Why is Interstellar Object 1I/2017 U1 (`Oumuamua) Rocky, Tumbling and Very Prolate?

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. This object was 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 star. I suggest that heating by the star's giant stage fluidized a precursor cometary object as volatiles escaped, causing it to assume the Jacobi ellipsoidal shape of a self-gravitating incompressible liquid. The collision that produced the inferred tumbling motion must 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 $\gtrapprox 5$ implies a lower bound on the density of 1.6 g/cm$^3$, excluding an icy interior unless it is almost entirely frozen CO$_2$. This object is the first Jurad to be discovered and may be related to accreting objects that pollute white dwarf atmospheres and may make Soft Gamma Repeaters.