A snow-line is the region of a protoplanetary disk at which a major volatile, such as water or carbon monoxide, reaches its condensation temperature. Snow-lines play a crucial role in disk evolution by promoting the rapid growth of ice-covered grains 1−6. Signatures of the carbon monoxide snow-line (at temperatures of around 20 kelvin) have recently been imaged with in the disks surrounding the pre-main-sequence stars TW Hydra 7−9 and HD163296 [3,10] , at distances of about 30 astronomical units (au) from the star. But the water snow-line of a protoplanetary disk (at temperatures of more than 100 kelvin) has not hitherto been seen, as it generally lies very close to the star (less than 5 au away for solar-type stars 11). Water-ice is important because it regulates the efficiency of dust and planetesimal coagulation 5 , and the formation of comets, ice giants and the cores of gas giants 12. Here we report ALMA images at 0.03-arcsec resolution (12 au) of the protoplanetary disk around V883 Ori, a protostar of 1.3 solar masses that is undergoing an outburst in luminosity arising from a temporary increase in the accretion rate 13. We find an intensity break corresponding to an abrupt change in the optical depth at about 42 au, where the elevated disk temperature approaches the condensation point of water, from which we conclude that-2-the outburst has moved the water snow-line. The spectral behaviour across the snow-line confirms recent model predictions 14 : dust fragmentation and the inhibition of grain growth at higher temperatures results in soaring grain number densities and optical depths. As most planetary systems are expected to experience outbursts caused by accretion during their formation [15,16] our results imply that highly dynamical water snow-lines must be considered when developing models of disk evolution and planet formation. V883 Ori is an FU Ori object identified as such by [17] from followup spectroscopy of deeply embedded sources from the Infrared Astronomical Satellite (IRAS). It is located in the Orion Nebula Cluster, which has a distance of 414±7 pc [18]. It has a disk mass of 0.3 M and a bolometric luminosity of 400 L [19]. We have obtained 230 GHz/1.3 mm (band-6) observations of V883 Ori using the Atacama Large Millimeter/submillimeter Array (ALMA) in four different array configurations with baselines ranging from 14 m to 12.6 km, which were taken in ALMA Cycle-2 and Cycle-3. These new ALMA observations include continuum and the 12 CO, 13 CO, and C 18 O J = 2-1 spectral lines. We use the C 18 O gas line to investigate the dynamics of the system at 0.2 (90 au) resolution and the continuum data to constrain the physical properties of the dust in the V883 Ori disk at 0.03 (12 au) resolution. In Figure 1 (top panel) we show our Cycle-3 continuum image at 0.03 resolution, the highest resolution ever obtained for a FU Ori object at millimeter wavelengths. We find that the V883 Ori disk has a two-region morphology, with a very bright inner disk (r ∼ 0.1 , 42 au) and a much more tenuous outer disk...

Context. The sticking of micron-sized dust particles caused by surface forces within circumstellar disks is the first stage in the production of asteroids and planets. The key components describing this process are the relative velocity between the dust particles in this environment and the complex physics of dust aggregate collisions. Aims. We present the results of a collision model based on laboratory experiments of these aggregates. We investigate the maximum aggregate size and mass that can be reached by coagulation in protoplanetary disks. Methods. We use the results of laboratory experiments to establish the collision model previously published by Güttler et al. The collision model is based on the assumptions that we model the aggregates as spheres with compact and porous "phases" and that there is a continuous transition between these two. We apply this collision model to the Monte Carlo method developed previously by Zsom & Dullemond and include Brownian motion, radial drift, and turbulence as contributors of relative velocity between dust particles. Results. We model the growth of dust aggregates at 1 AU in the midplane for three different gas densities. We find that the evolution of the dust does not follow the previously assumed growth-fragmentation cycles. Catastrophic fragmentation hardly occurs in the three disk models. Furthermore, we see long-lived, quasi-steady states in the distribution function of the aggregates caused by bouncing. We explore how the mass and the porosity depend on both the turbulence parameter and the critical mass ratio of dust particles. Upon varying the turbulence parameter, the system behaves in a non-linear way, and we find that the critical mass ratio has a strong effect on the particle sizes and masses. Particles reach Stokes numbers of roughly 10 −4 during the simulations. Conclusions. The particle growth is stopped by bouncing rather than fragmentation in these models. The final Stokes number of the aggregates is rather insensitive to the variations in the gas density and the strength of turbulence. The maximum mass of the particles is limited to ≈1 g (chondrule-sized particles). Planetesimal formation can proceed by the means of the turbulent concentration of these aerodynamically size-sorted, chondrule-sized particles.

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Water ice is one of the most abundant materials in dense molecular clouds and in the outer reaches of protoplanetary disks. In contrast to other materials (e.g., silicates), water ice is assumed to be stickier due to its higher specific surface energy, leading to faster or more efficient growth in mutual collisions. However, experiments investigating the stickiness of water ice have been scarce, particularly in the astrophysically relevant micrometer-sized region and at low temperatures. In this work, we present an experimental setup to grow aggregates composed of μm-sized water-ice particles, which we used to measure the sticking and erosion thresholds of the ice particles at different temperatures between 114 K and 260 K. We show with our experiments that for low temperatures (below ∼210 K), μm-sized water-ice particles stick below a threshold velocity of 9.6 m s −1 , which is approximately 10 times higher than the sticking threshold of μm-sized silica particles. Furthermore, erosion of the grown ice aggregates is observed for velocities above 15.3 m s −1 . A comparison of the experimentally derived sticking threshold with model predictions is performed to determine important material properties of water ice, i.e., the specific surface energy and the viscous relaxation time. Our experimental results indicate that the presence of water ice in the outer reaches of protoplanetary disks can enhance the growth of planetesimals by direct sticking of particles.