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Astron. Astrophys. 363, 323-334 (2000)

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8. Discussion

Orbital investigation allows to verify models of the nongravitational forces by comparing model of the perturbed motion of a comet with its real position on the orbit described by astrometric observations.

In all presented models of the nongravitational acceleration perturbing the orbital motion of the comet 43P/Wolf-Harrington the similar evolution of the time shift of maximum activity with respect to perihelion was detected. This temporal evolution over the whole examined interval of the comet's motion derived from three different kind of models was presented in Fig. 7. For the forced precession model the time displacements of the function [FORMULA] with respect to the perihelion described by parameters [FORMULA], [FORMULA] and [FORMULA] (Table 5) are presented as straight solid lines. Shifts of the maximum outgassing related to seasons effect resulting from Model A and Model B (Table 8) for the spotty nucleus model are marked in each apparition with big circles full and open, respectively. The character of variations of the activity shift which was originally negative then positive and again negative in the last apparition has been independently confirmed by three different models. Two of them represented by Model A, Model B and Model C successfully linked all apparitions of the comet. The third one was the asymmetric model used within short intervals of time covering three or four apparitions. The appropriate values of parameters [FORMULA] (Table 4) are drawn in Fig. 7 by straight dashed lines with crosses in the centres.

[FIGURE] Fig. 7. Time shift of the maximum activity with respect to the perihelion in the successive apparitions of the comet 43P/Wolf-Harrington obtained from different models. Dashed lines with centers denoted by crosses present the values of [FORMULA] found from orbital linkages of three consequtive apparitions by the asymmetrical model of the nongravitational acceleration. The values of parameters [FORMULA], [FORMULA] and [FORMULA] determined from linkage of all apparitions by forced precession model (Model C) are drawn as straight solid lines. The perihelion shift of the gas production curves derived from the model of discrete outgassing from the nucleus surface are presented by full circles for Model A and open circles for Model B.

From the present investigation follows that variations in the standard nongravitational parameter [FORMULA], can be interpreted as manifestations of a sudden redistribution of the momentum that is transferred to the nucleus by the sublimating mass. According to orbital solutions for the spotty nucleus an episodic activation of new emission region and deactivation of the existing one are responsible for the momentum changes. Likewise in a case of the forced precession model stepwise variations of the maximum function [FORMULA] with respect to perihelion have been obtained.

The outgassing pattern may be affected by changes in the surface insolation distribution. For a nonspherical nucleus the spin axis is forced to precess when the momentum transfered to the nucleus does not pass through the center of mass. Sekanina (1993b) pointed out that due to the nucleus precession the dormant sources of activity may be exposure to sunlight and simultaneusly the active regions may be protected from insolation. Exposition to sunlight of new sources of outgassing can, in turn, exert a torque that sustain nucleus precession. The irregular changes of the time shifts of [FORMULA] with respect to perihelion, obtained on the basis of the forced precession model can be interpreted as variations in the localizations of the active areas on the nucleus of the comet 46P/Wolf-Harrington. The variations in distribution of the active areas on the nucleus surface cause both the horizontal shifts and amplitude changes of the outgassing curves. Unfortunately the presented precessional model for uniform distribution of the active areas on the nucleus surface offers the perihelion shifts of the symmetrical production curve only. Probably the comet 43P/Wolf-Harington needs a model of the forced precession for the nucleus with activity concentrated in localized sources on the surface. Nevertheless, it seems that scenarios of the activity variations proposed here can explain the temporal variability of the nongravitional perturbations in the long-term of the comet's motion and are consistent if the variations of the shift perihelion of activity are considered.

Orbital investigation allows to verify models of the nongravitational forces by comparing model of the perturbed motion of a comet with its real position on the orbit described by astrometric observations.

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© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000
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