  Interior to the orbit of Mercury, between 0.07 and 0.21 AU, is a dynamically
stable region where a population of asteroids, known as Vulcanoids, may reside.
We present the results from our search for Vulcanoids using archival data from
the Heliospheric Imager-1 (HI-1) instrument on NASA's two STEREO spacecraft.
Four separate observers independently searched through images obtained from
2008-12-10 to 2009-02-28. Roughly, all Vulcanoids with e<=0.15 and i<=15deg
will pass through the HI-1 field of view at least twice during this period. No
Vulcanoids were detected. Based on the number of synthetic Vulcanoids added to
the data that were detected, we derive a 3 sigma upper limit (i.e. a confidence
level >0.997) that there are presently no Vulcanoids larger than 5.7 km in
diameter, assuming an R-band albedo of p_R=0.05 and a Mercury-like phase
function. The present-day Vulcanoid population, if it exists at all, is likely
a small remnant of the hypothetical primordial Vulcanoid population due to the
combined effects of collisional evolution and subsequent radiative transport of
collisional fragments. If we assume an extant Vulcanoid population with a
collisional equilibrium differential size distribution with a power law index
of -3.5, our limit implies that there are no more than 76 Vulcanoids larger
than 1 km.
