Abstract. Transient spectral absorption events monitored now for years towards the star β Pictoris have been interpreted as resulting from the transit across the line of sight of evaporating star-grazing kilometer-sized bodies (Falling Evaporating Bodies, or FEBs). Several Herbig Ae/Be stars of various ages have been observed to exhibit somehow similar absorption events that have been attributed to similar FEB events. We investigate here this question from a modeling point of view. Adapting the FEB simulation code we had developed earlier specifically for β Pic to the case of typical Herbig Ae/Be stars, we try to derive in which conditions FEB-like objects may generate detectable transient absorption events. We compare these conditions with those found in the case of β Pic. A major difference with β Pic is that Herbig Ae/Be stars have strong stellar winds (10 −9 -10 −7 M yr −1 ). Those winds appear to have a drastic interaction with the gaseous material escaped from the FEBs. With the presence of such stellar winds, the spectral signatures of FEBs are not detectable, unless their mass loss rate is huge. This translates into very large bodies (∼100 km size), instead of ∼15 km for β Pic FEBs. This appears unrealistic in terms of amount of planetesimal mass needed in the disks surrounding these stars. We discuss then the validity of the FEB hypothesis for specific example stars. It turns out that for the younger (a few 10 6 yr old) Herbig Ae/Be stars like AB Aur, with well identified winds > ∼ 10 −8 M yr −1 , the variable features sometimes observed are not likely to be due to FEBs, unless produced in wind free cavities. For older ( > ∼ 10 7 yr old) stars Herbig Ae/Be like HD 100546, the FEB scenario could still explain the spectral events observed, but either the wind must to be weaker than ∼10 −10 M yr −1 (which cannot be excluded so far), or the FEBs approach the star in wind free cavities.

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 > ∼ 4 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 β Pic, 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.

Abstract. Transient spectral redshifted absorption events in the spectrum of β Pictoris (β Pic) have been attributed to cometlike bodies falling toward the star (Falling Evaporating Bodies, or FEB) and evaporating in its immediate vicinity. Dynamical models shows that these bodies originate from circular orbits at > ∼ 4 AU. After an eccentricity increase due to planetary perturbations, they end up as star-grazers. The physics of the cloud generated by the body's sublimation is highly influenced by the physico-chemical conditions inside the body. Thus it is necessary to investigate the composition and the behavior of the materials within the body in order to explain the observed features. The physico-chemical evolution of the bodies has been simulated during all the phases before and during their FEB state. First the fluctuations in the gas production rates of volatile solids in the first phase when the refractory materials have not yet evaporated has been studied. Later on, the refractory crust of the object evaporates even if the core still contains volatile solids. The simulation then shows a phase lag between the production rate of volatiles and of refractory materials during the periastron passage. The influence of dynamical and physical parameters on the FEB phenomenon are also investigated. We find that only a heterogenous population of bodies can produce all the absorption features that have been observed so far. These results are important for the understanding of the FEB phenomenon and can lead to a revision of the FEB sublimation model.