Free-Floating planet Mass Function from MOA-II 9-year survey towards the Galactic Bulge

• DOI: 10.48550/arxiv.2303.08280
• Publication Date: 2023-01-01
• Authors: Sumi, Takahiro; koshimoto, Naoki; Bennett, David P.; Rattenbury, Nicho...; Abe, Fumio; Barry, Richard; Bhattacharya, Aparna; Bond, Ian A.; Fujii, Hirosane; Fukui, Akihiko; Hamada, Ryusei; Hirao, Yuki; Silva, Stela Ishi...; Itow, Yoshitaka; Kirikawa, Rintaro; Kondo, Iona; Matsubara, Yutaka; Miyazaki, Shota; Muraki, Yasushi; Olmschenk, Greg; Ranc, Clement; Satoh, Yuki; Suzuki, Daisuke; Tomoyoshi, Mio; Tristram, Paul . J.; Vandorou, Aikaterini; Yama, Hibiki; Yamashita, Kansuke
• Keywords: Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA); FOS: Physical sciences; Astrophysics - Astrophysics of Galaxies; Astrophysics - Earth and Planetary Astrophysics

17 pages, 7 figures, accepted for publication in AJ<br/>We present the first measurement of the mass function of free-floating planets (FFP) or very wide orbit planets down to an Earth mass, from the MOA-II microlensing survey in 2006-2014. Six events are likely to be due to planets with Einstein radius crossing times, $t_{\rm E}<0.5$days, and the shortest has $t_{\rm E} = 0.057\pm 0.016$days and an angular Einstein radius of $θ_{\rm E} = 0.90\pm 0.14μ$as. We measure the detection efficiency depending on both $t_{\rm E}$ and $θ_{\rm E}$ with image level simulations for the first time. These short events are well modeled by a power-law mass function, $dN_4/d\log M = (2.18^{+0.52}_{-1.40})\times (M/8\,M_\oplus)^{-α_4}$ dex$^{-1}$star$^{-1}$ with $α_4 = 0.96^{+0.47}_{-0.27}$ for $M/M_\odot < 0.02$. This implies a total of $f= 21^{+23}_{-13}$ FFP or very wide orbit planets of mass $0.33<br/>