MEASURING THE ABUNDANCE OF SUB-KILOMETER-SIZED KUIPER BELT OBJECTS USING STELLAR OCCULTATIONS

We present here the analysis of about 19,500 new star hours of low ecliptic latitude observations (vertical bar b vertical bar <= 20 degrees) obtained by the Hubble Space Telescope's Fine Guidance Sensors over a time span of more than nine years, which is in addition to the similar to 12,000 star hours previously analyzed by Schlichting et al. Our search for stellar occultations by small Kuiper Belt Objects (KBOs) yielded one new candidate event corresponding to a body with a 530 +/- 70 m radius at a distance of about 40 AU. Using bootstrap simulations, we estimate a probability of approximate to 5% that this event is due to random statistical fluctuations within the new data set. Combining this new event with the single KBO occultation reported by Schlichting et al. we arrive at the following results: (1) the ecliptic latitudes of 6 degrees.6 and 14 degrees.4 of the two events are consistent with the observed inclination distribution of larger, 100-km-sized KBOs. (2) Assuming that small, sub-kilometer-sized KBOs have the same ecliptic latitude distribution as their larger counterparts, we find an ecliptic surface density of KBOs with radii larger than 250 m of N(r > 250 m) = 1.1(-0.7)(+1.5) x 10(7) deg(-2); if sub-kilometer-sized KBOs have instead a uniform ecliptic latitude distribution for -20 degrees < b < 20 degrees then N(r > 250 m) = 4.4(-2.8)(+5.8) x 10(6) deg(-2). This is the best measurement of the surface density of sub-kilometer-sized KBOs to date. (3) Assuming the KBO size distribution can be well described by a single power law given by N(>r) proportional to r(1-q), where N(>r) is the number of KBOs with radii greater than r, and q is the power-law index, we find q = 3.8 +/- 0.2 and q = 3.6 +/- 0.2 for a KBO ecliptic latitude distribution that follows the observed distribution for larger, 100-km-sized KBOs and a uniform KBO ecliptic latitude distribution for -20 degrees < b < 20 degrees, respectively. (4) Regardless of the exact power law, our results suggest that small KBOs are numerous enough to satisfy the required supply rate for the Jupiter family comets. (5) We can rule out a single power law below the break with q > 4.0 at 2 sigma, confirming a strong deficit of sub-kilometer-sized KBOs compared to a population extrapolated from objects with r > 45 km. This suggests that small KBOs are undergoing collisional erosion and that the Kuiper Belt is a true analog to the dust producing debris disks observed around other stars.