## 3. Model of a cometary nucleus with discrete sources of outgassingThe insolation of active regions near the perihelion varies rapidly
not only due to variation of the heliocentric distance but also due to
changes of the subsolar point latitude. The model of the water ice
sublimation for insoled active areas, applied in this study, was
formulated by Sekanina (1988). In the model the absorbed solar energy
is spent on sublimation and thermal reradiation only. The emission
from each active region located on the surface of rotating cometary
nucleus is expressed as a product of the water sublimation rate
from the unit area at a subsolar
point and the dimensionless relative sublimation rate,
, at the Sun's local zenith distance
. A
total number of where is a third order
polynomial. The angle is the
critical angular distance of the Sun from the active spot, beyond
which the sublimation rate is negligibly low as compared with that at
the subsolar point and it depends on the
. In order to employ the law of
sublimation rate in orbital calculations the function
has been normalized to unit
heliocentric distance: , where
mol/km but differs in values of the
exponents Assuming that the variation in orbital position is negligible during one rotation of a cometary nucleus, the rotational-averaged orbital components of the nongravitational acceleration could be described as: where are directional cosinus of
the momentum transferred to a nucleus by the outgassing from
where For orbital computations the components of the nongravitational acceleration given by Eq. (4) have been transformed into the form: The expressions for were derived as: where Lifetime of each active region was limited by time of activation and deactivation . The expressions (6) were incorporated directly into the equations
of the comet's motion (Eq. (1)) which are solved by the recurrent
power series method. For the orbit improvement, the method of Sitarski
(1971, 1979a, 1979b) was applied. Parameters of the spotty-nucleus
model: ,
, © European Southern Observatory (ESO) 2000 Online publication: December 5, 2000 |