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

The first discovered interstellar object (ISO), `Oumuamua (1I/2017 U1) shows a dry and rocky surface, an unusually elongated shape, with short-to-long axis ratio c∕a ≲ 1∕6, a low velocity relative to the local standard of rest (~10 km s<sup>-1</sup>), non-gravitational accelerations and tumbles on a timescale of a few hours<sup>1-9</sup>. The inferred number density (~3.5 × 10<sup>13</sup>-2 × 10<sup>15</sup> pc<sup>-3</sup>) for a population of asteroidal ISOs<sup>10,11</sup> outnumbers cometary ISOs<sup>12</sup> by ≥10<sup>3</sup>, in contrast to the much lower ratio (≲10<sup>-2</sup>) of rocky/icy Kuiper belt objects<sup>13</sup>. Although some scenarios can cause the ejection of asteroidal ISOs<sup>14,15</sup>, 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 ≲ 1∕10, sizes a ≈ 100 m and rocky surfaces. Although volatiles with low sublimation temperature (such as CO) are concurrently depleted, H<sub>2</sub>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 kilometre-sized long-period comets from Oort clouds, kilometre-sized residual planetesimals from debris disks or planet-sized bodies at a few astronomical units, orbiting around low-mass main-sequence stars or white dwarfs. These provide abundant reservoirs to account for `Oumuamua's occurrence rate.