Falling Evaporating Bodies around Herbig stars. A theoretical study

Abstract

Abstract. Transient spectral absorption events monitored now for years towards the star β Pictoris have 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/Be stars 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 β Pic to the case of typical Herbig Ae/Be stars, 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 β Pic. A major difference with β Pic is that Herbig Ae/Be stars have strong stellar winds (10 −9 -10 −7 M 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 β Pic FEBs. 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 10 6 yr old) Herbig Ae/Be stars like AB Aur, with well identified winds > ∼ 10 −8 M yr −1 , the variable features sometimes observed are not likely to be due to FEBs, unless produced in wind free cavities. For older ( > ∼ 10 7 yr old) stars Herbig Ae/Be like HD 100546, the FEB scenario could still explain the spectral events observed, but either the wind must to be weaker than ∼10 −10 M yr −1 (which cannot be excluded so far), or the FEBs approach the star in wind free cavities.

Evidence for wind and accretion in the Herbig Be star HD 100546 from FUSE observations

“…They interpreted these transient spectral signatures as originating in the comae of star-grazing bodies like comets or asteroids, by analogy with the Falling Evaporating Bodies (FEB) model developed for the β Pictoris system (see Lagrange et al 2000, and references therein). However, Beust et al (2001) brought this interpretation into question for Herbig Ae/Be stars. Even though Beust et al (2001) concluded that this scenario could not be totally excluded in the case of the HD 100546 system, the presence of a structured wind as inferred by Vieira et al (1999) should prevent the observation of FEBs spectral signatures.…”

“…However, Beust et al (2001) brought this interpretation into question for Herbig Ae/Be stars. Even though Beust et al (2001) concluded that this scenario could not be totally excluded in the case of the HD 100546 system, the presence of a structured wind as inferred by Vieira et al (1999) should prevent the observation of FEBs spectral signatures.…”

“…On the other hand, the high velocities of the redshifted additional absorption components are close to the free-fall velocity, implying an origin from parent bodies very close to the star. In that case, according to Beust et al (2001), one should observe spectral variations in metallic ions, as refractory grains are expected to evaporate in the immediate vicinity of the star. The gas should be thus depleted in volatiles which is in contradiction with the present observations.…”

“…Similarities between these sporadic accreting gas features and those routinely observed in β Pictoris spectra led Grady et al (1997) to attribute them to the evaporation of infalling cometary bodies. This interpretation has been however recently called into question on account of the strong radiation field and the likely stellar wind of the star (Beust et al 2001). With its rich CS environment and its apparent spectral analogies with β Pictoris, HD 100546 is a prime target for our FUSE Circumstellar Disk team program .…”

HST/NICMOS2 coronagraphic observations of the circumstellar environment of three old PMS stars: HD 100546, SAO 206462 and MWC 480

“…The interpretation of such events in particular in terms of star-grazing comets (Grady et al 1997;Vieira et al 1999) remains however uncertain Beust et al 2000).…”