W.J. Merline, Southwest Research Institute, Boulder P.M. Tamblyn, Southwest Research Instutute, Boulder W.M. Owen, Jet Propulsion Lab, Pasadena C. Neyman, Keck Observatory, Hawaii et al IOTA meeting Rocklin, CA 2011 July 17 Keck Observations in preparation for the 2011 July occultation event by (90) Antiope

PURPOSE OF OBSERVATIONS Search for duplicity of star at level of less than 0.1 arcsec Get orbital phase (PA) and separation of binary close-in-time to event (the hope was to catch Antiope at exactly the phase or half phase of the occ event, meaning no computation would be needed to know exact configuration – we got close, but not exactly – 31 min off) Possible astrometry of star to guide field stations

Predictions for 2011 Jul 19, 10:25 UT, over central Calif. Previous prediction (French group, projecting forward about 2 years since last data input) (approx): PA ~ 26 deg Sep ~ 168 km Crude analysis by Merline, quick look: PA ~ 21 deg Sep ~ 145 km Improved formal analysis of data from Keck, Jul 13 & 15 UT, measured by P. Tamblyn, orbit fit by W. Owen: PA ~ 17 deg +/- 0.5 deg Sep ~ 161 km (there may be ~ 11 km uncertainty in scale here)

Much of PA difference could be explained if French predictions were in asteroid frame rather than Earth-Received-Time (but they say it was corrected). New prediction means, we think, that the chances of a gap observed between the two components is reduced, but the chances of a double event increase for some observers.

Duplicity of Star Both wider and narrower field of star searched for companion, see next 2 frames Definitively single

Astrometry of field star We had come up with a plan to do an image of the widest field allowed using the NIRC2 imager at Keck because we had not learned of anyone else who could get an image We had some tricks of using very narrow filters, coronagraph, etc. In the end we did not attempt any observations because there were no other stars within the small (40 arcsec) field that could be used to do any astrometry.