Albedos of Small Jovian Trojans

arXiv:0906.1786v1 [astro-ph.EP] 9 Jun 2009 appearing in Astron. J., July 2009, vol. 138, pp. 240-250 Albedos of Small Jovian Trojans Yanga R. Fern´andez Department of Physics, University of Central Florida, 4000 Central Florida Blvd, Orlando, FL 32816-2385 David Jewitt Institute for Astronomy, University of Hawaii, 2680 Woodlawn Dr, Honolulu, HI 96822 and Julie E. Ziffer Department of Physics, University of Southern Maine, 96 Falmouth St, Portland, ME 04104-9300 ABSTRACT We present thermal observations of 44 Jovian Trojan asteroids with diame- ters D ranging from 5 to 24 km. All objects were observed at a wavelength of 24 µm with the Spitzer Space Telescope. Measurements of the thermal emission and of scattered optical light, mostly from the University of Hawaii 2.2-meter telescope, together allow us to constrain the diameter and geometric albedo of each body. We find that the median R-band albedo of these small Jovian Tro- jans is about 0.12, much higher than that of “large” Trojans with D > 57 km (0.04). Also the range of albedos among the small Trojans is wider. The small Trojans’ higher albedos are also glaringly different from those of cometary nuclei, which match our sample Trojans in diameter, however they roughly match the spread of albedos among (much larger) Centaurs and trans-Neptunian objects. We attribute the Trojan albedos to an evolutionary effect: the small Trojans are more likely to be collisional fragments and so their surfaces would be younger. A younger surface means less cumulative exposure to the space environment, which suggests that their surfaces would not be as dark as those of the large, primordial Trojans. In support of this hypothesis is a statistically significant correlation of higher albedo with smaller diameter in our sample alone and in a sample that – 2 – includes the larger Trojans. This correlation of albedo and radius implies that the true size distribution of small Trojans is shallower than the visible magni- tude distribution alone would suggest, and that there are approximately half the Trojans with D > 1 km than previously estimated. Subject headings: minor planets — infrared: solar system 1. Introduction Jupiter’s Trojan asteroids inhabit two swarms centered on the L4 and L5 Lagrangian points located 5.2 AU from the Sun and from the planet. More than 2700 Trojans are known at the time of writing. Based on optical studies, the total population larger than 1 km in radius has been estimated by various workers: Jewitt et al. (2000) estimated ∼1.6×105 such objects in the L4 swarm; Szab´o et al. (2007) estimated ∼2.4×105 in both swarms combined; Yoshida & Nakamura (2005) estimated ∼2.4×105 in the L4 swarm; and Nakamura & Yoshida (2008) estimated ∼0.63×105 in the L4 swarm and ∼0.34×105 in the L5. The magnitude- derived size distribution resembles a broken power law (Jewitt et al. 2000), and is such that the bulk of the mass (approximately 10−4 M⊕, where M⊕= 6×1024 kg is the mass of the Earth) is contained within the largest objects. By number and by mass, the Trojan pop- ulation is only slightly inferior to the population of the main-belt asteroids. However, the observational attention given to the Trojans so far is miniscule compared to that lavished on the main-belt objects, and many of the basic properties of Jupiter’s Trojans remain poorly known. The Trojans have been reviewed alongside the irregular satellites of Jupiter, to which they may be closely related, by Jewitt et al. (2004) and separately by Dotto et al. (2008). Scientific interest in the Trojans focuses both on their origin and on their composition. How and when they were trapped in 1:1 mean-motion resonance with Jupiter remains un- known. Capture at a very early epoch in association with planet formation and capture much later, in a dynamical clearing phase in the Solar system, are both under current considera- tion (Morbidelli et al. 2005; Marzari & Scholl 2007). The snow-line in the Solar system was most likely inside the orbit of Jupiter (Garaud & Lin 2007), so if they formed in-situ or at a more distant location in the Sun’s protoplanetary disk, the Trojans could have incorporated water as bulk ice. In this sense, the Trojans might share as much in common with the nuclei of comets as with the classical, rocky asteroids. Observationally, the measured Tro- jans resemble the nuclei of short-period comets in their optical colors (Jewitt & Luu 1990; Fornasier et al. 2007) and albedos (Fern´andez et al. 2003, Paper I), tending to reinforce by association the possibility that they might be comet-like, ice-rich bodies. On the other hand, low-resolution spectral observations in the near infrared have uniformly failed to reveal ab- – 3 – sorption bands that could be attributed to water ice or, indeed, to show any absorption bands at all (Luu et al. 1994; Dumas et al. 1998; Emery & Brown 2003; Yang & Jewitt 2007). The low albedos, neutral to reddish optical colors and featureless near infrared spectra are com- patibl

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