Exploring the Enigma of 4709 Ennomos

Large Trojan asteroids are characteristically dark, having albedos that are typically in the range 0.03 to 0.08 (Fernandez et al., 2003). One notable exception is 4709 Ennomos with an unusually high measured albedo of about 0.13 (Fernandez et al., 2003). This corresponds to an albedo of more than 10 standard deviations above the mean of the group of 32 large Trojans sampled by Fernandez et al (2003). There are two main explanations for the anomalous albedo: Ennomos's surface composition may truly be different from similarly-sized Trojans and be richer in more highly-reflective species, or the assumptions that go into the modeling used to derive diameter and albedo are inapplicable to Ennomos because of unusual physical or thermal properties. For the first hypothesis, so far only upper limits to compositional signatures have been found (e.g. Yang and Jewitt 2007). In this work we address the second hypothesis. One plausible explanation is that Ennomos’ rotation period is sufficiently fast or its thermal inertia is sufficiently high so as to preclude the use of a zero-thermal memory thermal model (Lebofsky and Spencer 1989, Harris 1998) i.e. the model actually used to calculate its albedo. An alternative explanation is that shape or topographic anomalies conspired to reduce the thermal emission, causing the model - which assumes a spherical body - to underestimate the diameter. To address these issues, we obtained BVRI time-series CCD photometry of Ennomos with the University of Hawaii's 88 inch telescope on February 8, 9, and 10, 2003. The goals were to determine Ennomos’ rotation period, basic shape, and visible colors, and we will present these results. We will also discuss what the results imply about the nature of Ennomos's surface.