Tidal fragmentation as the origin of 1I/2017 U1 (‘Oumuamua)

Abstract

The first discovered interstellar object (ISO), `Oumuamua (1I/2017 U1) shows a dry and rocky surface, an unusually elongated short-to-long axis ratio $c/a \lesssim 1/6$, a low velocity relative to the local standard of rest ($\sim 10$ km s$^{-1}$), non-gravitational accelerations, and tumbles on a few hours timescale. The inferred number density ($\sim 3.5 \times 10^{13} - 2 \times 10^{15}$ pc$^{-3}$) for a population of asteroidal ISOs outnumbers cometary ISOs by $\geq 10^3$, in contrast to the much lower ratio ($\lesssim 10^{-2}$) of rocky/icy Kuiper belt objects. Although some scenarios can cause the ejection of asteroidal ISOs, a unified formation theory has yet to comprehensively link all `Oumuamua's puzzling characteristics and to account for the population. Here we show by numerical simulations that `Oumuamua-like ISOs can be prolifically produced through extensive tidal fragmentation and ejected during close encounters of their volatile-rich parent bodies with their host stars. Material strength enhanced by the intensive heating during periastron passages enables the emergence of extremely elongated triaxial ISOs with shape $c/a \lesssim 1/10$, sizes $a \sim 100$ m, and rocky surfaces. Although volatiles with low sublimation temperature (such as CO) are concurrently depleted, H$_2$O buried under surfaces is preserved in these ISOs, providing an outgassing source without measurable cometary activities for `Oumuamua's non-gravitational accelerations during its passage through the inner Solar System. We infer that the progenitors of `Oumuamua-like ISOs may be km-sized long-period comets from Oort clouds, km-sized residual planetesimals from debris disks, or planet-size bodies at a few AU, orbiting around low-mass main-sequence stars or white dwarfs. These provide abundant reservoirs to account for `Oumuamua's occurrence rate.