A Detection Method for Small Kuiper Belt Objects:
The Search for Stellar Occultations

Françoise Roques and Michel Moncuquet

Abstract:

We explore the possibility of detecting small Kuiper Belt Objects (KBO) by serendipitous observation of stellar occultations: We show that such unpredictable occultations may allow us to detect a population of very small objects (typically of $\sim$100m radius at 40AU), invisible by any other observational method, as long as (i)the assumed population fills up a sufficient area on the sky plane, (ii)the instrumental sensitivity and acquisition frequency are high enough and (iii)the observed star has a small angular radius. This result is basically due to the diffractive broadening of the geometric shadow of small (assumed numerous) occulting objects. This diffractive broadening is more pronounced for smaller stellar disks and better photometric precision. Assuming there exist about $10^{11}$ objects of radius $\rho > 1$km, located between 30 and 50 AU near the Ecliptic, and that the differential size distribution varies as $\rho^{-q}$ with the index $q = 4$ extending down to decameter-sized objects, we expect a number of valid occultations (i.e. a $4\sigma$ event) between a few to several tens per night, if we may obtain an r.m.s. signal fluctuation $\sigma \;\rlap{\lower 2.5pt \hbox{$\sim$}}\raise 1.5pt\hbox{$<$}\;1\%$ and observe a star in the ecliptic with an angular radius $\;\rlap{\lower 2.5pt \hbox{$\sim$}}\raise 1.5pt\hbox{$<$}\;$ 0.01 mas. Since this occultation rate is very sensitive to the index slope q and plummets when $q \;\rlap{\lower 2.5pt \hbox{$\sim$}}\raise 1.5pt\hbox{$<$}\;3$, a KBO occultation observation campaign could provide a decisive constraint on the actual slope of the KBO size distribution for sub-kilometer-sized objects. Blue O class stars are the best candidates for detecting KBOs since they have the smallest angular radius for a given visual magnitude. The occultation events are typically very brief ( $\;\rlap{\lower 2.5pt \hbox{$\sim$}}\raise 1.5pt\hbox{$<$}\;$ 1s) and they are shorter but more numerous when observed in the antisolar direction, so rapid photometry(>1Hz) is required and high speed photometry( $\;\rlap{\lower 2.5pt \hbox{$\sim$}}\raise 1.5pt\hbox{$>$}\;20$Hz) is preferred. The French space mission Corot will provide an excellent opportunity to observe occultations by KBOs using high precision photometry.