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10/18/18: SO/NOAO Joint Colloquium Series: Karen Meech, Univ. of Hawaii


Title: The Story of 1I/’Oumuamua, the First Visitor from Another Star System

On October 19, 2017 the Pan-STARRS1 telescope discovered a rapidly moving object. Additional as-trometry obtained with pre-discovery observations on October 18 through data obtained with the Cana-da-France-Hawaii-Telescope on October 22 showed that the object had the highest hyperbolic eccentricity ever detected, confirming that this object clearly originated from outside the solar system. 1I/2017 U1 passed perihelion on September 9, 2017 and had made its Earth close approach at 63 lunar radii on Octo-ber 14. The official name of ‘Oumuamua, meaning visitor from the distant past, was approved by the IAU on November 6. Beginning on October 22 there was an intense effort to secure observing resources to characterize the object. Because it was receding rapidly from the Earth and Sun, within a week of dis-covery the brightness had dropped by a factor of 10 and in less than a month it had dropped by a factor of 100. Thus, there was a period of just over a week where the target could be relatively easily character-ized. Deep images of ‘Oumuamua showed no hint of cometary activity, with limits on the amount of dust that could be present at less than 7-8 orders of magnitude that of a typical comet at similar distances. Light curve observations showed that the object was rotating with an instantaneous rotation period of 7.34 hours, and a light curve range of 2.5 magnitudes, implying an extremely elongated axis ratio perhaps as large as 10:1. Assuming a low albedo typical of comets (4%) this implies a size of 800x80x80 m. Howev-er, as more time series data were obtained, it was evident that ‘Oumuamua was in an excited spin state with the long axis precessing around the total angular momentum vector with an average period of 8.67±0.34 hr. The timescale for damping an excited spin in a body this size is very long, so the spin state may reflect the violent process of ejection of ‘Oumuamua from its host planetary system. The color of ‘Oumuamua was found to be quite red with a spectral slope of 23%±3% per 100 nm, consistent with comet surfaces, the dark side of Iapetus, and other minerals.

Our final experiment was to combine position measurements obtained from the Hubble Space Telescope with ground based measurements to do a detailed study of the orbit. Our analysis of 207 as-trometric positions showed that the orbit cannot be fit by a purely gravity-only trajectory, but are well matched (at the 30-sigma level) by the addition of a radial acceleration. We explored several explanations for the non-gravitational motion, and found that cometary outgassing is the most physically plausible, but requires that ‘Oumuamua has a somewhat different nature from solar system comets. In this talk I’ll share the story of the discovery of ‘Oumuamua and discuss what we know about our first known interstellar visitor – including new information from papers in press.