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Friday, December 13, 2019
2:00 PM - 3:00 PM
Cahill 370

TAPIR Seminar

Evidence that 1I/2017 U1 ('Oumuamua) was composed of molecular hydrogen ice
Darryl Seligman, Graduate Student and Gruber Fellow, Department of Astronomy, Yale University,
Speaker's Bio:
Darryl grew up in the Philadelphia area and attended the University of Pennsylvania where he majored in physics and mathematics. His research interests include hydrodynamics and magnetohydrodynamics of solar system formation and stellar astrophysics. He is the Co-Founder of a science outreach organization called OpenLabs ( In his free time he enjoys playing the guitar, traveling, playing sports and being in the outdoors.

'Oumuamua (I1 2017) was the first macroscopic ($l\sim100\,{\rm m}$) body observed to traverse the inner solar system on an unbound hyperbolic orbit. Its light curve displayed strong periodic variation, and it showed no hint of a coma or emission from molecular outgassing. Astrometric measurements indicate that 'Oumuamua experienced non-gravitational acceleration on its outbound trajectory, but energy balance arguments indicate this acceleration is inconsistent with a water ice sublimation-driven jet of the type exhibited by solar system comets. Here, we show that all of 'Oumaumua's observed properties can be explained if it contained a significant fraction of molecular hydrogen (H$_{2}$) ice. H$_{2}$ sublimation at a rate proportional to the incident solar flux generates a surface-covering jet that reproduces the observed acceleration. Mass wasting from sublimation leads to monotonic increase in the body axis ratio, explaining 'Oumuamua's shape. Back-tracing 'Oumuamua's trajectory through the Solar System permits calculation of its mass and aspect ratio prior to encountering the Sun. We show that H$_{2}$-rich bodies plausibly form in the coldest dense cores of Giant Molecular Cloud Cores, where number densities are of order $n\sim10^5$, and temperatures approach the $T=3\,{\rm K}$ background. Post-formation exposure to galactic cosmic rays implies a $\tau \sim 100$ Myr age, explaining the kinematics of 'Oumuamua's inbound trajectory.

For more information, please contact JoAnn Boyd by phone at x4280 or by email at