The Vega phenomenon around G dwarfs

In this paper, we present and discuss photometric infrared measurements tak en with ISOPHOT at 60 mu m for a sample of 30 G dwarfs, using C100 3X3 mini maps. This sample was selected from the set of more than 1600 G dwarfs whic h will be scrutinized for radial-velocity variations by the Geneva group, a iming at the detection of extra-solar planets. In our sample, 5 stars displ ay an infrared excess at 60 mu m. 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 aroun d main-sequence stars. They concluded that most stars which arrive on the m ain-sequence still possess a disk, and that this disk then decays in the ne xt 400 Myr. The 5 stars in our sample which have an infrared excess, are pr obably older than 3 Gyr, however, from which we suggest that the disks arou nd cool stars may survive longer than those among earlier-type objects. For two stars in our sample: the fractional luminosity of the disk is signific antly higher than for typical Vega-type stars, and approaches the exception al value observed for beta Pie. To investigate the correlation between an infrared excess and planets/compa nion stars, we combine our results with the first results of the CORALIE su rvey. None of the until now detected radial-velocity variables display an i nfrared excess. That the stars for which an infrared excess is found lack a companion, can however not be concluded at the present stage.