EDP Sciences
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Volume 372, Number 2, June III 2001
Page(s) 616 - 626
Section Stellar atmospheres
DOI http://dx.doi.org/10.1051/0004-6361:20010528

A&A 372, 616-626 (2001)
DOI: 10.1051/0004-6361:20010528

The physico-chemical history of Falling Evaporating Bodies around $\beta $ Pictoris: investigating the presence of volatiles

C. Karmann1, H. Beust1 and J. Klinger2

1  Laboratoire d'Astrophysique de l'Observatoire de Grenoble, Université J. Fourier, BP 53, 38041 Grenoble Cedex 9, France
2  Laboratoire de Planetologie de Grenoble, Université J. Fourier, BP 53, 38041 Grenoble Cedex 9, France

(Received 22 December 2000 / Accepted 12 March 2001)

Transient spectral absorption events have been monitored for many years toward the star $\beta $ 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 $\ga
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 $\beta $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.

Key words: stars: $\beta $Pic -- methods: numerical -- comets: general -- stars: circumstellar matter -- planetary systems: formation

Offprint request: C. Karmann, ckarmann@obs.ujf-grenoble.fr

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