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Astron. Astrophys. 343, 137-150 (1999)

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Episodic accretion around the Herbig Ae star BF Orionis *

Evidence for the presence of extra-solar comets

D. de Winter  **  1,2, C.A. Grady 3, M.E. van den Ancker 4, M.R. Pérez 5 and C. Eiroa 1

1 Departemento Física Teórica, C-XI, Facultad de Ciencias, Universidad Autónoma de Madrid, Cantoblanco, E-28049 Madrid, Spain
2 Centro de Astrofísica de Universidade do Porto, Rua Campo Alegre 823, 4150 Porto, Portugal
3 Eureka Scientific, 2452 Delmar St., Suite 100, Oakland, CA 94602, USA
4 Astronomical Institute "Anton Pannekoek", University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
5 Space Applications Corp., 9315 Largo Drive West, Suite 250, Largo MD 20774, USA

Received 5 June 1998 / Accepted 2 December 1998

Abstract

The results of a monitoring programme of high and intermediate resolution spectra covering He I 5876 Å, Na I [FORMULA] and H[FORMULA] 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 [FORMULA] 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[FORMULA] 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 H[FORMULA] 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 I [FORMULA] low velocity absorption components and the observed flip over of the relative strength of the blue and red peak of H[FORMULA] simultaneous with the change of blue to redshifted absorption components in both the Na I [FORMULA] 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 [FORMULA] 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.

Key words: comets: general – stars: emission-line – stars: individual: Beta Pic – stars: individual: BF Ori – stars: individual: UX Ori – stars: pre-main sequence

* Based on observations made at the European Southern Observatory, La Silla, Chile.
** Current address: Instituto de Astrofísica de Canarias, C/ Via Láctea s/n, E-38200 La Laguna, Tenerife, Spain

Send offprint requests to: D. de Winter (dolf@astro1.ft.uam.es; dwinter@iac.es)

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© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999
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