Incidence and survival of remnant disks around main-sequence stars

We present photometric ISO 60 and 110 um measurements, complemented by some IRAS data at 60 mum, of a sample of 84 nearby main-sequence stars of spect ral class A, F, G and K in order to determine the incidence of dust disks a round such main-sequence stars. Fifty stars were detected at 60 mum; 36 of these emit a flux expected from their photosphere while 14 emit significant ly more. The excess emission we attribute to a circumstellar disk like the ones around Vega and beta Pictoris. Thirty four stars were not detected at all; the expected photospheric flux, however, is so close to the detection limit that the stars cannot have an excess stronger than the photospheric f lux density at 60 mum. Of the stars younger than 400 Myr one in two has a d isk; for the older stars this is true for only one in ten. We conclude that most stars arrive on the main sequence surrounded by a disk; this disk the n decays in about 400 Myr. Because (i) the dust particles disappear and mus t be replenished on a much shorter time scale and (ii) the collision of pla netesimals is a good source of new dust, we suggest that the rapid decay of the disks is caused by the destruction and escape of planetesimals. We sug gest that the dissipation of the disk is related to the heavy bombardment p hase in our Solar System. Whether all stars arrive on the main sequence sur rounded by a disk cannot be established: some very young stars do not have a disk. And not all stars destroy their disk in a similar way: some stars a s old as the Sun still have significant disks.