%0 Unpublished work %T The interstellar object 'Oumuamua as a fractal dust aggregate %+ PoreLab [Oslo] %+ Centre for Earth Evolution and Dynamics [Oslo] (CEED) %+ Institute of Theoretical Astrophysics [Oslo] %+ Institut de physique du globe de Strasbourg (IPGS) %A Flekkøy, Eirik G. %A Luu, Jane X. %A Toussaint, Renaud %Z Astrophysical Journal Letters, accepted, in press - 8 pages, 3 figures %8 2019-11-05 %D 2019 %Z 1910.07135 %Z Physics [physics]/Astrophysics [astro-ph]/Earth and Planetary Astrophysics [astro-ph.EP] %Z Physics [physics]/Astrophysics [astro-ph]Preprints, Working Papers, ... %X The first known interstellar object 'Oumuamua exhibited a nongravitational acceleration that appeared inconsistent with cometary outgassing, leaving radiation pressure as the most likely force. Bar the alien lightsail hypothesis, an ultra-low density due to a fractal structure might also explain the acceleration of 'Oumuamua by radiation pressure (Moro-Martin 2019). In this paper we report a decrease in 'Oumuamua's rotation period based on ground-based observations, and show that this spin-down can be explained by the YORP effect if 'Oumuamua is indeed a fractal body with the ultra-low density of $10^{-2}$ kg m$^{-3}$. We also investigate the mechanical consequences of 'Oumuamua as a fractal body subjected to rotational and tidal forces, and show that a fractal structure can survive these mechanical forces. %G English %L insu-02348185 %U https://insu.hal.science/insu-02348185 %~ INSU %~ CNRS %~ UNIV-STRASBG %~ SITE-ALSACE