Mean-motion resonances as a source for infalling comets toward beta Pictoris

Repeated time-variable redshifted absorption features in the spectrum of beta Pictoris (beta Pic> have been attributed to comet-like bodies falling toward the star, when evaporating in its immediate vicinity. This model explains now a large number of observational characteristics, but the exact mechanism that could generate these numerous star-grazers is still controversial, even if planetary perturbations are thought to be the basic process. The different models proposed up to now are here reviewed, and we discuss in particular a recent one, involving the effect of secular resonances in the beta Pic system. We stress that it seems highly improbable that such a mechanism could apply to the beta Pic case, because the extremely strong power of secular resonances is connected to the very specific structure of the Solar System. Therefore, the secular resonance mechanism is highly non-generic. Conversely, we propose a model involving the eccentricity-pumping effect of mean-motion resonances with a massive planet on a moderately eccentric orbit. We show in particular that the 4:1 mean-motion resonance is a very active source of star-grazers as soon as the eccentricity of the perturbing planet is greater than or similar to 0.05, while the 3:1 mean-motion resonance is less efficient. We stress that this mechanism is very generic. These theoretical predictions are confirmed by numerical integrations using the Extended Schubart Integrator. The time-scale of the process is discussed, and we show that if the eccentricity of the perturbing planet fluctuates, due to secular perturbations, this time-scale is compatible with the age of beta Pic's system. (C) 1996 Academic Press, Inc.