$ (with $v_\infty$ the ISO's incoming speed far away from the binary), which is slightly enhanced for $e_p>0$. Energy changes $ΔE\gtrsim qv_c^2$, on the other hand, require close interactions when $σ\propto (v_\infty ΔE)^{-2}$ hardly depending on $e_p$. Finally, at $ΔE\gtrsim v_c^2$, the cross-section drops to zero, depending on the planet's radius $R_p$ through the Safronov number $Θ=qa_p/R_p$. We also derive the cross-sections for collisions of ISOs with planets or moons.