WISE/NEOWISE Observations of the Jovian Trojan Population: Taxonomy

Abstract

We present updated/new thermal model fits for 478 Jovian Trojan asteroids observed with the Wide-field Infrared Survey Explorer (WISE). Using the fact that the two shortest bands used by WISE, centered on 3.4 and $4.6\mu$m, are dominated by reflected light, we derive albedos of a significant fraction of these objects in these bands. While the visible albedos of both the C-, P- and D-type asteroids are strikingly similar, the WISE data reveal that the albedo at $3.4\mu$m is different between C-/P- and D-types. The albedo at 3.4$\mu$m can be thus be used to classify the objects, with C-/P-types having values less than 10%, and D-types have values larger than 10%. Classifying all objects larger than 50km shows that the D-type objects dominate both the leading cloud ($L_4$), with a fraction of 84%, and trailing cloud ($L_5$), with a fraction of 71-80%. The two clouds thus have very similar taxonomic distribution for these large objects, but the leading cloud has a larger number of of these large objects, $L_4/L_5 = 1.34$. The taxonomic distribution of the Jovian Trojans are found to be different than that of the large Hildas, which is dominated by C- and P-type objects. At smaller sizes, the fraction of D-type Hildas starts increasing, showing more similarities with the Jovian Trojans. If this similarity is confirmed through deeper surveys, it could hold important clues to the formation and evolution of the two populations. The Jovian Trojans does have similar taxonomic distribution to that of the Jovian irregular satellites, but lacks the ultra red surfaces found among the Saturnian irregular satellites and Centaur population.