Abstract
Abstract The recently discovered first interstellar object 1I/2017 U1 (‘Oumuamua) has brightness that varies by a factor of 10, a range greater than that of any Solar system asteroid, a spectrum characteristic of type D asteroids, and no evidence of evaporating volatiles, contrary to expectation for exo-Oort clouds. ‘Oumuamua is possibly the first example of the proposed ‘Jurads’, objects depleted in volatiles and ejected from planetary systems during the post-main sequence evolution of their parent stars. I suggest that heating by the star’s giant stage fluidized a precursor object as well as driving off any volatiles, causing it to assume the Jacobi ellipsoidal shape of a self-gravitating incompressible liquid. The collision that produced the inferred tumbling motion may have occurred thousands of years after the formation of 1I/2017 U1 ‘Oumuamua. Jacobi ellipsoids have a unique relation among rotation rate, density and axial ratio. The inferred axial ratio ⪆5 suggests a lower bound on the density of 1.6 g cm−3, apparently excluding an icy interior unless it is almost entirely frozen CO2. ‘Oumuamua may be related to accreting objects that pollute white dwarf atmospheres and that may make Soft Gamma Repeaters.
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