**31st Annual Meeting of the DPS, October 1999**

*Session 16. Comet Nuclei Posters*

Poster Group I, Monday-Wednesday, October 11, 1999, , Kursaal Center
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## [16.05] The rotational state of Comet Hale-Bopp

*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.

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