  The Keck Interferometer Nuller (KIN) was used to survey 25 nearby main
sequence stars in the mid-infrared, in order to assess the prevalence of warm
circumstellar (exozodiacal) dust around nearby solar-type stars. The KIN
measures circumstellar emission by spatially blocking the star but transmitting
the circumstellar flux in a region typically 0.1 - 4 AU from the star. We find
one significant detection (eta Crv), two marginal detections (gamma Oph and
alpha Aql), and 22 clear non-detections. Using a model of our own Solar
System's zodiacal cloud, scaled to the luminosity of each target star, we
estimate the equivalent number of target zodis needed to match our
observations. Our three zodi detections are eta Crv (1250 +/- 260), gamma Oph
(200 +/- 80) and alpha Aql (600 +/- 200), where the uncertainties are 1-sigma.
The 22 non-detected targets have an ensemble weighted average consistent with
zero, with an average individual uncertainty of 160 zodis (1-sigma). These
measurements represent the best limits to date on exozodi levels for a sample
of nearby main sequence stars. A statistical analysis of the population of 23
stars not previously known to contain circumstellar dust (excluding eta Crv and
gamma Oph) suggests that, if the measurement errors are uncorrelated (for which
we provide evidence) and if these 23 stars are representative of a single class
with respect to the level of exozodi brightness, the mean exozodi level for the
class is <150 zodis (3-sigma upper-limit, corresponding to 99% confidence under
the additional assumption that the measurement errors are Gaussian). We also
demonstrate that this conclusion is largely independent of the shape and mean
level of the (unknown) true underlying exozodi distribution.
