On The Origin of 1I/'Oumuamua

1I/'Oumuamua is the first interstellar interloper to have been detected. Because planetesimal formation and ejection of predominantly icy objects are common by-products of the star and planet formation processes, we address whether 1I/'Oumuamua could be representative of this background population of ejected objects. To do so, we compare the mass density of interstellar objects inferred from its detection to that expected from planetesimal disks under two scenarios: circumstellar disks around single stars and wide binaries, and circumbinary disks around tight binaries. Our study makes use of a detailed study of the PanSTARRS survey volume; takes into account that the contribution from each star to the population of interstellar planetesimals depends on stellar mass, binarity, and planet presence; and explores a wide range of possible size distributions for the ejected planetesimals, based on solar system models and observations of its small-body population. We find that 1I/'Oumuamua is unlikely to be representative of a population of isotropically distributed objects, favoring the scenario that it originated from the planetesimal disk of a young nearby star whose remnants are highly anisotropic. Comparing the fluxes of meteorites and micrometeorites observed on Earth to those inferred from this population of interstellar objects, we conclude that it is unlikely that one of these objects is already part of the collected meteorite samples. The second hypothesis we test is whether 1I/'Oumuamua could be representative of a background population of exo-Oort cloud objects ejected under the effect of post-main sequence mass loss and stellar encounters. We consider the 0.08--8 Msun mass range, take into account the dependencies with stellar mass, Galactocentric distance, and evolutionary state, and consider a wide range of size distributions for the ejected objects. Our conclusion is that 1I/'Oumuamua is likely not representative of this background population, strengthened by the consideration that our estimate is likely an overestimate because it assumes exo-Oort clouds are frequent but the parameter space to form them is actually quite restricted. We discuss whether the number density of free-floating, planetary-mass objects derived from gravitational microlensing surveys could be used as a discriminating measurement regarding 1I/'Oumuamua's origin (given their potential common origin). We conclude that this is challenged by the mass limitation of the surveys and the resulting uncertainty of the mass distribution. The detection of interlopers may be one of the few observational constraints of the low end of the mass distribution of free-floaters, with the caveat that it might not be appropriate to assume they are representative of an isotropic background population, which makes the derivation of a number density very challenging.

11 Dec 2018
Dr. Amaya Moro-Martin