The physico-chemical history of Falling Evaporating Bodies aroundβPictoris: investigating the presence of volatiles

Abstract

Transient spectral absorption events have been monitored for many years toward the star β 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 AU, becoming star-grazers due to planetary perturbations. The physics of the evaporation of those bodies is widely influenced by their physico-chemical properties, especially the presence of volatile matter. We investigate here this question from a modeling point of view, adapting for that case the models designed for solar comets. We simulate the physico-chemical evolution of the FEB progenitors on circular orbits for a time comparable to the supposed age of βPic, and constrain the quantity of volatiles present in them in relationship with its 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 generation by planetary perturbations, and show that it is probably less than ~10 AU. Finally, we show that demanding the FEB progenitors to be icy in a major part of their volume at those distances is probably an unrealistic constraint, and that they more likely look like asteroids with perhaps 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.