Beta-Pic stars

The formation of planetary systems is a very old theoretical question, while only recently observations can constrain models. Starting with the study of the fractionation processes in giant interstellar molecular clouds, observations now tackle the star formation process through pre-main sequence stars as well as young stellar objects. Circumstellar material is most of the time probably present around T-Tauri and Herbig Ae and Be stars, often confined to relatively dense disks of gas and dust even directly imaged in front of the Orion nebula. Thanks to near-infrared to mm observations of radiation thermally emitted by cold (typically 100 K) circumstellar matter, more evolved systems are also now known to be surrounded by solid grains in orbit around them. Called the Vega phenomenon, these IR excesses over the photospheric flux discovered by IRAS are estimated to occur in about 50% of the main sequence G dwarfs and in up to 20% of the more massive A and F ones. Amongst the four most spectacular cases - beta Pictoris, Fomalhaut, Vega and epsilon Eridani - beta Pictoris soon became the leading example not only because of its largest excess, but mainly because it was the first object around which a dust disk was imaged. A giant planet has even possibly be detected within its disk through an occultation of the star. Since three years, several giant planets have been indirectly detected around other main sequence stars.

The beta Pictoris disk was first imaged coronographically, with anti-blooming CCDs or through coronography with adaptive optics, in the visible and near-IR stellar scattered light. It is seen almost edge-on from the Earth and extends to more than 1000 AU from the star. The size spectrum of the dust grains peaks near the radiative blow-out limit of about 1 micron. Particles smaller than about 5 mm have a total mass of the order of one Earth mass, insufficient to form planets. But planets may already have formed. Thanks to the favorable geometry, the circumstellar gas has been extensively studied through absorption spectroscopy. The presence of red absorption components of some atomic lines, e.g. CAII, constitutes a strong evidence for planetesimal-sized bodies (comets) evaporating during the passages in the close vicinity of the star. These passages could result from orbital perturbations due to giant planets. These planets may also assist with the creation of the central gap in the disk and the moderate to large asymetries present in both the vertical and the azimuthal distribution of the dust. Also, the IR spectrum is similar to those of comets in the solar system.

The dust disk of beta Pictoris as well as its gaseous spectral signatures long remained the only ones detected around a main sequence star, despite many searches in nearby stars. However, a new disk has finally been imaged around a binary system, and very recently, probable disks were revealed at mm wavelengths around Vega and Fomalhaut. Also, few stars have been shown to present some visible spectral similarities with beta Pictoris, while ISO spectra appears very rich and similar to those of meteorites and comets. In the prevailing view, beta Pic and members of its class are evolving, young planetary systems undergoing a more or less rapid collisional grinding and/or gravitational clearing epoch. This stage is thought to last several hundreds million years. With these objects, we can now study our solar system in its early stages.