Small Jupiter Trojans Survey with Subaru/Hyper Suprime-Cam

Abstract

We observed the L4 Jupiter Trojans (L4 JTs) swarm using the Hyper Suprime-Cam attached to the 8.2~m Subaru telescope on March 30, 2015 (UT). The survey covered $\sim$26~deg$^{2}$ of sky area near the opposition and around the ecliptic plane with the 240-sec exposure time in the $r$-band filter through the entire survey. We detected 631 L4 JTs in the survey field with the detection limit of $m_r$~=~24.4~mag. We selected 481 objects with absolute magnitude $H_r$~$<$~17.4 mag and heliocentric distance $r$~$<$~5.5~au as an unbiased sample and then used them to estimate the size distribution. Assuming a geometric albedo of 0.07 \citep{grav12}, the size range of our unbiased sample is $\sim$~2~--~20~km in diameter ($D$). We can fit a single-slope power law to the cumulative size distribution and then found the best-fit index ($b$) is $b$~=~1.84~$\pm$~0.05 in $N(>D)~\propto~D^{-b}$. The slope value ($\alpha $) of corresponding absolute magnitude distribution ($N(H)~\propto~10^{\alpha H}$) is 0.37~$\pm$~0.01. This $\alpha$ is consistent with that of the faint end slope presented by \cite{WB15}. The size distribution obtained from this survey is slightly different from previous survey's result with a similar size range \citep*{YN05,YN08,WB15}, which reported a broken power-law or double power-law slopes in their cumulative size distribution. Our results insists that the slope of $b$~=~1.84 continues from $H$~=~14.0 to at least $H$~=~17.4. Since this work contains the largest L4 JT samples and one magnitude deeper than the study by \cite{WB15}, we believe that our study obtained the robustest size distribution of small JTs so far. Combining the cataloged L4 JTs and our survey, we finally show the entire size distribution of L4 JTs up to $H_r$~=~17.4~mag.