  We present multiband photometric observations of nine Centaurs. Five of the
targets are known active Centaurs (167P/CINEOS, 174P/Echeclus, P/2008 CL94,
P/2011 S1, and C/2012 Q1), and the other four are inactive Centaurs belonging
to the redder of the two known color subpopulations (83982 Crantor, 121725
Aphidas, 250112 2002 KY14, and 281371 2008 FC76). We measure the optical colors
of eight targets and carry out a search for cometary activity. In addition to
the four inactive Centaurs, three of the five active Centaurs showed no signs
of activity at the time of observation, yielding the first published color
measurements of the bare nuclei of 167P and P/2008 CL94 without possible coma
contamination. Activity was detected on P/2011 S1 and C/2012 Q1, yielding
relatively high estimated mass loss rates of $140\pm20$ and $250\pm40$ kg/s,
respectively. The colors of the dormant nuclei are consistent with the
previously-published colors, indicating that any effect of non-geometric
scattering from Centaur dust or blanketing debris on the measured colors is
minimal. The results of our observations are discussed in the context of the
cause of Centaur activity and the color distributions of active and inactive
Centaurs. We suggest that the relative paucity of red Centaurs with
low-perihelion orbits may not be directly due to the blanketing of the surface
by unweathered particulates, but could instead be a result of the higher levels
of thermal processing on low-perihelion Centaurs in general.

Earth and Planetary Astrophysics