Color and Size Distributions of Small Jupiter Trojans

Abstract

We conducted a two-band imaging survey observation using the Subaru Telescope and its wide-field camera, Suprime-Cam, to study the visible colors and size distribution of Jupiter's Trojan asteroids. The survey covered an area around Jupiter's L4 Lagrange point totaling 9.2 square degrees. We detected 120 Trojan asteroids in this survey. From these Trojan asteroids, we extracted 44 unbiased samples with absolute magnitudes in the $g^\prime$ band ranging from 12.9 to 16.9 mag (corresponding to diameter ranges of approximately $\sim$3 - 16 km assuming an albedo of 0.05) and analyzed their $g^\prime - i^\prime$ color and size distributions. Large Jupiter Trojan asteroids are known to be classified into two color groups, ''red'' and ''less red''. We found that such bimodality in the color distribution is absent for small Jupiter Trojan asteroids, which is consistent with previous studies. Previous studies have also shown that these two groups have different slopes in the magnitude distributions from each other, which was explained by conversion of red objects to less-red fragments through catastrophic disruptions. In contrast, we found that the size frequency distributions of our two sample groups divided by the color of $g^\prime - i^\prime$ = 0.7 (in AB magnitude) are quite similar. Our results can provide new insights into collisional evolution of color and size distribution of small Jupiter Trojans.

Figures (7)

• Figure 1: Locations of our survey fields. The size of each rectangle corresponds to the field of view of Suprime-Cam. The solid line is the ecliptic plane, and the dashed lines show ecliptic latitudes with an interval of 1$\hbox{$^\circ$}$. The background image is from Pan-STARRS.
• Figure 2: Examples of JT detection efficiencies measured from 90-s exposures taken with a $g^\prime$ band filter on May 24, 2017 UT, under sunny conditions $\sim$ 1$\hbox{$.\!\!^{\prime\prime}$}$0 seeing. Each panel with the detector ID (in parentheses the name of the CCD chip) in the upper left corner corresponds to the actual layout of the CCD mosaic on the Suprime-Cam focal plane. CCDs #0, #3, #6, and #8 are located at the corners of the FoV of the Suprime-Cam. The solid lines are best-fit curves (see text).
• Figure 3: Lower left: The distribution of heliocentric distance ($R$) vs. $g^\prime$-band absolute magnitude ($H_g$) of the detected JT asteroids. The uncertainty of the estimated heliocentric distances is $\sim$0.11 au, which causes the $H_g$ errors of $\sim$0.1 mag as the primary factor for most objects. The filled circles are the objects with $R$$<$ 5.6 au and $H_g$$<$ 16.9 mag (separated by the dashed lines) defined as the unbiased sample. The detection limit of our data, namely the apparent magnitude of $m_g$ = 23.9 mag, is indicated by the dotted line. Upper left and lower right: the $R$ and $H_g$ histograms of the full sample (open bars) and the unbiased sample (solid bars), respectively.
• Figure 4: The histogram of the $g^\prime - i^\prime$ color distribution of the unbiased JT sample. Error bars are based on Poisson errors. The dashed line shows the color of the Sun bertin1996. The squares with solid lines show the mode values and color ranges, respectively, of the C-, X-, and D-type asteroids derived from the SDSS-based Asteroid Taxonomy database hasselmann2011.
• Figure 5: The normalized absolute magnitude distributions of the "less red" (circles) and "red" (squares) objects separated by $g^\prime -i^\prime$ = 0.70 mag. The dashed line shows the power-law function for the absolute magnitude distribution of L4 JTs obtained by yoshida2017.