  Comet 17P/Holmes underwent a dramatic outburst in October 2007, caused by the
sudden fragmentation of its nucleus and the production of a large quantity of
grains scattering sunlight. We report on 90 GHz continuum observations carried
out with the IRAM Plateau de Bure interferometer on 27.1 and 28.2 October 2007
UT, i.e., 4-5 days after the outburst. These observations probed the thermal
radiation of large dust particles, and therefore provide the best constraints
on the mass in the ejecta debris. The thermal emission of the debris was
modelled and coupled to a time-dependent description of their expansion after
the outburst. The analysis was performed in the Fourier plane. Visibilities
were computed for the two observing dates and compared to the data to measure
their velocity and mass. Optical data and 250-GHz continuum measurements
published in the literature were used to further constrain the dust kinematics
and size distribution. Two distinct dust components in terms of kinematic
properties are identified in the data. The large-velocity component, with
typical velocities V0 of 50-100 m/s for 1 mm particles, displays a steep size
distribution with a size index estimated to q = -3.7 (\pm0.1), assuming a
minimum grain size of 0.1 \mum. It corresponds to the fast expanding shell
observed in optical images. The slowly-moving "core" component (V0 = 7-9 m/s)
detected near the nucleus has a size index |q| < 3.4 and contains a higher
proportion of large particles than the shell. The dust mass in the core is in
the range 0.1-1 that of the shell. Using optical constants pertaining to porous
grains (50% porosity) made of astronomical silicates mixed with water ice (48%
in mass), the total dust mass Mdust injected by the outburst is estimated to
4-14 x 10**11 kg, corresponding to 3-9% the nucleus mass.
