The Vega phenomenon around G dwarfs ?

In this paper, we present and discuss photometric infrared measurements taken with ISOPHOT at 60 μm for a sample of 30 G dwarfs, using C100 3X3 minimaps. This sample was selected from the set of more than 1600 G dwarfs which will be scrutinized for radial-velocity variations by the Geneva group, aiming at the detection of extra-solar planets. In our sample, 5 stars display an infrared excess at 60 μm. We have compared our results with those of Habing et al. (1999, in prep.), who have investigated the incidence and the survival of remnant disks around main-sequence stars. They concluded that most stars which arrive on the main-sequence still possess a disk, and that this disk then decays in the next 400 Myr. The 5 stars in our sample which have an infrared excess, are probably older than 3 Gyr, however, from which we suggest that the disks around cool stars may survive longer than those among earlier-type objects. For two stars in our sample, the fractional luminosity of the disk is significantly higher than for typical Vega-type stars, and approaches the exceptional value observed for β Pic. To investigate the correlation between an infrared excess and planets/companion stars, we combine our results with the first results of the CORALIE survey. None of the until now detected radial-velocity variables display an infrared excess. That the stars for which an infrared excess is found lack a companion, can however not be concluded at the present stage.