Probing the Radial Distribution of the Kuiper Belt using Stellar

Introduction: The Trans-Neptunian Objects (TNOs) are the frozen leftovers from the formation period of the outer solar system [1]. The current total mass in the Kuiper Belt is estimated to be around 0.03-0.3 Earth masses [2], but there is evidence that a much larger mass (10-40 M Earth) was originally present at the time of formation [3]. The direct observation of the TNOs does not allow reaching two important informations, the population of the small objects and the external part of the disc. However, by extrapolation of the density of matter in the solar system and by comparison with known cir-cumstellar discs, one can think that the distribution of sizes is extended to objects of size much lower than one kilometer and that the Kuiper belt spreads beyond 50 AU. The occultation method: A very powerful method to detect tiny and invisible objects by the direct method is to seek their transit in front of a star, that is the ser-endipitous stellar occultation method ([4] and references therein). Stellar occultation can detect kilometric objects beyond the orbit of Neptune. Serendipitous occultations, on the other hand, have no other competing methods, as the magnitudes of the corresponding objects, V~35 or fainter, is unreachable through classical ground-based imaging [4]. Such oc-cultations reveal the vertical and radial distribution of the TNOs as far as 50 AU and beyond. Also, it provides information on size distribution down to hectome-ter-sized objects. Observations and data analysis: We conducted a survey for serendipitous occultations in 17-20 May 2005 using the high-speed, triple-beam imaging pho-tometer ULTRACAM [5], mounted at the visitor focus of the 8.2-m Very Large Telescope in Chile. The stars were chosen to ensure a small angular diameter, which is critical to obtain diffraction's fringes. A total of ~4.0 million frames were obtained on the nights between 17/05/2005 and 20/05/2005, simultaneously in g' (0.48 µm) and r' (0.63 µm) SDSS filters. The reduced data consist in two lightcurves of the two observed stars in each field. thus totalizing 5 runs or continuous lightcurves. This leads to nearly 19 hours of data for each star. We used the same method for the analysis as described in [6]. This means that we search for deviant points from the mean standard deviation of the light-curve. and found no occultation event. However these results bring strong constraints on the Kuiper belt structure.