Infrared surveys indicate that the dust content in debris disks gradually declines with stellar age. We simulated the long-term collisional depletion of debris disks around solar-type (G2 V) stars with our collisional code. The numerical results were supplemented by, and interpreted through, a new analytic model. General scaling rules for the disk evolution are suggested. The timescale of the collisional evolution is inversely proportional to the initial disk mass and scales with radial distance as r 4.3 and with eccentricities of planetesimals as e −2.3 . Further, we show that at actual ages of debris disks between 10 Myr and 10 Gyr, the decay laws of the dust mass and the total disk mass are different. The reason is that the collisional lifetime of planetesimals is size-dependent. At any moment, there exists a transitional size, which separates larger objects that still retain the "primordial" size distribution set in the growth phase from smaller objects whose size distribution is already set by disruptive collisions. The dust mass and its decay rate evolve as that transition affects objects of everlarger sizes. Under standard assumptions, the dust mass, fractional luminosity, and thermal fluxes all decrease as t ξ with ξ = −0.3...−0.4. Specific decay laws of the total disk mass and the dust mass, including the value of ξ, largely depend on a few model parameters, such as the critical fragmentation energy as a function of size, the primordial size distribution of largest planetesimals, as well as the characteristic eccentricity and inclination of their orbits. With standard material prescriptions and a distribution of disk masses and extents, a synthetic population of disks generated with our analytic model agrees quite well with the observed Spitzer/MIPS statistics of 24 and 70 µm fluxes and colors versus age.

We present 7-mm continuum observations of 14 low-mass pre-main-sequence stars in the Taurus-Auriga star-forming region obtained with the Very Large Array with ∼1. 5 resolution and ∼0.3 mJy rms sensitivity. For 10 objects, the circumstellar emission has been spatially resolved. The large outer disk radii derived suggest that the emission at this wavelength is mostly optically thin. The millimetre spectral energy distributions are characterised by spectral indices α mm = 2.3 to 3.2. After accounting for contributions from free-free emission and corrections for optical depth, we determine dust opacity indices β in the range 0.5 to 1.6, which suggest that millimetre-sized dust aggregates are present in the circumstellar disks. Four of the sources with β > 1 may be consistent with submicron-sized dust as found in the interstellar medium. Our findings indicate that dust grain growth to millimetre-sized particles is completed within less than 1 Myr for the majority of circumstellar disks.Key words. stars: pre-main sequence -stars: planetary systems: protoplanetary disks -planetary systems: formation IntroductionAlthough dust grains constitute only a minor fraction of the disk material around young stars (∼1 Myr), they play a pivotal role in the complicated multi-stage process of planet formation. The growth of submicron-sized dust particles as found in the interstellar medium (ISM) to larger aggregates by coagulation (collisional sticking) is the first essential step towards building larger rocky bodies (planetesimals) that may eventually accrete into terrestrial planets and planetary cores (Beckwith et al. 2000).Statistical analysis of the near-infrared excess of young stellar clusters shows that disk emission disappears within a few Myr (Strom et al. 1989;Haisch et al. 2001). Similarly, sub-mm and mm observations suggest a decrease in the amount of cold circumstellar dust during the post T-Tauri phase (Carpenter et al. 2005). The decline of dust emission can be interpreted as a consequence of gradual mass loss through disk dissipation and/or opacity changes due to particle growth. It is important to stress that such studies only trace the evolution of the dust disk. The evolution of the gaseous disk, containing the bulk of the disk mass, is at present only poorly understood.For many years there has been little sound observational evidence for dust grain growth in disks around pre-mainsequence stars. The analysis of spectral energy distributions showed that the sub-mm/mm fluxes of T Tauri stars decline more slowly towards longer wavelengths than expected for ISM-sized dust (e.g. Beckwith & Sargent 1991). It was tempting to interpret a shallow spectral slope as an indication of the presence of particles much larger than in the ISM ( 0.1 µm). It was soon realised, however, that spatially unresolved disk observations cannot be used to distinguish between small, optically thick disks containing sub-micron ISM dust and extended, optically thin disks with larger dust particles. Spatially resolved images are needed t...

