Abstract. We present photometric ISO 60 and 170 um measurements, complemented by some IRAS data at 60 µm, of a sample of 84 nearby main-sequence stars of spectral class A, F, G and K in order to determine the incidence of dust disks around such main-sequence stars. Fifty stars were detected at 60 µm; 36 of these emit a flux expected from their photosphere while 14 emit significantly more. The excess emission we attribute to a circumstellar disk like the ones around Vega and β 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 flux density at 60 µm. Of the stars younger than 400 Myr one in two has a disk; 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 then decays in about 400 Myr. Because (i) the dust particles disappear and must be replenished on a much shorter time scale and (ii) the collision of planetesimals 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 suggest that the dissipation of the disk is related to the heavy bombardment phase in our Solar System. Whether all stars arrive on the main sequence surrounded 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 as old as the Sun still have significant disks.

Comet Hale-Bopp (C/1995 O1) was observed at wavelengths from 2.4 to 195 micrometers with the Infrared Space Observatory when the comet was about 2.9 astronomical units (AU) from the sun. The main observed volatiles that sublimated from the nucleus ices were water, carbon monoxide, and carbon dioxide in a ratio (by number) of 10:6:2. These species are also the main observed constituents of ices in dense interstellar molecular clouds; this observation strengthens the links between cometary and interstellar material. Several broad emission features observed in the 7- to 45-micrometer region suggest the presence of silicates, particularly magnesium-rich crystalline olivine. These features are similar to those observed in the dust envelopes of Vega-type stars.

Abstract. ESA's Infrared Space Observatory (ISO) was used to perform a deep survey with ISOCAM through three massive gravitationally lensing clusters of galaxies. The total area surveyed depends on source flux, with nearly seventy square arcminutes covered for the brighter flux levels in maps centred on the three clusters Abell 370, Abell 2218 and Abell 2390. We present maps and photometry at 6.7 µm (hereafter 7 µm) and 14.3 µm (hereafter 15 µm), showing a total of 145 mid-infrared sources and the associated source counts. At 15 µm these counts reach the faintest level yet recorded. Almost all of the sources have been confirmed on more than one infrared map and all are identified with counterparts in the optical or near-infrared. Detailed models of the three clusters have been used to correct for the effects of gravitational lensing on the background source population. Lensing by the clusters increases the sensitivity of the survey, and the weakest sources have lensing corrected fluxes of 5 and 18 µJy at 7 and 15 µm, respectively. Roughly 70% of the 15 µm sources are lensed background galaxies. Of sources detected only at 7 µm, 95% are cluster galaxies for this sample. Of fifteen SCUBA sources within the mapped regions of the three clusters seven were detected at 15 microns. The redshifts for five of these sources lie in the range 0.23 to 2.8, with a median value of 0.9. Flux selected subsets of the field sources above the 80% and 50% completeness limits were used to derive source counts to a lensing corrected sensitivity level of 30 µJy at 15 µm, and 14 µJy at 7 µm. The source counts, corrected for the effects of completeness, contamination by cluster sources and lensing, confirm and extend earlier findings of an excess by a factor of ten in the 15 µm population with respect to source models with no evolution. The observed mid-infrared field sources occur mostly at redshifts between 0.4 and 1.5. For the counts at 7 µm, integrating in the range 14 µJy to 460 µJy, we resolve (0.49± 0.2)×10 −9 W m −2 sr −1 of the infrared background light into discrete sources. At 15 µm we include the counts from other extensive ISOCAM surveys to integrate over the range 30 µJy to 50 mJy, reaching two to three times deeper than the unlensed surveys to resolve (2.7 ± 0.62) × 10 −9 W m −2 sr −1 of the infrared background light. These values correspond to 10% and 55%, respectively, of the upper limit to the infrared background light, derived from photon-photon pair production of the high energy gamma rays from BL-Lac sources on the infrared background photons. However, the recent detections of TeV gamma rays from the z = 0.129 BL Lac H1426+428 suggest that the value for the 15 µm background reported here is already sufficient to imply substantial absorption of TeV gamma rays from that source.Key words. surveys -galaxies: clusters: individual: Abell 370 -galaxies: clusters: individual: Abell 2218 -galaxies: clusters: individual: Abell 2390 -gravitational lensing -infrared: galaxiesSend offprint requests to: L. Metcalfe, e-mail: lmetca...

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We examine far-infrared and submillimeter spectral energy distributions for galaxies in the Infrared Space Observatory Atlas of Bright Spiral Galaxies. For the 71 galaxies where we had complete 60-180 lm data, we fitted blackbodies with À1 emissivities and average temperatures of 31 K or À2 emissivities and average temperatures of 22 K. Except for high temperatures determined in some early-type galaxies, the temperatures show no dependence on any galaxy characteristic. For the 60-850 lm range in eight galaxies, we fitted blackbodies with À1 , À2 , and À (with variable) emissivities to the data. The best results were with the À emissivities, where the temperatures were $30 K and the emissivity coefficient ranged from 0.9 to 1.9. These results produced gas-to-dust ratios that ranged from 150 to 580, which were consistent with the ratio for the Milky Way and which exhibited relatively little dispersion compared with fits with fixed emissivities.