to 400 km. Such objects are significantly larger than comets like P/Halley (effective diameter ศ11.3 km; Keller et al.
We present visible and near-infrared broadband photometry of the Centaur objects 1995 GO and 5145 Pholus. Our data 1987). confirm the extremely red V ุ J color and nearly solar J ุ
We know almost nothing about the physical and chemi-H and H ุ K colors of 5145 Pholus. Our V ุ J data reveal cal properties of the TNOs that have been discovered so that 1995 GO is as red as the reddest D-type asteroid, 1748 far. It is possible, perhaps likely, that they are similar in Mauderli, but not nearly as red as 5145 Pholus. We suggest bulk and surface composition to the short-period comets that the red color of 1995 GO is probably due to the presence that are believed to be derived from this part of the Solar of organic-rich materials produced by eons of particle bombard-System. However, because the known TNOs are much ment and ultraviolet irradiation of surface grains, a process larger than nearly all known comets and span a fairly wide that may be widespread among the Centaur and trans-nepturange in heliocentric distance, some or all of them may be nian object populations. We show that the V ุ J data for very different from known comets. Composition may be the Centaurs suggest that V ุ J increases with increasing semimajor axis. If this correlation is correct, then the red colors a function of heliocentric distance, providing a signature of the Centaurs 5145 Pholus and 1993 HA 2 may be typical of of formation mechanisms, as may be true for objects in the fairly pristine Centaur objects. In contrast, the progressively asteroid belt. Surface composition also may be a function of less red colors of 1995 GO and 2060 Chiron may be the result exposure time to high-energy bombardment by photons of an increased rate of resurfacing of these objects as they and cosmic ray particles.
migrate progressively closer to the Sun. ยฉ 1997 Academic Press
Cometary and TNO material may have been subject to a variety of chemical and physical processes during its evolution from submicrometer-sized icy grains in the dense
1. Introduction
molecular cloud core that preceded the solar nebula to final incorporation into comet and TNO nuclei. Specifically, Perhaps one of the more exciting discoveries in the field ultraviolet photons and strong stellar winds from a T Tauriof planetary astronomy in recent years has been the deteclike Sun may have synthesized complex carbon-bearing tion of trans-neptunian objects (TNOs) (Jewitt and Luu molecules from simpler ice components in the mantles of 1995). The TNOs discovered so far are observed in low the grains prior to accretion into larger comet and TNO eccentricity orbits (e ฯฝ 0.2), with semimajor axes of 30 to nuclei (Tegler et al. 1993, Weintraub et al. 1994, Tegler et 50 AU, and are thought to populate the inner region of al. 1995). In addition, such nuclei may have been subject the Kuiper Belt, the primordial reservoir of dynamically evolved (short period, low inclination) comets. The ob-to additional surface processing after their assembly. Laboratory experiments using keV and MeV particle accelera-served TNOs have effective diameters ranging from 100 456