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...

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Collisions between centimeter-to decimeter-sized dusty bodies are important to understand the mechanisms leading to the formation of planetesimals. We thus performed laboratory experiments to study the collisional behavior of dust aggregates in this size range at velocities below and around the fragmentation threshold. We developed two independent experimental setups with the same goal to study the effects of bouncing, fragmentation, and mass transfer in free particle-particle collisions. The first setup is an evacuated drop tower with a free-fall height of 1.5 m, providing us with 0.56 s of microgravity time so that we observed collisions with velocities between 8 mm s −1 and 2 m s −1 . The second setup is designed to study the effect of partial fragmentation (when only one of the two aggregates is destroyed) and mass transfer in more detail. It allows for the measurement of the accretion efficiency as the samples are safely recovered after the encounter. Our results are that for very low velocities we found bouncing as could be expected while the fragmentation velocity of 20 cm s −1 was significantly lower than expected. We present the critical energy for disruptive collisions Q , which showed up to be at least two orders of magnitude lower than previous experiments in the literature. In the wide range between bouncing and disruptive collisions, only one of the samples fragmented in the encounter while the other gained mass. The accretion efficiency in the order of a few percent of the particle's mass is depending on the impact velocity and the sample porosity. Our results will have consequences for dust evolution models in protoplanetary disks as well as for the strength of large, porous planetesimal bodies.

Water ice is abundant in protoplanetary disks. Its sticking properties are therefore important during phases of collisional growth. In this work, we study the sticking and rolling of 1.1 mm ice grains at different temperatures. We find a strong increase in sticking between 175 K to 200 K which levels off at higher temperatures. In terms of surface energy this is an increase with a factor of 63.4, e.g. from γ = 0.0029J/m 2 to γ = 0.19J/m 2 , respectively. We also measured critical forces for inelastic rolling. The critical rolling distance is constant with a value of 0.19 mm. In view of planetesimal formation at low temperatures in protoplanetary disks, the surface energy is not larger than for silicate dust and ice aggregation will share the same shortcommings. In general, water ice has no advantage over silicates for sticking and collisional growth might not favor ice over silicates.

In laboratory experiments we determine the mass gain and loss in central collisions between centimetre‐ to decimetre‐size SiO2 dust targets and submillimetre‐ to centimetre‐size SiO2 dust projectiles of varying mass, size, shape and at different collision velocities up to ∼56.5 m s−1. Dust projectiles much larger than 1 mm lead to a small amount of erosion of the target but decimetre targets do not break up. Collisions produce ejecta, which are smaller than the incoming projectile. Projectiles smaller than 1 mm are accreted by a target even at the highest collision velocities. This implies that net accretion of decimetre and larger bodies is possible. Independent of the original size of a considered projectile, after several collisions, all fragments will be of submillimetre size which might then be (re)accreted in the next collision with a larger body. The experimental data suggest that collisional growth through fragmentation and reaccretion is a viable mechanism to form planetesimals.

The outcome of the first stage of planetary formation, which is characterized by ballistic agglomeration of preplanetary dust grains due to Brownian motion in the free molecular flow regime of the solar nebula, is still somewhat speculative. We performed a microgravity experiment flown onboard the space shuttle in which we simulated, for the first time, the onset of free preplanetary dust accumulation and revealed the structures and growth rates of the first dust agglomerates in the young solar system. We find that a thermally aggregating swarm of dust particles evolves very rapidly and forms unexpected open-structured agglomerates. PACS numbers: 96.35.Cp, 61.43.Hv, 81.10.Mx It is now widely accepted that planets form from the nebula of gas and dust that comprises nascent solar systems. Inelastic, adhesive collisions between these dust particles eventually form kilometer-sized bodies, called planetesimals, which then collide under the influence of their mutual gravity to form planets [1][2][3][4][5][6]. After condensation of the micron-sized dust grains in the cooling gas, these initially collide with each other due to thermal (Brownian) motion, and, by adhesion due to van der Waals forces, form aggregates. The agglomeration rate of freshly condensed [7,8] preplanetary dust grains is determined by three factors: the collision cross section, the collision velocity, and the sticking probability of the dust particles, which are mutually interdependent. Laboratory experiments with micron-sized solid particles and dust agglomerates thereof have shown that, for moderate collision velocities y c # 1 m s 21 , the sticking probability is always unity [9][10][11]. The collision cross section and the collision velocity strongly depend on the morphology of the interacting preplanetary dust aggregates. Open-structured, fluffy particles generally have a larger cross section than compact grains, but couple also much better to the gas motion, so that relative velocities between fluffy agglomerates are suppressed. The gas-grain interaction is best described by the dust particles' response time to the gas motion, t f . In the free molecular flow regime, t f~m s a , where m and s a are the mass and the geometrical cross section (i.e., the projected area) of the dust aggregate. Aggregation models [1,12] for the Brownian motion-driven dust growth predict a scenario in which dust clusters of similar mass predominantly contribute to the agglomeration process. This leads to the evolution of a quasimonodisperse distribution of aggregate masses and to a relation between aggregate mass and size s ofwith an exponent ("fractal dimension") in the range of D f ഠ 1.8 2.1. For quasimonodisperse systems, the mean aggregate mass grows by a power law in time, m͑t͒~t z , and is related to the mass dependence of the collision cross section s c~m m and the collision velocity y c~m n of the aggregating particles through m 1 n z21 z (e.g., see the review in [13]). Here, z, m, and n are the respective exponents of the assumed power law functions for ...