2018AJ....156..270S

2013 FY27 is the ninth intrinsically brightest Trans-Neptunian Object (TNO). We used ALMA at thermal wavelengths and Magellan in the optical to determine 2013 FY27's size and albedo for the first time and compare it to other dwarf planets. We found 2013 FY27 has a geometric albedo of p_V=0.17_–0.030^+0.045 and effective diameter of D=765_–85⁺⁸⁰ km. This puts 2013 FY27 in the transition region between the largest TNOs that have higher albedos and densities than smaller TNOs. No short-term light curve was found, with variations <0.06 ± 0.02 mag over hours and days. The Sloan colors of 2013 FY27 are g-r = 0.76 ± 0.02 and r-i = 0.31 ± 0.03 mag, giving a moderately red color. This is different than the neutral or ultra-red colors found for the 10 largest TNOs, making 2013 FY27 one of the largest moderately red TNOs, which are only seen, and in abundance, at diameters less than 800 km. This suggests something different might be associated with TNOs larger than 800 km. Moderately red colors might indicate old or ice-poor surfaces with TNOs larger than 800 km having fresher or more volatile-rich surfaces. TNOs larger than 800 km could be more differentiated, giving them different surface compositions. A satellite at 0.''17 and 3.0 ± 0.2 mag fainter than 2013 FY27 was found through Hubble Space Telescope observations. Almost all the largest TNOs have satellites, which now includes 2013 FY27. Assuming a similar albedo, the satellite is ∼186 km in diameter, making the primary D=742_–83⁺⁷⁸ km.