Episodic accretion around the Herbig Ae star BF orionis - Evidence for thepresence of extra-solar comets

The results of a monitoring programme of high and intermediate resolution s pectra covering He I 5876 Angstrom, Na I D-2,D-1 and H alpha of the isolate d Herbig Ae star BF Ori are presented. We detect the presence of blue and r edshifted emission and absorption components of these lines which vary from day-today with correlated changes suggesting a similar origin. The appearance, strength and variations of the redshifted Na I D absorption component on a time scale of days show variable accretion activity similar to that seen toward the Herbig Ae star UX Ori and beta Pic, suggesting eva poration of star-grazing bodies. We estimate for one event that such a body is kilometer sized, evaporates at a distance of about 0.4 AU from the cent ral star and has a mass comparable to comets in the solar system. A dependence was found of the H alpha line profile on the photometric brigh tness of BF Ori similar to that observed for UX Ori. It is evidence for obs curation of a dense dusty body located in the outer disk regions as no extr a absorption components from a gaseous content and no direct influences on the cometary activity were observed. More complex variations of the H alpha profile could be explained in part by absorption of star-grazer material, equal to the absorption at the sodium lines, and in part by obscuration of its Line forming region by the cometesimal. More evidences for detections o f revolving clumpy material are: observed changes in the velocity direction of the very strong Na I D-2,D-1 low velocity absorption components and the observed flip over of the relative strength of the blue and red peak of H alpha simultaneous with the change of blue to redshifted absorption compone nts in both the Na I D-2,D-1 and He I lines. In case of orbiting bodies, th e estimated period lies between 60 and 100 days with a distance from 0.35 t o 0.57 AU, respectively. The detection of possible orbiting and comet-like objects in the disk of BF Ori, a 3(-1)(+2) Myr old pre-main sequence A5-6 IIIe star, making it a pos sible progenitor of the HR 4796 (protoplanetary) disk system, suggests the existence of structures similar to those probably present in the solar syst em at a time of formation of planetesimals. The estimated much higher than cosmic abundances of refractory (Na) over volatile (H, He) gases for the de tected bodies supports this suggestion.