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

TL;DR

`Oumuamua's extreme brightness variation, shape, and spectral features suggest it is a rocky, tumbling, highly elongated object likely shaped by stellar heating and collision, representing a new class of interstellar objects called Jurads.

Contribution

This paper proposes a novel explanation for `Oumuamua's shape and properties as a Jurad, a volatile-depleted object shaped by stellar heating and collision, expanding understanding of interstellar objects.

Findings

`Oumuamua is likely a rocky, highly elongated Jacobi ellipsoid.

Its density is estimated to be at least 1.6 g/cm³.

It may be related to objects polluting white dwarf atmospheres.

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.