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A. Molina (Universidad de Granada, Spain), F. Moreno (Instituto de Astrofisica de Andalucia, CSIC,Spain)
The knowledge of the cometary rotation gives important information on many physical parameters of these bodies. We have carried out a search to determine the rotational state of Comet Hale-Bopp based on the classical rigid body theory, and taking into account observational results. A torque-free symmetrical top as a model for the nucleus has been considered, which is a good first approximation when several parameters affecting the rotation are still not well known for Comet Hale-Bopp. After considering two possible periods (11.3 hours and 20-24 days) we conclude that the motion of precession should be nearly perpendicular of the direction of the long axis of the comet, and then it should be in conflict with the observed stability of the shell structures. This result in independent of the axes ratio for reasonable values, therefore a complex rotational state is not compatible with observation. From the analysis of stability of a 11.3 hours simple rotation, a wobbling 18 to 30 hour period should be detected if the nucleus is assumed to be a symmetric ellipsoid. Hence, the case of a simple rotation as a spherical top with a 11.3 hour period seems to be the most plausible possibility.