Roman R. Rafikov^1,2
The Astrophysical Journal 861, 35 Link to Article [https://doi.org/10.3847/1538-4357/aac5ef]
¹Centre for Mathematical Sciences, Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Wilberforce Road, Cambridge CB3 0WA, UK
²Institute for Advanced Study, Einstein Drive, Princeton, NJ 08540, USA

Discovery of the first interstellar asteroid (ISA)—1I/2017 ‘Oumuamua—raised natural questions regarding its origin, some related to its lack of cometary activity, suggesting refractory composition. Here we explore the possibility that ‘Oumuamua-like ISAs are produced in tidal disruption events (TDEs) of refractory planetoids (asteroids, terrestrial planets, etc.) by white dwarfs (WDs). This idea is supported by spectroscopic observations of metal-polluted WDs, indicating the predominantly volatile-poor composition of the accreted material. We show that such TDEs sourced by realistic planetary systems (including a population of 10³ km planetoids and massive perturbers—Neptune-to-Saturn mass planets) can eject up to 30% of planetary mass involved in TDEs to interstellar space. Collisional fragmentation, caused by vertical collapse of the disrupted planetoid’s debris inside the WD Roche sphere, channels most of its mass into 0.1–1 km fragments, similar to ‘Oumuamua. Such a size spectrum of ISAs (very different from the top-heavy distributions expected in other scenarios) implies that planetary TDEs can account for a significant fraction (up to ~30%) of ISAs. This figure is based on existing observations of WD metal pollution, which are de-biased using realistic models of circum-WD planetary systems. Such ISAs should exhibit kinematic characteristics of old, dynamically hot Galactic populations. ISA ejection in individual planetary TDEs is highly anisotropic, resulting in large fluctuations of their space density. We also show that other ISA production channels involving stellar remnants—direct ejection by massive planets around the WDs and supernova explosions—have difficulty explaining ‘Oumuamua-like ISAs.