Visible and near-infrared photometry of the centaur objects 1995 GO and 5145 Pholus

We present visible and near-infrared broadband photometry of the Centaur objects 1995 GO and 5145 Pholus. Our data confirm the extremely redV-Jcolor and nearly solarJ-HandH-Kcolors of 5145 Pholus. OurV-Jdata reveal that 1995 GO is as red as the reddest D-type asteroid, 1748 Mauderli, but not nearly as red as 5145 Pholus. We suggest that the red color of 1995 GO is probably due to the presence of organic-rich materials produced by eons of particle bombardment and ultraviolet irradiation of surface grains, a process that may be widespread among the Centaur and trans-neptunian object populations. We show that theV-Jdata for the Centaurs suggest thatV-Jincreases with increasing semimajor axis. If this correlation is correct, then the red colors of the Centaurs 5145 Pholus and 1993 HA₂may be typical of fairly pristine Centaur objects. In contrast, the progressively less red colors of 1995 GO and 2060 Chiron may be the result of an increased rate of resurfacing of these objects as they migrate progressively closer to the Sun.