CBET #2937 - 2011/12/5 -  2005 YU_55

Dennis Bodewits, Univ. of Maryland, College Park (UMD); Sergio Campana, INAF—Osservatorio Astron. di Brera, Merate; Jamie Kennea, Penn. State Univ., University Park; Jian-Yang Li, UMD; Samantha Oates, MSSL, Univ. Coll. London, Holmbury St Mary; Stefan Immler, UMD/NASA GSFC; and Neil Gehrels, NASA GSFC, Greenbelt,

report ultraviolet photometry of 2005 YU_55, taken with the Ultraviolet/Optical Telescope (UVOT) on board the Swift Gamma Ray Observatory during the asteroid’s close encounter with Earth. Tracking was not attempted. Instead, fluxes were measured using ‘event-mode’, allowing reconstructing an image of the asteroid as it moved over the detector. The object was observed 3 times with the solar blind uvm2 filter (center 260 nm, FWHM 70 nm) and once with the v filter (center 546.8 nm; FWHM 75 nm). We measured magnitudes of 16.07 ± 0.05 in uvm2 at UT 2011-11- 8.0717; 15.53 ± 0.02 in uvm2 at 2011-11-8.0717; 18.68 ± 0.08 in uvm2 at 2011-11-12.1569; and 14.27 ± 0.02 in v at 2011-11-12.0304. The best fit to the UV data suggests an absolute UV magnitude of H_uv = 25.6 and G = -0.13. Using the same phase curve for the v-band, we find an absolute magnitude H_v = 21.2. Assuming the spherical equivalent diameter reported by Merline et al. (CBET 9242) implies a geometrical albedo of 0.064 in visual, and 0.044 at 245 nm (the effective wavelength of the uvm2 filter; see Li et al. 2011 Icarus 216 p640). We conclude that the albedo, color, and phase curve are consistent with values characteristic for C-type asteroids (cf. Roettger & Buratti, 1994, Icarus 112 p496).