The Large-Grained Dust Coma of 174P/Echeclus

On 2005 December 30, Y.-J. Choi and P. R. Weissman discovered that the formerly dormant Centaur 2000 EC98 was in strong outburst. Previous observations by P. Rousselot et al. spanning a 3-year period indicated a lack of coma down to the 27 mag arcsec^-2 level. We present Spitzer Space Telescope MIPS observations of this newly active Centaur—now known as 174P/Echeclus (2000 EC98)—or 60558 Echeclus—taken in 2006 late February. The images show strong signal at both the 24 and 70 μm bands and reveal an extended coma about 2‧ in diameter. Analyses yield estimates of the coma signal contribution that are in excess of 90% of the total signal in the 24 μm band. Dust production estimates ranging from 1.7-4 × 10² kg s^-1 are on the order of 30 times that seen in other Centaurs. Simultaneous visible-wavelength observations were also obtained with Palomar Observatory's 200-inch telescope, the 1.8-m Vatican Advanced Technology Telescope, the Bohyunsan Optical Astronomy Observatory (BOAO) 1.8-m telescope, and Table Mountain Observatory's 0.6-m telescope, revealing a coma morphology nearly identical to the mid-IR observations. The grain size distribution derived from the data yields a log particle mass power-law with slope parameter α = -0.87 ± 0.07, and is consistent with steady cometary activity, such as that observed during the Stardust spacecraft’s encounter at 81P/Wild 2, and not with an impact-driven event, such as that caused by the Deep Impact experiment.