HCN and CN in Comet 2P/Encke: Models of the non-isotropic, rotation-modulated coma and CN parent life time

Axisymmetric models of the outgassing of a cometary nucleus have been constructed. Such models can be used to describe a nucleus with a single active region. The models may include a solar zenith angle dependence of the outgassing. They retrieve the outgassing flux at distances from the nucleus where collisions between molecules are unimportant, as function of the angle with respect to the outgassing axis. The observed emissions must be optically thin. Furthermore the models assume that the outflow speed at large distance from the nucleus does not depend on direction. The value of the outflow speed is retrieved. The models are applied to CN images and HCN spectra of Comet 2P/Encke, obtained nearly simultaneously in November 2003 with the 2 m optical telescope on Mount Rozhen, Bulgaria, and with the 10 m Heinrich Hertz Submillimeter Telescope on Mount Graham, Arizona, USA. According to Sekanina (1988), Astron. J. 95, 911-924, at that time a single outgassing source was active. Input parameters to the models like the rotation period of the nucleus and a small correction to Sekanina’s rotation axis are determined from a simpler jet position angle model. The rotation is prograde with a sideric period of 11.056 ± 0.024 h, in agreement with literature values. The best fit model has an outflow speed of 0.95 ± 0.04 km s⁻¹. The same value has been derived from the corkscrew appearing in the CN images. The location of the outgassing axis is at colatitude δ_a = 7.4° ± 2.9° and longitude λ_a = 235° ± 17° (a definition of zero longitude is provided). Comet Encke’s outgassing corresponds approximately to the longitudinally averaged solar input on a spherical nucleus (i.e. very likely comes from deeper layers) but with some deficiency of outgassing at mid-latitudes and non-zero outgassing from the dark polar cap. The presence of gas flow from the dark polar cap is explained as evidence of gas flow across the terminator. The models rely mostly on the CN images. The HCN spectra are more noisy. They provide information how to determine the best fit outflow velocity and the sense of rotation. The model HCN spectra are distinctly non-Gaussian. Within error limits they are consistent with the observations. Models based solely on the HCN spectra are also presented but, because of the lower quality of the data and the unfavorable observing geometry, yield inferior results. As a by-product we determine the CN parent life time from our CN observations. The solar EUV and Lyα radiation field at the time of our observations is taken into account.