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[8.03] HST observations of Kuiper Belt binaries

J. L. Margot (Cornell University), M. E. Brown (Caltech), C. A. Trujillo (Gemini North Observatory), R. Sari (Caltech)

We report preliminary results from a Hubble Space Telescope (HST) program designed to characterize the orbital and physical properties of six confirmed KBO binaries [Margot et al. 2003]. Our results show that KBOs in our sample have much smaller sizes and larger albedos than expected, possibly requiring continuous collisional resurfacing and implying a Kuiper Belt that is less massive than previously assumed.

Table 1 describes a subset of four KBO binaries that can be clearly resolved only with HST (1999 TC36, 1998 SM165, 2001 QC298, 1997 CQ29). The orbital period P and semi-major axis a yield the total mass of the binary M. Uncertainties listed are three times the formal errors of the fit or 1%, whichever is greater. The primary to secondary radius ratio R_p/R_s is based on flux measurements in HST's F606W filter. We used the absolute magnitudes as reported by JPL's Horizons system, assumed that binary components have identical albedos and densities, and derived the densities corresponding to geometric albedos of 5%, 10%, and 20%. Most KBOs in our sample must have albedos in excess of the radiometric average of 8% [Altenhoff et al., 2004] otherwise their density would be implausibly low. For unit density, the primary radii/geometric albedos are (147 km, 23%), (116 km, 15%), (117 km, 7.6%), and (42 km, 41%). Detection of such small KBOs in the IR/mm is challenging.

	P [days]	a [km]	M [1018 kg]	Rp/Rs	\rho5	\rho10	\rho20
TC	50.38 ± 0.5	7640 ± 460	13.9 ± 2.5	2.7	0.1	0.3	0.8
SM	130.1 ± 1	11310 ± 110	6.78 ± 0.24	3.0	0.2	0.5	1.5
QC	19.23 ± 0.2	3690 ± 70	10.8 ± 0.7	1.2	0.5	1.5	4.3
CQ	309.2 ± 3	8320 ± 240	0.48 ± 0.04	1.2	-	0.1	0.3

References\\ Margot, Brown, Trujillo, Sari, HST General Observer Prgm 9746, 2003. \\ Altenhoff, Bertoldi, Menten, A&A 415, 2004.

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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.