T. G. Müller 1,3, J. S. V. Lagerros 2, M. Burgdorf 3, T. Lim 3, P. Morris 3, A. Salama 3, B. Schulz 3, & B. Vandenbussche 3,
1 Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
2 Astronomiska observatoriet, Box 515, SE-75120 Uppsala, Sweden
3 ISO Science Operations Centre, Astrophysics Division, Space Science Department of ESA, Villafranca, P.O. Box 50727, E-28080 Madrid, Spain
The asteroid (1) Ceres is the largest and one of the best known objects of the
main belt.
At around
,
where the thermal emission of main belt asteroids
typically peaks, Ceres is more than one order of magnitude brighter than the
brightest stars. It is therefore an excellent object for an extensive
study with ISO at wavelengths between 2 and
.
Since Ceres is a typical C-type asteroid, we believe that our results
are valid for a large fraction of all main-belt asteroids.
We present observations from SWS, LWS and PHT at wavelengths between 5and
.
One SWS spectrum of Ceres, taken in October 1997, covers the range from
5.3 to
,
including the maximum of the thermal emission. We
determine the influence of the infrared beaming and discuss the consequences
for the surface structure, which influences the beaming.
LWS observed Ceres more than 10 times in fixed-grating position, providing
each time 10 photometric data points between 46 and
.
Additionally
a full grating scan from 43 to
has been obtained.
We find a emissivity decrease towards longer wavelengths, which might be due
to sub-surface scattering processes.
ISOPHOT photometry was obtained between 50and
.
By combining the flux density variations at different
phase angles before and after opposition, we could derive the thermal
inertia of Ceres and confirm a very low value of
.
Together with groundbased and ISO observations of other asteroids we
derived standard physical parameters, which we believe are useful for the
study of other less well known main-belt asteroids.
Our discussions are based on a thermophysical model code (J. Lagerros, AA
310, 1996).