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

7. Discussion and conclusions

We discussed the evidence for the presence of large bodies km-size or larger, around  Pic. Falling evaporating bodies have been observed directly in the form of transient absorption features in spectral lines. The presence of orbiting evaporating bodies has been derived from the fact that the gaseous disk, e.g., observed in CO lines, and the dust disk have a destruction rate which is short compared to the age of the star. This requires a continuous replenishment of the gas and dust, e.g., by orbiting evaporating bodies. More massive planets are generally thought to be present but their presence is inferred only indirectly. The main argument is the perturbation of the orbits of smaller bodies, to explain the large number of evaporating bodies. The asymmetry of the dust disk close to the star might also be due to the presence of a planet.

An exceptional photometric event was recorded on JD 4918 (= Nov 10, 1981). The lightcurve shows an achromatic brightening by about 0.06 mag. in about ten days, and a slow decrease to the normal magnitude on about the same timescale. At maximum brightness, the lightcurve shows a short dip with a duration of less than a day. In Sect. 3 we discussed several possible explanations which in principle might explain the lightcurve but turn out to be not acceptable. The two explanations which can explain the general features of the lightcurve as well as the magnitude of the effect are: occultation by a planet surrounded by a dust-free zone, and an orbiting dust cloud with dust that has a strongly forward peaked phase function. In this paper we discussed the first model.

Because we know that the disk is seen almost edge on, with a tilt angle less than about 3 degrees (Kalas & Jewitt 1995) the probability of occultation is not negligible. We have modeled the lightcurve with a model consisting of a dust-ring around  Pic at the distance of the planet. The interaction with the planet has cleared the dust in a zone around the star. Part of this dust is now concentrated near the two Lagrangian points at 60 degrees angle from the line star-planet. The dust-ring is assumed to have an extinction of at least 0.06 mag. The clearing zone has about the same width as the dust ring and orbits the star with the same period as the planet. When the clearing zone enters the line of sight to the star, the extinction of the star decreases and the star gets brighter. When the planet, in the middle of the clearing zone, passes in front of the star its occultation produces the dip in the light curve. The detailed modeling indicates a most probable distance of the planet of 5 AU, and a radius 0.22 , which is twice the size of Jupiter. This is larger than the size of gaseous planets. So either the planet is not gaseous (which is unlikely) or the planet is surrounded by a large ring that also contributes to the occultation. The calculated azimuthal distribution of the dust in the ring around the star with the concentration at the Lagrangian points, might explain the slow brightening of the star from 1979 to 1982.

This model is attractive because it can explain the three phases of the brightness variations of  Pic : the slow brightening, the 20-day brightening in November 1981 and the dip in the lightcurve on Nov. 19 1981. However it requires a few characteristics that are uncertain: (1) the inner dust disk around  Pic must be flat and have an extinction of about 0.06 mag. in the visual within a radial extent of the order of the diameter of the clearing zone, which is about 1 AU; (2) the orbit of the planet must pass exactly in front of the star. For a distance of 5 AU, and the upper limit for the tilt angle of the disk around  Pic of 3 degrees, this planet occultation has a probability of 2%.

In a separate paper we investigate the alternative explanation for the brightness variations of  Pic in terms of an orbiting forward scattering cloud (Lamers et al. 1997, Paper II).

© European Southern Observatory (ESO) 1997

Online publication: March 24, 1998

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