  We report on high contrast mid-infrared observations of Fomalhaut obtained
with the Keck Interferometer Nuller (KIN) showing a small resolved excess over
the level expected from the stellar photosphere. The measured null excess has a
mean value of 0.35% +/- 0.10% between 8 and 11 microns and increases from 8 to
13 microns. Given the small field of view of the instrument, the source of this
marginal excess must be contained within 2AU of Fomalhaut. This result is
reminiscent of previous VLTI K-band observations, which implied the presence of
a ~ 0.88% excess, and argued that thermal emission from hot dusty grains
located within 6 AU from Fomalhaut was the most plausible explanation. Using a
parametric 2D radiative transfer code and a Bayesian analysis, we examine
different dust disk structures to reproduce both the near and mid-infrared data
simultaneously. While not a definitive explanation of the hot excess of
Fomalhaut, our model suggests that the most likely inner few AU disk geometry
consists of a two-component structure, with two different and spatially
distinct grain populations. The 2 to 11 microns data are consistent with an
inner hot ring of very small (~ 10 to 300 nm) carbon-rich grains concentrating
around 0.1AU. The second dust population consists of larger grains (size of a
few microns to a few tens of microns) located further out in a colder region
where regular astronomical silicates could survive, with an inner edge around
1AU. From a dynamical point of view, the presence of the inner concentration of
sub-micron sized grains is surprising, as such grains should be expelled from
the inner planetary system by radiation pressure within only a few years. This
could either point to some inordinate replenishment rates (e.g. many grazing
comets coming from an outer reservoir) or to the existence of some braking
mechanism preventing the grains from moving out.
