Episodic accretion around the Herbig AE star BF Orionis. Evidence for the presence of extra-solar comets

D. de Winter, C. A. Grady, M. E. van den Ancker, M. R. Pérez, C. Eiroa

Abstract
The results of a monitoring programme of high and intermediate resolution spectra covering He i 5876 Angstroms, Na iD_{2,1} and Hα 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 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 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 Hα line profile on the photometric 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 activity were observed. More complex variations of the Hα 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 of revolving clumpy material are: observed changes in the velocity direction of the very strong Na iD_{2,1} low velocity absorption components and the observed flip over of the relative strength of the blue and red peak of Hα simultaneous with the change of blue to redshifted absorption components in both the Na iD_{2,1} and He i lines. In case of orbiting 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 (protoplanetary) disk system, suggests the existence of structures similar to those probably present in the solar system at a time of formation of planetesimals. The estimated much higher than cosmic abundances of refractory (Na) over volatile (H, He) gases for the detected bodies supports this suggestion. Based on observations made at the European Southern Observatory, La Silla, Chile.

Astronomy and Astrophysics
Volume 343, Page 137
March 1999

>> ADS