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Astron. Astrophys. 319, 995-1006 (1997)

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7. Conclusions

The activity of comet 29P/Schwassmann-Wachmann 1 has been studied by means of numerical modelling. A comet nucleus model with a 3D space resolution and an improved description of the gas diffusion has been developed. The model takes into account the phase changes due to crystallisation and sublimation of volatile species inside a porous nucleus composed of ices and dust. The received solar heat flux generates heat and gas diffusion which is simulated in both radial and meridional directions. A 3D solution is obtained by gathering a set of 2D solutions whose boundary conditions depend on the third space dimension (hour angle). The gas diffusion coefficients are determined using the Chapman-Enskog method (Chapman & Cowling 1960).

Various configurations have been assumed and simulated. Special attention was taken to the tilt of the nucleus spin axis. An initial amorphous ice composition and surface erosion was considered. Several important conclusions can be made on the basis of the model results:

  1. The orientation of the nucleus rotation axis of P/SW1 is very important for the thermodynamic evolution of the nucleus. Outbursts are more likely for an obliquity of [FORMULA] than for an obliquity of [FORMULA]. Furthermore, the CO gas production rates are correlated with the apsides. This behaviour is in good agreement with the results of an independent data analysis published by Cabot et al. (1996).
  2. Erosion of surface material seems to be necessary to keep amorphous water ice close to the surface. Only in the case where [FORMULA] outbursts have been found along all 6 revolutions. A dust erosion rate of a few hundred kilogramme per second is likely to maintain an outburst regime if the dust to ice mass ratio is about 1. This erosion rate is in agreement with the observed dust production rate given by Fulle (1992).
  3. The range of the modelled CO gas production rates is consistent with the measured production rates of about [FORMULA]. The CO sublimation front can be expected within the first 10 metres below the surface. If no surface material is eroded, the activity decreases continuously.

These results give an important hint for the presence of amorphous water ice in cometary nuclei. The crystallisation of amorphous water ice and the release of trapped gases can account for outbursts. This explanation is compatible with the interpretation of comet Halley's outburst in 1991 at 14 AU (see Schmitt et al. 1991; Prialnik & Bar-Nun 1992).

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998