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SCUBA sheds new light on the size of Kuiper Belt objectsHervé Aussel
Kuiper Belt Objects (KBOs) are a population of planetary bodies beyond the orbit of Neptune. About 400 of these transneptunian objects are known as of today since the discovery of the first of them in 1992. It is suspected that the Kuiper belt hosts tens of thousands of 100 km size objects and billions of 1 km size. These bodies might be primitive remnants from the early accretion phase of the solar system. They are also believed to be the source of short period comets. Last, many share dynamical properties with Pluto and have for this reason been christened plutinos. These have played a major role in the recent debate on the nature and significance of the ``planet'' Pluto. Hence, the study of transneptunian objects is a current hot topic of planetary science. The crucial parameter that needs to be determined when studying a planetary body is its size. It is usually obtained by measuring the amount of sunlight reflected by the body. Unfortunately, this quantity is not only proportional to the square of the size, but also to the albedo of the body. Until now, the sizes of KBOs has been estimated by assuming the same albedo as cometary nuclei, which reflect 4% of the light they receive. A method to determine simultaneously the size and the albedo of a body is to observe at the same time in the optical (to measure the reflected light) and in the infrared (to measure the thermal emission). This method has been applied to many asteroids belt, by observing the thermal emission in the atmospheric windows at 10 and 20 microns. These measurements were successful because asteroids are close to the sun and therefore hot enough to radiate in the mid-infrared. KBOs are much farther and colder : with a temperature near 45 K, the Planck maximum at 70 microns is inaccessible from the ground infrared windows. But these cold objects are within the reach of the most sensitive instruments in the submillimeter windows. Using the Submillimeter Common User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope, we have observed the (at the time) brightest KBO 20000 Varuna on the nights of December 30 and 31, 2000 [1]. The observations were done using SCUBA photometric mode, and lead to the detection of the transneptunian body with a flux of 2.81 +/-0.85 mJy at 850 microns. At the same time, we obtained R band images of our target using the University of Hawaii 2.2m telescope. Figure 1 shows the displacement of 20000 Varuna during our observations : by observing on two different nights, we minimized the possibility of detecting a background galactic or extragalactic object. Moreover, we carefully checked that our detection could not be caused by cirrus fluctuation at this low galactic latitude. Combining our submillimeter and optical fluxes allowed us to determine an albedo of 7+3-2%, almost twice as large as the cometary nuclei one that was assumed for these objects previously. Our diameter determination is 900+125-145km, making of Varuna at the third largest transneptunian object after Pluto (2400 km) and Charon (1200 km). We note that this values are relatively insensitive to the parameters of the model we had to assume, essentialy the emissivity of Varuna. Recently, a potentially larger KBO, 2001 KX76 has been discovered, but no direct measurements of its albedo or size have yet been reported. More information on transneptunian objects and Varuna can be found on Dave Jewitt page at http://www.ifa.hawaii.edu/faculty/jewitt/kb.html.
FIGURE 1: R band images of 20000 Varuna taken at the 2.2m telescope of the University of Hawaii at the same time that SCUBA photometry was obtained.
back to:> September 2001 Newsletter Index
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