GeminiFocus 2017 Year in Review | Page 7

that the object is exceptionally elon- gated in shape. “What we found was a rapidly rotating object, at least the size of a football field, that changed in brightness quite dramatically,” said Kar- en Meech of the University of Hawai‘i’s Institute for Astronomy, and the leader of the discovery team that first obtained Gemini DDT observations of the object. “This change in brightness hints that ‘Oumuamua could be ten times lon- ger than it is wide — something which has never been seen in our own Solar System.” The best current estimates are that the object has the dimensions, roughly speaking, of five football fields laid end to end (omitting endzones). The research led by Meech combines observations from the Gemini South and Very Large Telescopes in Chile, as well as from Pan-STARRS, Keck 2, Can- ada-France-Hawai‘i, and the United Kingdom Infrared Telescope in Hawai‘i. The study was published in the Novem- ber 20th online issue of Nature. Although the shape of ‘Oumuamua is like nothing seen in our Solar System, its color is more conventional. “Our first interstellar planetesimal is just slightly redder than reflected sunlight,” said Michele Bannister, an astronomer at the Astrophys- ics Research Centre of Queen’s University in Belfast, and the leader of another team that obtained Gemini DDT observations of ‘Ou- muamua. “This is fascinating, as we might have expected it would be deep red from spending a long time travelling between stars, where cosmic rays would alter organic molecules on its surface. Instead, its colour looks a lot like those of tiny minor planets in our own Solar System that orbit in Jupiter’s Trojan clouds, or some that orbit beyond Neptune.” Bannister adds, “Gemini’s ability to observe near-simultaneously in the optical and near- January 2018 / 2017 Year in Review infrared with rapid instrument-switching was ideal, as ‘Oumuamua turned out to be strongly variable in brightness, and we had to quantify that to properly measure its co- lours.” A paper presenting the results from Bannister’s team has been accepted by The Astrophysical Journal Letters. A preprint is available online. An additional surprise from ‘Oumuamua was highlighted in a study by the third team that obtained Gemini observations during the event. The team, led by Piotr Guzik and Michal Drahus of the Astronomical Obser- vatory of Jagiellonian University in Kraków, obtained 442 individual exposures all in the same red filter. “Thanks to the long, continu- ous observations at Gemini, we found that GeminiFocus Figure 2. Variations in the brightness (top) and magnitude (bottom) of ‘Oumuamua, as measured in the r-band filter with GMOS on successive nights in late October. The rotation period is determined with high precision to be 7.5483 hours, but the light curve does not repeat exactly from one rotation to the next. This indicates the object is “tumbling.” Figure reproduced from Drahus et al., https://arxiv. org/abs/1712.00437. 5