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Astron. Astrophys. 343, 916-922 (1999)

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1. Introduction

The search for extra-solar planets through photometric variation is a well-known problem analyzed in many aspects for several years (Schneider et al. 1990; Schneider 1996). However, planets are not the only objects detectable by their effect on stars' brightness, comets can also induce photometric variations. In our study of [FORMULA]Pictoris photometric variations (Lecavelier des Etangs et al. 1995), we concluded that the variations observed on November 1981 could be due to the passage in front of the star of either a planet or a dusty cloud (Lecavelier des Etangs et al. 1997, Lamers et al. 1997). If the latter is the correct explanation, this cloud of dust must be a cometary tail for two reasons. First, because the lifetime of the dust in such a system is very short and one must find a way to produce the dust continuously. Second, because the cloud shape is constrained by the observed light curve; the cloud cannot be spherical, it should have a sharp edge in the part pointing toward the star and a huge cloud of dust in the opposite direction: exactly like a cometary tail (Lecavelier des Etangs, 1996).

In the case of the solar system, the observations of stellar occultation by comets have been discussed by Combes et al. (1983). Observations of extinction and polarization of star light by dust of cometary tails in the solar system have been published (Dossin 1962, Ninkov 1994, Rosenbush et al. 1994).

But the solar system is only one particular planetary system at a given age. It is known that cometary activity was formerly much more important, and the well-known case of [FORMULA]Pictoris shows that during the first [FORMULA] years, a planetary system is expected to show large cometary activity (Ferlet & Vidal-Madjar 1995, Vidal-Madjar et al. 1998). Moreover, presence of comets around stars can be considered as an indirect signature for the presence of gravitational perturbations, and possibly caused by planets.

In this paper, we deal with the important possibility of detecting cometary activity from a photometric survey. We take the COROT space mission as an example of what will be achievable in the very near future (Baglin et al. 1997). COROT, which primary aim is stellar seismology, will be launched in early 2002. It will allow a survey of about 30 000 stars with a photometric accuracy of few [FORMULA] during several months. Here, we predict the probabilities of detecting comets using such a photometric survey.

In Sect 2, we describe the model of the cometary occultation, and then give the expected light curve in Sect. 3. Estimates of the number of comets which could be detected are given in Sect. 4. The conclusion is in Sect. 5.

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© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999