## 2. Model for dust ejectionCometary dust is released from the nucleus surface by ice sublimation with negligible initial velocity. Then dust is dragged out by the expanding gas in the coma, and at about ten nuclear radii the grain reaches its terminal velocity (Crifo 1991). For the largest particles, the deceleration due to nucleus gravity must be taken into account (Wallis 1982). Detailed dust-gas interaction models (Gombosi 1986, Crifo 1987) allow to obtain the time and size dependent dust ejection velocity, but they require precise data on the nucleus size, on the gas loss rate, and on the dust size distribution. These quantities are affected by large uncertainties, and are unavailable for the comets we consider. Moreover, no in-situ experiment has test such models. Ground-based observations require multi-parametric models to extract information on the dust velocity, and provide results which can be compared with the outputs of dust-gas drag theory. All models describing dust comae and tails must take into account the time and size dependence of the dust ejection velocity as free parameters. In the majority of these models, the dust velocity is an input parameter, and the sensitivity of the model results on it was never tested. It follows that the successfull fits of these models with observations never provided any information on the reliability of the adopted dust ejection velocity. To our knowledge, two models only consider the dust velocity as an output parameter, thus providing a significant test of dust-gas drag theory: dust tail models (Finson & Probstein 1968, Kimura & Liu 1977, Richter & Keller 1987, Fulle 1987, Fulle 1989) and neck-line models (Fulle & Sedmak 1988, Cremonese & Fulle 1989). Tail models provide the time dependence of the dust velocity, whereas its size dependence must be treated as an input parameter. Then it is necessary to test the sensitivity of the model output on this input parameter. Neck-line models provide directly the size dependence of the dust velocity and the velocity value at a fixed time. For the large sizes typical of meteoroids, dust-gas drag theory
provides a dust velocity size dependence which can well be
approximated by a power law with index of the
dust diameter The kernel of the inverse tail model is an automatic least square
fit of the observed tail images with the model tail images. The free
parametric function which allows to minimize the residual of such
least square fit is the product of the dust loss rate times the
time-dependent size distribution, which are the automatic linear
outputs of the model. However, the tail model images too (which are
built-up by means of a Monte-Carlo procedure considering keplerian
dust dynamics) depend on other free parameters, in particular on the
dust ejection velocity. Changes of this non-linear free parameter
allow to optimize the data fit and the stability of the linear output.
Therefore, inverse Monte-Carlo tail models provide self-consistent
estimates of all physical quantities describing a cometary dust
environment which do not depend on dust-gas drag models. The performed
tests showed a low sensitivity of the model outputs on the power index
Inverse tail models can provide information on the dust velocity in
limited time and size ranges, because the input images have limited
size and spatial resolution. We will test the sensitivity of meteoroid
orbit evolution on the dust size and ejection anisotropy at a fixed
ejection time. Due to the inverse tail model limitations, we will
analyse such sensitivity on a dust diameter change of a factor 10
only: from about 10 © European Southern Observatory (ESO) 1997 Online publication: May 26, 1998 |