Falling Evaporating Bodies around Herbig stars. A theoretical study

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

Corresponding author: H. Beust, beust@obs.ujf-grenoble.fr

Received: 19 April 2000
Accepted: 24 November 2000

Abstract

Transient spectral absorption events monitored now for years towards the star βPictorishave been interpreted as resulting from the transit across the line of sight of evaporating star-grazing kilometer-sized bodies (Falling Evaporating Bodies, or FEBs). Several Herbig Ae/Bestars of various ages have been observed to exhibit somehow similar absorption events that have been attributed to similar FEB events. We investigate here this question from a modeling point of view. Adapting the FEB simulation code we had developed earlier specifically for βPic to the case of typical Herbig Ae/Bestars, we try to derive in which conditions FEB-like objects may generate detectable transient absorption events. We compare these conditions with those found in the case of βPic. A major difference with βPic is that Herbig Ae/Bestars have strong stellar winds (10^-9- yr^-1). Those winds appear to have a drastic interaction with the gaseous material escaped from the FEBs. With the presence of such stellar winds, the spectral signatures of FEBs are not detectable, unless their mass loss rate is huge. This translates into very large bodies (∼100 km size), instead of ∼15 km for βPicFEBs. This appears unrealistic in terms of amount of planetesimal mass needed in the disks surrounding these stars. We discuss then the validity of the FEB hypothesis for specific example stars. It turns out that for the younger (a few 10⁶ yr old) Herbig Ae/Bestars like AB Aur, with well identified winds yr^-1, the variable features sometimes observed are not likely to be due to FEBs, unless produced in wind free cavities. For older (yr old) stars Herbig Ae/Belike HD 100546, the FEB scenario could still explain the spectral events observed, but either the wind must to be weaker than ∼10 yr^-1 (which cannot be excluded so far), or the FEBs approach the star in wind free cavities.