Draft version August 25, 2020 Typeset using L A T E X twocolumn style in AASTeX63 ’Oumuamua as a Cometary Fractal Aggregate: the "Dust Bunny" Model Jane X. Luu, 1, 2 Eirik G. Flekkøy, 3 and Renaud Toussaint 4, 3 1 Centre for Earth Evolution and Dynamics, Department of Geosciences, University of Oslo 2 Institute of Theoretical Astrophysics, University of Oslo P.O. Box 1047, Blindern, NO-0316 Oslo, Norway 3 PoreLab, the Njord Centre Department of Physics, University of Oslo, P. O. Box 1048 Blindern, N-0316, Oslo, Norway 4 Institut de Physique du Globe de Strasbourg, University of Strasbourg (Received July 8, 2020; Accepted August 14, 2020) Submitted to ApJL Abstract The first known interstellar object, 1I/2017 U1 ’Oumuamua, displayed such unusual properties that its origin remains a subject of much debate. We propose that ’Oumuamua’s properties could be ex- plained as those of a fractal dust aggregate (a "dust bunny") formed in the inner coma of a fragmenting exo-Oort cloud comet. Such fragments could serve as accretion sites by accumulating dust particles, resulting in the formation of a fractal aggregate. The fractal aggregate eventually breaks off from the fragment due to hydrodynamic stress. With their low density and tenuously bound orbits, most of these cometary fractal aggregates are then ejected into interstellar space by radiation pressure. Keywords: Comets, comae, Oort cloud objects 1. INTRODUCTION Two interstellar objects have visited the solar system in the last three years, 1I/’Oumuamua (MPEC 2017- U181 (2017)) and 2I/Borisov (Guzik et al. (2019)), and the two could not be more different. Widely expected to be a comet, ’Oumuamua showed no sign of cometary activity (Jewitt et al. (2017); Meech et al. (2017)), but its orbit nevertheless exhibited nongravitational accel- eration (Micheli et al. (2018)). In contrast, 2I/Borisov is most definitely a comet, with cometary emission (Opitom et al. (2019)), and optical colors similar to those of solar system comets (Jewitt & Luu (2019a); Fitzsimmons et al. (2019); Guzik et al. (2020)). The stark difference between the two interstellar bodies sug- gests that whereas Borisov is probably a comet formed in a planetary disk, ’Oumuamua’s origin lies elsewhere. Several models have been proposed to explain ’Oumua- mua’s origin, including tidal fragmentation (Ćuk (2018); Corresponding author: J. Luu jane.luu@geo.uio.no Zhang & Lin (2020)), extinct fragment of a comet-like planetesimal (Raymond et al. (2018)), and an hydrogen- rich body formed in the dense core of a giant molecu- lar cloud (Seligman & Laughlin (2020)). Flekkøy et al. (2019) showed that ’Oumuamua’s unusual shape and ul- tra low density (ρ = 10 -2 kg/m 3 , Moro-Martín (2019)) were consistent with a fractal aggregate with fractal di- mension D f =2.3 - 2.4. This fractal dimension is typ- ical of an intermediate stage of planetesimal formation (Suyama et al. (2008)), before the planetesimal becomes a compressed body with D f approaching the normal Eu- clidean dimension of 3. It hints at an abbreviated accre- tion process; such a process could happen in the densest region of a cometary coma, near the nucleus’s surface. In this work we build on the fractal framework for ’Oumuamua; we show how the object could have formed in the inner coma of an exo-Oort cloud comet, then es- caped to interstellar space. The paper is organized as follows. Section 2 describes the conditions for accretion in the inner coma. The evolution of the fractal aggregate is described in Section 3, and in Section 4 we estimate the statistics for a population of cometary fractal aggre- gates. arXiv:2008.10083v1 [astro-ph.EP] 23 Aug 2020