Context. Debris discs are a consequence of the planet formation process and constitute the fingerprints of planetesimal systems. Their solar system counterparts are the asteroid and Edgeworth-Kuiper belts. Aims. The DUNES survey aims at detecting extra-solar analogues to the Edgeworth-Kuiper belt around solar-type stars, putting in this way the solar system into context. The survey allows us to address some questions related to the prevalence and properties of planetesimal systems. Methods. We used Herschel/PACS to observe a sample of nearby FGK stars. Data at 100 and 160 μm were obtained, complemented in some cases with observations at 70 μm, and at 250, 350 and 500 μm using SPIRE. The observing strategy was to integrate as deep as possible at 100 μm to detect the stellar photosphere. Results. Debris discs have been detected at a fractional luminosity level down to several times that of the Edgeworth-Kuiper belt. The incidence rate of discs around the DUNES stars is increased from a rate of ∼12.1% ± 5% before Herschel to ∼20.2% ± 2%. A significant fraction (∼52%) of the discs are resolved, which represents an enormous step ahead from the previously known resolved discs. Some stars are associated with faint far-IR excesses attributed to a new class of cold discs. Although it cannot be excluded that these excesses are produced by coincidental alignment of background galaxies, statistical arguments suggest that at least some of them are true debris discs. Some discs display peculiar SEDs with spectral indexes in the 70-160 μm range steeper than the Rayleigh-Jeans one. An analysis of the debris disc parameters suggests that a decrease might exist of the mean black body radius from the F-type to the K-type stars. In addition, a weak trend is suggested for a correlation of disc sizes and an anticorrelation of disc temperatures with the stellar age.

We have carried out a sensitive search for gas emission lines at infrared and millimeter wavelengths for a sample of 15 young sun-like stars selected from our dust disk survey with the Spitzer Space Telescope. We have used mid-infrared lines to trace the warm (300-100 K) gas in the inner disk and millimeter transitions of 12 CO to probe the cold (∼20 K) outer disk. We report no gas line detections from our sample. Line flux upper limits are first converted to warm and cold gas mass limits using simple approximations allowing a direct comparison with values from the literature. We also present results from more sophisticated models following Gorti and Hollenbach (2004) which confirm and extend our simple analysis. These models show that the [S i] line at 25.23 µm can set constraining limits on the gas surface density at the disk inner radius and traces disk regions up to a few AU. We find that none of the 15 systems have more than 0.04 M J of gas within a few AU from the disk inner radius for disk radii from 1 AU up to ∼40 AU. These gas mass upper limits even in the 8 systems younger than ∼30 Myr suggest that most of the gas is dispersed early. The gas mass upper limits in the 10-40 AU region, that is mainly traced by our CO data, are < 2 M ⊕ . If these systems are analogs of the Solar System, either they have already formed Uranus-and Neptune-like planets or they will not form them beyond 100 Myr. Finally, the gas surface density upper limits at 1 AU are smaller than 0.01% of the minimum mass solar nebula for most of the sources. If terrestrial planets form frequently and their orbits are circularized by gas, then circularization occurs early.

HST NICMOS PSF-subtracted coronagraphic observations of HD 181327 have revealed the presence of a ringlike disk of circumstellar debris seen in 1.1 m light scattered by the disk grains, surrounded by a diffuse outer region of lower surface brightness. The annular disk appears to be inclined by 31N7 AE 1N6 from face-on, with the disk major-axis P.A. at 107 AE 2 . The total 1.1 m flux density of the light scattered by the disk (at 1B2 < r < 5B0) of 9:6 AE 0:8 mJy is 0:17% AE 0:015% of the starlight. Seventy percent of the light from the scattering grains appears to be confined in a 36 AU wide annulus centered on the peak of the radial surface brightness (SB) profile 86:3 AE 3:9 AU from the star, well beyond the characteristic radius of thermal emission estimated from IRAS and Spitzer flux densities, assuming blackbody grains (%22 AU). The 1.1 m light scattered by the ring (1) appears bilaterally symmetric, (2) exhibits directionally preferential scattering well represented by a Henyey-Greenstein scattering phase function with g HG ¼ 0:30 AE 0:03, and (3) has a median SB (over all azimuth angles) at the 86.3 AU radius of peak SB of 1:00 AE 0:07 mJy arcsec À2. No photocentric offset is seen in the ring relative to the position of the central star. A low SB diffuse halo is seen in the NICMOS image to a distance of $4 00 . Deeper 0.6 m Hubble Space Telescope (HST ) ACS PSF-subtracted coronagraphic observations reveal a faint (V % 21:5 mag arcsec À2 ) outer nebulosity at 4 00 < r < 9 00 , asymmetrically brighter to the north of the star. We discuss models of the disk and properties of its grains, from which we infer a maximum vertical scale height of 4Y8 AU at the 87.6 AU radius of maximum surface density, and a total maximum dust mass of collisionally replenished grains with minimum grain sizes of %1 m of %4M Moon .