The Story of 1I/丹umuamua, The First Visitor from Another Star System

Karen Meech , Inst. Astron., Univ. Hawaii (USA)
On October 19, 2017 the Pan-STARRS1 telescope discovered a rapidly moving object. Additional astrometry obtained with pre-discovery observations on October 18 through data obtained with the Canada-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 October 14. The official name of 前umuamua, 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 discovery 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 characterized. Deep images of 前umuamua 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, but certainly larger than 5:1. Spitzer observations suggest an average diameter somewhere between 98-440 km depending on model-dependent surface thermal properties. As more time series data were obtained, it was evident that 前umuamua was in an excited spin state with the long axis precessing around the total angular momentum vector with an average period of 8.67ア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 前umuamua from its host planetary system. The color of 前umuamua was found to be quite red with a spectral slope of 23%ア3% per 100 nm, consistent with comet surfaces, the dark side of Iapetus, and other minerals. Precision astrometric measurements obtained from the Hubble Space Telescope and the ground allowed us to do a detailed study of the orbit. Analysis of 207 astrometric 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 前umuamua has a somewhat different nature from solar system comets. Many attempts were made to trace 前umuamua back to it痴 home system, the most detailed after the release of the Gaia 2 catalog, but no convincing candidates have been found. Whether this will ever be feasible depends on how long ago it was ejected. 前umuamua has challenged many of our assumptions about what small bodies from another solar system would look like, and has triggered an avalanche of papers, some highly speculative. In this talk I値l share the story of the discovery of 前umuamua and discuss what we know about our first known interstellar visitor including new information from papers in press.
Heidelberg Joint Astronomical Colloquium
28 May 2019, 16:15
Philosophenweg 12, gro゚er Hrsaal

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