Accretion around the Herbig Ae star BF Ori; evidence for the presence of extra-solar comets

The results of a monitoring programme of high and intermediate resolution spectra covering He i 5876 ˚ A, NaiD2;1 and H of the isolated Herbig Ae star BF Ori are presented. We detect the presence of blue and redshifted emission and absorption components of these lines which vary from day-to- day with correlated changes suggesting a similar origin. The appearance, strength and variations of the redshifted Nai D absorption component on a time scale of days show vari- able accretion activity similar to that seen toward the Herbig Ae star UX Ori and Pic, suggesting evaporation 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 central star and has a mass comparable to comets in the solar system. A dependence was found of the H line profile on the photo- metric brightness of BF Ori similar to that observed for UX Ori. It is evidence for obscuration of a dense dusty body located in the outer disk regions as no extra absorption components from a gaseous content and no direct influences on the cometary activ- ity were observed. More complex variations of the H profile could be explained in part by absorption of star-grazer mate- rial, 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 of revolving clumpy material are: observed changes in the velocity direction of the very strong NaiD2;1 low velocity absorption components and the observed flip over of the relative strength of the blue and red peak of H simultaneous with the change of blue to redshifted absorption components in both the NaiD2;1 and Hei lines. In case of orbit- ing bodies, the estimated period lies between 60 and 100 days with a distance from 0.35 to 0.57 AU, respectively. The detection of possible orbiting and comet-like objects in the disk of BF Ori, a 3 +2 1 Myr old pre-main sequence A5-6 IIIe star, making it a possible progenitor of the HR 4796 (protoplan- etary) disk system, suggests the existence of structures similar to those probably present in the solar system at a time of forma- tion of planetesimals. The estimated much higher than cosmic abundances of refractory (Na) over volatile (H, He) gases for the detected bodies supports this suggestion.