A Lightcurve and Color Analysis of Asteroid 4709 Ennomos

We will present results from our study of the Jovian Trojan asteroid 4709 Ennomos, an asteroid with an unusually high estimated albedo. Large Trojan asteroids (radius > 25 km) have a mean V-band geometric albedo of 0.041 with very little variation (standard deviation = 0.007 ; Fernandez et. al. 2003). Smaller Trojan asteroids, with radius < 25 km, have both higher albedo (mean = 0.12) and wider variation (standard deviation = 0.065; Fernandez et. al. 2010). Asteroid 4709 Ennomos has a radius of about 38 km and a geometric albedo of about 0.15: several standard deviations above the mean albedo of other large Trojans, but very similar to the albedos of small Trojans. One plausible explanation of Ennomos’ apparently high albedo is that its rotation period may be sufficiently fast so as to invalidate the use of a low-thermal memory thermal model to calculate its size and albedo--the model used for Ennomos. To test this hypothesis, we obtained time series CCD photometry of Ennomos’ light curve using the University of Hawaii 88-inch telescope on UT February 8 through 10, 2003. Analysis of Ennomos’ light curve and rotation period will determine if an isothermal latitude model is more appropriate. Since asteroids of Ennomos’ size, both Trojans and Main-Belt, tend to be relatively slow rotators, a high rotation speed would be unusual. We therefore also consider some of the other hypotheses to explain Ennomos’ high albedo. For example, comparing Ennomos’ colors to those of other asteroid groups can give clues to the reason for an elevated albedo. To this end, we also obtained BVRI colors of Ennomos during our 2003 observing run. We will present a comparison between Ennomos’ colors, other published large Trojan and small Trojan colors (e.g. Jewitt & Luu 1990), and small asteroid colors (e.g. Karlsson et al. 2009).