5. Explanations which could work
5.1. A cloud of forward scattering dust
Although a backward scattering dust cloud cannot explain the observed brightness variations of Pic (sect. 4.2), forward scattering can be much more efficient. If the dust particles produce optical refraction, the phase function in the forward direction might reach very high values of . Thus, the forward scattering by a dust cloud of few stellar radii in size located at about one AU can cause detectable light variations when it passes in front of the star. Combined with occultation by the optical thick part of the cloud, such a model might explain both the brightening of Pic and the dip in the lightcurve. This model will be investigated in a separate paper (Lamers et al. 1997, Paper II).
5.2. A hole in the dust disk due to a planet
The observed photometric variations of Pic are compatible with the transit of a planet in front of the star (Lecavelier des Etangs et al. 1995), if the sphere of influence of this putative planet is free of dust.
We propose that the enhancement of brightness of Pic around JD 4918 may be due to the passage of this cleared out zone in front of Pic. The cleared region will have a radius of the "Hill-radius", which is the distance from the planet at which the planet's gravity equals the tidal force of the star's gravity. With the stellar mass given by Lanz et al. (1995): , we have
where is the mass of the planet. To cover a distance equal to its Hill radius, the planet needs a time :
Note that this time is of the order of the duration of the brightening observed around JD 4918, within the uncertainty of such simple approximation. So the brightening of Pic could be due to the passage of a cleared zone in the dust disk around the star, if the system is seen edge-on.
The central dip in the lightcurve could be due to occultation by the planet itself. An occultation by the supposed planet crossing a diameter of Pic will last a time , again in reasonable agreement with the observed duration of the dip in the light curve. Occultation will decrease the brightness of the star by (where and are the radius of the planet and Jupiter). Thus, the measurements of JD 4918 could be explained by an occultation of the star by a planet whose radius is slightly larger than that of Jupiter (Lecavelier des Etangs et al. 1995). Below we describe a detailed model.
© European Southern Observatory (ESO) 1997
Online publication: March 24, 1998