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 mu m 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 mu m band. Dust production estimates ranging from 1.7 - 4 x 10(2) 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 alpha = -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.