Shigeru Takahashi

Abstract Recent observations of the Edgeworth-Kuiper belt object (EKBO) 2001 $\mathrm{QG}_{298}$ (Sheppard, Jewitt 2004) have shown that the lightcurve of this object has a very large amplitude ($1.14 \pm 0.04 \,\mathrm{mag}$), indicating that it is either of an elongated shape or of a binary structure with two components of similar sizes nearly in contact with each other. On the basis of these interesting published data, we employed Roche binary lightcurve simulations to construct a shape model of EKBO 2001 $\mathrm{QG}_{298}$. The shape parameters of the best-fitted model were $260\ (164) \times 205 (130) \times 185\ (116) \,\mathrm{km}$ for the primary, and $265 (168)\times 160\ (102) \times 150\ (94) \,\mathrm{km}$ for the secondary in the case of an albedo of 0.04 (0.10). An additional result of this calculation is that the average bulk density of the contact binary system could be estimated to be $630 \,\mathrm{kg} \,\mathrm{m}^{-3}$. This value is similar to that of several icy moons of Saturn with a diameter of less than 200 km. We have also used the Jacobi ellipsoidal approximation to compute the shape of one of the largest EKBOs, Varuna. The corresponding shape parameters are $a : b : c = 1.00 : 0.76 : 0.50$. The lower limit of the bulk density is $\rho \ge 1000 \,\mathrm{kg} \,\mathrm{m}^{-3}$. These results are in good agreement with the published values of Jewitt and Sheppard (2002), and are consistent with their suggestion that larger icy bodies have higher densities (Sheppard, Jewitt 2002).