Abstract
We report on the analysis of several sequences of broadband visible images of comet C/2012 S1 (ISON) taken with the Wide Field Camera 3 of the Hubble Space Telescope on 2013 April 10, May 8, October 9, and November 1 in an attempt to detect and characterize its nucleus. Whereas the overwhelming coma precluded the detection of the nucleus in the first two sequences, the contrast was sufficient in early October to unambiguously retrieve the signal from the nucleus. Two images taken within a few minutes led to similar V magnitudes for the nucleus of 21.97 and 22.0 with a 1σ uncertainty of 0.065. Assuming a standard value for the geometric albedo (0.04) and a linear phase function with a coefficient of 0.04 mag deg{sup –1}, these V values imply that the nucleus radius is 0.68 ± 0.02 km. Although this result does depend on these two assumptions, we argue that the radius most likely lies in the range 0.6-0.9 km. This result is consistent with the constraints derived from the water production rates reported by Combi et al. The last sequence of images in 2013 November revealed temporal variation of the innermost coma. If attributed to a single rotating jet, thismore » coma brightness variation suggests the rotational period of the nucleus may be close to ∼10.4 hr.« less
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