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Assessing the Hydrogen Ice Hypothesis for 1I/2017 ('Oumuamua)

Presentation #232.07 in the session “Molecular Clouds”.

Published onJun 18, 2021
Assessing the Hydrogen Ice Hypothesis for 1I/2017 ('Oumuamua)

The unique physical and dynamical properties of 1I/2017 (ʻOumuamua) have yet to be explained by a composition of volatiles commonly found in the Solar System’s small-bodies. Furthermore, its passage within 0.2AU of the Earth implies that the Galaxy produces a large number of these interstellar objects. One interpretation, albeit an exotic one, is that ‘Oumuamua contained a substantial component of molecular hydrogen ice. While H2 ice could satisfy the observational constraints and would explain ‘Oumuamua as the first-detected member of a novel class of small-bodies, the possible formation scenario is unknown. We examine the feasibility of assembling ‘Oumuamua-like objects in the starless cores of Giant Molecular Clouds, the only known environments which could possibly reach the extreme conditions required for H2 deposition. Taking ‘Oumuamua as representative of a Galactic population, we discuss the relevant processes that would be required to produce this reservoir of interstellar objects. Via order-of-magnitude arguments on the energy balance in starless cores, simple analytic and numerical models of accretion, and the classic Kolmogorov framework of turbulence, we characterize the potential barriers at size regimes ranging from micron-sized dust to possible kilometer-scale progenitors of ‘Oumuamua. We find that the harshest obstacle against formation is the thermodynamic requirement for solid molecular hydrogen: temperature approaching that of the cosmic microwave background. However, if starless cores can cool via excellent shielding and pockets of adiabatic expansion, then the resulting characteristic “cloud products” may resemble ‘Oumuamua. Finally, we discuss the astrophysical implications if the H2 hypothesis is correct.

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