Radar observations of asteroid 2063 Bacchus

We report Doppler-only (cw) and delay-Doppler radar observations of Bacchus obtained at Goldstone at a transmitter frequency of 8510 MHz (3.5 cm) on 1 996 March 22, 24, and 29. Weighted, optimally filtered sums of cw and delay -Doppler echoes achieve signal-to-noise ratios of similar to 80 and similar to 25, respectively, and cower about 180 degrees of rotation phase (period = 14.90 h; Pravec et al, 1998), Our cw observations place up to four 2-Hz- resolution cells on Bacchus at echo powers greater than two standard deviat ions of the noise. Delay-Doppler observations typically place about ten 0.5 -mu s (75-m)x 1-Hz cells on Bacchus above the same threshold. A weighted su m of all cw spectra gives an OC radar cross section of 0.12(-0.02)(+0.06) k m(2) and a circular polarization ratio of 0.21 +/- 0.01. The dispersion of the echoes in time delay indicates a lower bound on Bacchus' maximum pole-o n breadth of 0.6 km that is consistent with the echo bandwidth (6+/-2 Hz) a nd rotation period. Echo spectra on March 22 and delay-Doppler images on al l three days show a central deficit of echo power that provides strong evid ence for a bifurcation in the shape. Inversion of delay-Doppler images, cw spectra, and optical lightcurves obtained at Ondrejov Observatory yields si ngle-lobe and two-lobe models that define lower and upper bounds on the deg ree of bifurcation. Both shape models have a prominent central concavity, m odestly asymmetric shapes, and similar physical dimensions, spin vectors, a nd radar and optical geometric albedos, We adopt the more conservative sing le-lobe shape model as our working model and explore its implications. It h as a radar-derived sidereal rotation period P-sid = 15.0 +/- 0.2 h and a no rth pole within a few tens of degrees of ecliptic longitude lambda = 24 deg rees and ecliptic latitude beta = -26 degrees; retrograde rotation is likel y, It has dimensions of 1.11 x 0.53 x 0.50 km, an effective diameter (the d iameter of a sphere with the same volume as the model) D-eff = 0.63(-0.06)( +0.13) km, and radar and optical geometric albedos <(sigma)over cap> = 0.33 (-0.11)(+0.25) and p(v) = 0.56(-0.18)(+0.12), respectively, that are larger than most estimated for other asteroids. Bacchus' low circular polarizatio n ratio and high radar albedo are consistent with nearly regolith-free ordi nary chondrite and basaltic achondrite compositions, but its high optical g eometric albedo seems inconsistent with an ordinary chondrite composition a nd may favor a V-class composition. Bacchus has less structural complexity at centimeter-to-decimeter spatial scales and its near-surface is more dens e (either more metal, lower porosity, or both) than the average radar-detec ted near-Earth asteroid. (C) 1999 Academic Press.