Numerical results. (a) The shrinkage of the cometary nucleus due to water ice sublimation. The time variations of the radii of the primitive region and the dust mantle are shown by dashed and solid curves, respectively. (b) The change in angular velocity of the cometary nucleus associated with the shrinkage due to water ice sublimation. The spin-up rate in the vertical axis is the angular velocity when the radius of the primitive region contracts to the value given by the horizontal axis as the ratio with respect to the initial angular velocity.