  The seven known main belt comets (MBCs) have orbital characteristics of main
belt asteroids yet exhibit dust ejection like comets. In order to constrain
their physical and orbital properties we searched the Thousand Asteroid Light
Curve Survey (TALCS; Masiero et al. 2009) for additional candidates using two
diagnostics: tail and coma detection. This was the most sensitive MBC survey
effort to date, extending the search from MBCs with H~18 (D~1 km) to H~21
(D~150 m).
  We fit each of the 924 TALCS objects to a PSF model incorporating both a coma
and nuclear component to measure the fractional contribution of the coma to the
total surface brightness. We determined the significance of the coma detection
using the same algorithm on a sample of comparable null detections. We did not
identify any MBC candidates with this technique to a sensitivity limit on the
order of cometary mass loss rate of about 0.1 kg/s. Our tail detection
algorithm identified statistically significant flux in a segmented annulus
around the candidate object. We show that the technique can detect tail
activity throughout the asteroid belt to the level of the currently known MBCs.
Although we did not identify any MBC candidates with this technique, we find a
statistically significant detection of faint activity in the entire ensemble of
TALCS asteroids. This suggests that many main belt asteroids are active at very
low levels.
  We set 90% upper confidence limits on the number distribution of MBCs as a
function of absolute magnitude, semimajor axis, eccentricity, and inclination.
There are <~ 400000 MBCs in the main belt brighter than H_V=21 (~150 m) and the
MBC:MBA ratio is <~ 1:400. We further comment on the ability of observations to
meaningfully constrain the snow line's location. Under some reasonable and
simple assumptions we claim 85% confidence that the contemporary snow line lies
beyond 2.5 AU.

Earth and Planetary Astrophysics