β Pictoris: its evolutionary status

Abstract

Since the discovery of a large infrared excess in β Pic, this star has been intensively studied, and is currently considered as one of the best candidates for being the centre of an extrasolar planetary system. Such a star will be surrounded by an asymmetric, edge-on dust disc extending beyond 1(f au from the star. The suspected existence of a planetary system around β Pic has recently been strongly reinforced by the discovery of a central cleared zone about 40 au from the star. A disc warp has also been detected, the most straightforward explanation of which is the presence of at least a giant planet orbiting β Pic. Further evidence comes from the low gas-todust ratio, which may be due to the planetary system formation process. Redshifted and highly variable circumstellar absorption lines of several ions have been interpreted as resulting from the evaporation of comets falling on to β Pic. Recently, the very existence of the dust disc was employed to attribute a small age to β Pic. However, the amount of dust produced by the evaporating comets has been shown to be sufficient to replenish the disc, making the arguments in favour of small ages no longer meaningful. In the frame of the cometary hypothesis, we show that the analysis of the rate of events that would have been observed in the Solar system at different evolutionary stages argues in favour of a large age for β Pic. However, the estimation of stellar ages employing cometary fluxes should be treated with caution, on account of the diversity of possible planetary systems. We also present a new analysis of the evolutionary status of β Pic in the framework of stellar evolution, showing that, with the present uncertainties, there is, a continuum of possible ages for this star. However, employing the data deduced from the cometary flux, we obtain a consistent scenario of a large stellar age.