The physico-chemical history of Falling Evaporating Bodies around beta Pictoris: investigating the presence of volatiles

Transient spectral absorption events have been monitored for many years tow ard the star beta Pictoris and have been interpreted as resulting from the transit across the line of sight of evaporating comet-sized bodies (Falling Evaporating Bodies, or FEBs). The model shows that these bodies come from circular orbits at greater than or similar to 4AU, becoming star-grazers du e to planetary perturbations. The physics of the evaporation of those bodie s is widely influenced by their physico-chemical properties, especially the presence of volatile matter. We investigate here this question from a mode ling point of view, adapting for that case the models designed for solar co mets. We simulate the physico-chemical evolution of the FEB progenitors on circular orbits for a time comparable to the supposed age of beta Pic, and constrain the quantity of volatiles present in them in relationship with it s age and to the semi-major axis of the orbits. We also constrain this semi -major axis by investigating its influence on the dynamical model of FEB ge neration by planetary perturbations, and show that it is probably less than similar to 10 AU. Finally, we show that demanding the FEB progenitors to b e icy in a major part of their volume at those distances is probably an unr ealistic constraint, and that they more likely look like asteroids with per haps a small icy nucleus rather than fully icy comets. This result leads to a revision of the FEB evaporation model that has been assumed up to now.