Title:

                  Falling Evaporating Bodies as a Clue to Outline the Structure of the β Pictoris Young Planetary
                  System
 Authors:

                  Beust, Hervé; Morbidelli, Alessandro
 Affiliation:

                  AA(Laboratoire d'Astrophysique, Observatoire de Grenoble, Grenoble Cedex 9, France),
                  AB(Observatoire de la Côte d'Azur, Nice Cedex 4, France)
 Journal:

                  Icarus, Volume 143, Issue Icarus, pp. 170-188. (Icarus Homepage)
 Publication Date:

                  01/2000
 Origin:

                  ICAR
 Abstract Copyright:

                  (c) 2000: Academic Press
 Bibliographic Code:

                  2000Icar..143..170B
 

                                            Abstract

Transient redshifted events monitored in the spectrum of β Pictoris have been interpreted for many years as resulting from
the evaporation of numerous comet-like bodies in the vicinity of this stars. This motivated the investigation of dynamical
mechanisms responsible for the origin of these star-grazing comets. Among various ideas, a model involving mean-motion
resonances with a jovian-like planet was proposed a few years ago and applied to the β Pictoris case (H. Beust and A.
Morbidelli 1996 Icarus 120, 358-370). According to this model, the 4:1 and possibly the 3:1 mean-motion resonances are able to
generate numerous star-grazers from an initially dynamically cold disk of planetesimals. In this paper, detailed numerical
simulations of this dynamical process over a large number of particles are presented, showing in particular that the model is robust
toward the presence of additional planets around the star. The question of the evaporation rate of the comet-like bodies is also
investigated, showing that, in order to explain the observed spectral events, the comet-like bodies should be larger than 10-20 km,
rather than 1 km as previously conjectured. This in turn makes it possible to estimate the typical density of the planetesimal disk
required to explain the observed spectral phenomena, i.e., ~108 bodies per astronomic unit in the resonance at 4 AU from the star.

Apart from the main redshifted spectral events, a few blueshifted events were observed over the past few years. These spectral
events are clearly distinct from the main redshifted ones and cannot be considered as outliers, although they are far less numerous.
In the simple framework of the mean-motion resonance model, these events, which should correspond to bodies moving on
differently oriented orbits, should not be expected. We show that assuming the presence of a terrestrial-like planet, well inside the
orbit of the jovian planet, may generate these additional events. Close encounters with a terrestrial planet may extract some
particles from the resonance, and bring some of them to star-grazing orbits, but with a different orbital orientation, so that they
generate blueshifted events.

The question of the refilling of the resonance is investigated. Two basic models may be invoked: First, collisions among
planetesimals may replenish the resonance. This appears to be possible, but the mass density of the planetesimal disk in the vicinity
of the resonance needs to be ~10 M⊕ per astronomic unit or more, which is hardly realistic. Planetary migration may be a
second possible mechanism. A migrating resonance can capture new bodies as it sweeps the disk. We show that this model is
realistic only if the migration velocity is high enough, i.e., compatible with models invoking a tidal interaction with the disk, but
the reality of this mechanism in the β Pictoris disk must be questioned.