Significant interstellar object production by close stellar flybys

Context. Within just two years, two interstellar objects (ISOs) – 1I/‘Oumuamua and 2I/Borisov – have been discovered, the first of their kind. Large quantities of planetesimals form as a by-product of planet formation. Therefore, it seems likely that ISOs are former planetesimals that became somehow unbound from their parent star. The discoveries raise the question of the dominant ISO formation process. Aims. Here, we concentrate on planetesimals released during another star’s close flybys. Such close flybys happen most frequently during the first 10 Myr of a star’s life. Here, we quantify the amount of planetesimals released during close stellar flybys, their ejection velocity and likely composition. Methods. We numerically study the dependence of the effect of parabolic flybys on the mass ratio between the perturber and parent star, the periastron distance, inclination, and angle of periastron. Results. As expected, close prograde flybys of high-mass stars produce the most considerable amount of ISOs. Especially flybys of stars with M > 5 M⊙ on trajectories closer than 250 AU can lead to more planetesimals turning into ISOs than remaining bound to the parent star. Even strongly inclined orbits do not significantly reduce the ISO production; only retrograde flybys lead to a significantly lower ISO production. For perturbers slightly more massive than the parent star, there is a competition between ISO production and planetesimals being captured by the perturber. Whenever ISOs are produced, they leave their parent system typically with velocities in the range of 0.5–2 km s−1. This ejection velocity is distinctly different to that of ISOs produced by planet scattering (~4–8 km s−1) and those shed during the stellar post-main-sequence phase (~0.1–0.2 km s−1). Using the typical disc truncation radius in various cluster environments, we find that clusters like the Orion nebula cluster are likely to produce the equivalent of 0.85 Earth-masses of ISOs per star. In contrast, compact clusters like NGC 3603 could produce up to 50 Earth-masses of ISOs per star. Our solar-system probably produced the equivalent of 2–3 Earth masses of ISOs, which left our solar system at a mean ejection velocity of 0.7 km s−1. Conclusions. Most ISOs produced by flybys should be comet-like, similar to Borisov and unlike ‘Oumuamua. ISOs originating from compact long-lived clusters would often show a deficiency in CO. As soon as a statistically significant sample of ISOs is discovered, the combined information of their observed velocities and composition might help in constraining the dominant production process