We describe interferometric observations of the asteroid (41) Daphne in the
thermal infrared obtained with the Mid-Infrared Interferometric Instrument
(MIDI) of the Very Large Telescope Interferometer (VLTI). We derived the size
and the surface thermal properties of (41) Daphne by means of a thermophysical
model (TPM), which is used for the interpretation of interferometric data for
the first time. From our TPM analysis, we derived a volume equivalent diameter
for (41) Daphne of 189 km, using a non-convex 3-D shape model derived from
optical lightcurves and adaptive optics images (B. Carry, private
communication). On the other hand, when using the convex shape of Kaasalainen
et al. (2002. Icarus 159, 369-395) in our TPM analysis, the resulting volume
equivalent diameter of (41) Daphne is between 194 and 209 km, depending on the
surface roughness. The shape of the asteroid is used as an a priori information
in our TPM analysis. No attempt is made to adjust the shape to the data. Only
the size of the asteroid and its thermal parameters (albedo, thermal inertia
and roughness) are adjusted to the data. We estimated our model systematic
uncertainty to be of 4% and of 7% on the determination of the asteroid volume
equivalent diameter depending on whether the non-convex or the convex shape is
used, respectively. In terms of thermal properties, we derived a value of the
surface thermal inertia smaller than 50 J m-2 s-0.5 K-1 and preferably in the
range between 0 and 30 J m-2 s-0.5 K-1. Our TPM analysis also shows that Daphne
has a moderate macroscopic surface roughness.Comment: 44 pages, 8 figures, 3 table
