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Astron. Astrophys. 328, 311-320 (1997)

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2. Evidence of large bodies around [FORMULA]  Pic

[FORMULA]  Pic is a very good candidate to search for planets. Indeed, its gas and dust disk presents a large number of characteristics of a young planetary system seen edge-on from the Earth. Here we present a list of arguments for the presence of large bodies.

2.1. Comets seen through their ejecta

The thermal emission from the central part of the [FORMULA]  Pic disk shows a silicate emission feature at 10 [FORMULA] (Telesco & Knacke 1991). This feature is different from all known emissions except that it is remarkably similar to those observed in comets which have emissions characteristic of crystalline silicates (Knacke et al. 1993). This seems to indicate that possibly some cometary-like objects are already present within the [FORMULA]  Pic system.

Independently, the stable component of the gaseous disk has also been subject to intense investigations (Ferlet & Vidal-Madjar 1995). Among many results, we point out the detection of CO absorption by HST-GHRS (Vidal-Madjar et al. 1994) which gives evidence to the presence of evaporating bodies. The CO molecule is destroyed very rapidly by the ambient interstellar UV light at frequencies [FORMULA] s-1, and thus the CO gas must be supplied by a permanent internal source. An obvious possibility is continuous quiescent evaporation of comets at several tens of AU from the star.

The presence of a dust disk is also very challenging: the time scale of dust destruction is much smaller than the age of the star (Weissman 1984, Backman & Paresce 1993). This means that the disk must be continuously replenished either by collisions between larger bodies or, more probably, by evaporating bodies. Indeed, Lecavelier des Etangs et al. (1996a) have shown that a disk which is continuously supplied by orbiting (slowly) evaporating bodies (OEB) perturbed by a planetary system, can have the same spatial distribution of dust as the distribution observed around [FORMULA]  Pic. This new model may be able to explain the main characteristics of the disk, such as the power law of the radial dust distribution and asymmetries observed at large distances. It is compatible with the CO detection and requires the presence within the disk of orbiting and evaporating large bodies at tens of AU from the star.

2.2. Comets seen individually: falling evaporating bodies

Spectroscopy revealed strongly variable absorption lines in the visible (Ferlet et al. 1987, Lagrange et al. 1987) and the UV (Vidal-Madjar 1994 et al., and refs. therein). These very sporadic and mostly redshifted absorption lines are now very well explained as due to matter falling onto the star (at velocities up to 400 km s-1). Complete dynamical simulations have reproduced the observed redshifted events as due to evaporating comets, of sizes more than a kilometer, falling towards [FORMULA]  Pic. (Beust et al. 1991a). This Falling-Evaporating-Bodies (FEB) scenario has predicted unusual line ratios which have indeed been observed (Vidal-Madjar et al. 1994). Finally, hydrodynamical calculations of the flow of metallic ions around their evaporating parent bodies give a possible explanation for the very presence of AlIII and CIV lines (Deleuil et al. 1993). The ions can be produced through collisions within the coma, because the stellar radiation of an A5V star is unable to create these highly ionized species through photoionization (Beust & Tagger 1993). All these different aspects of the observed spectral variations can hardly be explained by any other model, and none was ever proposed. So there is strong evidence that at least kilometer size bodies are already present within the [FORMULA]  Pic system.

2.3. Massive bodies needed

The exact mechanism that could generate these numerous star-grazers (Ferlet et al. 1993) is still controversial, but planetary perturbations are thought to be the basic process. Several explanations have been proposed. The hypothesis of an unique family of fragments from a single large body like comet D/Shoemaker-Levy 9's fragments, (i.e. SL9's fragment falling into Jupiter) cannot explain all the observational constraints (Beust et al. 1996). Therefore, one needs to assume that many bodies must be gravitationally perturbed by one or several planets.

A single perturbation by a single planet requires a massive planet with a large eccentricity ([FORMULA]). Although it could a priori seem unrealistic, a massive planet with an eccentricity of 0.6 has recently been discovered in orbit about 16 Cyg B by Cochran et al. (1996). Alternatively, long term perturbations by massive planets have been suggested through two different mechanisms: secular resonances or mean motion resonances. A secular resonance occurs when the precession frequency of the longitude of periastron or alternatively of the longitude of the ascending node of the perturbed body is commensurate with one of the frequencies of the planetary system. A mean motion resonance occurs simply when the mean motion is commensurate with the mean motion of a planet. Both could provide the observed asymmetry in the longitude of periastron of the FEBs ([FORMULA], see discussion in Beust et al. 1996). On one hand, secular resonances can be very efficient, such as the so-called [FORMULA] resonance with Saturn in our solar system (Levison 1994), but unfortunately a general theory is still needed. On the other hand, Beust & Morbidelli (1996) have shown that mean motion resonances can generate star-grazers with all the observed characteristics of the FEBs.

The distribution of dust around [FORMULA]  Pic also might give indirect evidence for the existence of planets. The dust distribution is very asymmetric close to the star and the central part of the disk is relatively free of dust (Lagage & Pantin 1994). These dust distributions and inhomogeneities could be explained by planetary perturbation (Scholl et al. 1993, Roques et al. 1994). However, an explanation for the observed asymmetries of a factor larger than 3 between the infrared emission of the two extensions of the disk is still needed (Lecavelier des Etangs et al. 1996b).

In conclusion, there are many indirect indications that a planetary system is already formed around [FORMULA]  Pic with a multitude of bodies with a large range of sizes. The large number of comet-like bodies (FEBs) even suggests the presence of massive planets. However, up to now there is no direct evidence for the existence of planets around [FORMULA]  Pic.

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© European Southern Observatory (ESO) 1997

Online publication: March 24, 1998