Photometric and polarimetric observations and model simulations of (216) Kleopatra

Abstract

We performed photometric and polarimetric observations, on November 8 and 9, 1999, of an M-type main belt asteroid, (216) Kleopatra by using the HBS spectropolarimeter installed at Dodaira observatory, National Astronomical Observatory of Japan (NAOJ). Photometric amplitude of lightcurve in the V band was 0.12 mag, and the averaged degree of polarization was −1.01±0.1%. It seems that the polarimetric data might also show a slight change in the degree of polarization (∼0.2%) at the second minimum of the photometric lightcurve, but we could not confirm that the feature was real because of the large errors of data. With the assumption that the surface is uniform, we have carried out lightcurve simulations based on shape models by Ostro et al. (2000), Tanga et al. (2001) and Roche binary (Cellino et al., 1985). The results of simulations were compared to the configurations of lightcurves which had been obtained at different 4 geometric positions (1980, 1982, 1987 and 1999). The model by Cellino et al. (1985) reproduced almost all the data points without the 1987 observations within ∼0.05 mag., which is the best result among the 3 models. The model by Tanga et al. (2001) well reproduced the lightcurves, but failed in reproducing the 1982 amplitude (difference diff ∼ 0.2 mag.). We also confirmed that the model by Ostro et al. (2000) could not explain the observed lightcurves.

Dumb-bell-shaped equilibrium figures for fiducial contact-binary asteroids and EKBOs

“…With an equivalent radius of 62.5 km, the overall extension of the dumb-bell solution is 283 km x 89 km x 81 km, with a central waist of 38 km, which is in rough agreement with the results of Tanga et al (2001). Takahashi et al (2004b) focused on the question of Kleopatra's shape through the Roche approximation formalism. They obtained a Roche solution with a mass ration of 0.84 and a weighting parameter of 05-0.6 ( Table 2).…”

Triplicity and physical characteristics of Asteroid (216) Kleopatra

“…Hestroffer et al (2002a) suggested that (216) Kleopatra most likely is very elongated, as suggested before; however, the authors could not determine whether the asteroid did consist of two components separated by a small gap, or whether it was indeed one elongated object. Takahashi et al (2004), from lightcurve simulations, tend to favor the bilobed model of Tanga et al (2001). These works presented serious caveats and evidence showing that the dog-bone shape model needs to be somehow revised and improved or at the very least compared with new imaging data with even higher spatial resolution.…”

Polarimetric Study of Near-Earth Asteroid (1566) Icarus

“…A notable example is (4) Vesta (Dollfus et al 1989), which showed a 0.1% polarimetric variation, and the maximum of the polarization coincides with the lightcurve minimum, suggesting that albedo variation exists on the surface per the controlled polarization degree and visible magnitude. Similarly, (3) Juno, (9) Metis, and (216) Kleopatra showed rotational variations of 0.15-0.27 %, ∼0.1 %, and ∼0.2 %, respectively (Takahashi et al 2009;Nakayama et al 2000;Takahashi et al 2004). Although the measurement accuracy is too limited to detect such a small variations in the polarization degree, we suggest that Icarus has a quite homogeneous albedo in contrast with these asteroids because our measurement was made at a large phase angle, while these previous detections were made at a small phase angle where the polarization degree itself has small values (1/6∼1/10 of Icarus's |P r |, i.e., 0.5 |P r | 1.0%).…”