Most protoplanetary discs are thought to undergo violent and frequent accretion outbursts, during which the accretion rate and central luminosity are elevated for several decades. This temporarily increases the disc temperature, leading to the sublimation of ice species as snowlines move outwards. In this paper, we investigate how an FUor-type accretion outburst alters the growth and appearance of dust aggregates at different locations in protoplanetary discs. We develop a model based on the Monte Carlo approach to simulate locally the coagulation and fragmentation of icy dust particles and investigate different designs for their structure and response to sublimation. Our main finding is that the evolution of dust grains located between the quiescent and outburst water snowlines is driven by significant changes in composition and porosity. The time required for the dust population to recover from the outburst and return to a coagulation/fragmentation equilibrium depends on the complex interplay of coagulation physics and outburst properties, and can take up to 4500 yr at 5 au. Pebble-sized particles, the building blocks of planetesimals, are either deprecated in water ice or completely destroyed, respectively resulting in drier planetesimals or halting their formation altogether. When accretion outbursts are frequent events, the dust can be far from collisional equilibrium for a significant fraction of time, offering opportunities to track past outbursts in discs at millimetre wavelengths. Our results highlight the importance of including accretion outbursts in models of dust coagulation and planet formation.

<p>The growth of dust grains to dust aggregates is a very important process in the chain of events from dust to planetesimals in protoplanetary discs (PPDs). Not only the size, shape, and porosity play an important role in the collisional growth process, but also the collision speed, the type of gas coupling and the charge of the dust particles [1, 2].</p><p>The ICAPS (Interactions in Cosmic and Atmospheric Particle Systems) campaign provides an experimental approach to protoplanetary dust growth and all the above parameters under realistic PPD conditions. The first ICAPS experiment flew onboard the TEXUS-56 sounding rocket and consisted of a vacuum chamber with a cloud of micrometer-sized SiO<sub>2</sub> spheres embedded in a rarefied gas inside. The dust particles could be manipulated using temperature and external electric fields. During flight, the particles were observed using two overview cameras and a high-speed camera attached to a long-distance microscope. In total, three electrical scans were conducted to measure the charge distribution of the dust particles. Two of these scans (E1, E3) were applied immediately after the two dust injections, while a longer one (E2) was performed after the Brownian growth phase. Each of these scans consisted of two equal-length phases of different field polarisation. The analysis of the image recordings provided precise particle tracks and velocities as well as the mass and size of the dust aggregates [3]. From the change in velocity, when the external electric field was present, it was also possible to derive the particle charge.</p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.f2915fd1838261365872561/sdaolpUECMynit/2202CSPE&app=m&a=0&c=fcd08095f5871b045e61cda7944a351d&ct=x&pn=gnp.elif&d=1" alt="" width="1074" height="358"></p><p><strong>Fig.1:   </strong>Charge per monomer plotted against the cumulative normalized frequency of tracked particles during the electrical scans immediately after the first dust injection (E1), after the Brownian motion phase (E2), and after the second injection (E3), respectively. The duration between E1 and E2 was 164s. In this period, the mean charge per monomer grain decreased to less than 40% of the initial value.</p><p>In <strong>Fig. 1,</strong> the electric charge per monomer grain is plotted as a cumulative normalized frequency distribution of all particles tracked during each scan. Over the duration of 164 s between E1 and E2, a reduction of the mean charge per monomer grain to less than 40% of the initial value was observed. This finding is an indication that there was a relatively large number of distributed charges immediately after the injection, which allowed rapid agglomeration due to the charge-enhanced collision cross-section. At a later stage of the experiment run, the agglomeration was likely mainly driven by Brownian motion and dipole-dipole interactions. It is planned that the evolution of grain charging during agglomeration will be explored in more detail as part of the Laplace campaign, which will use a similar setup for a variety of experiments on the ISS. </p><p> </p><p><strong>References</strong></p><p>[1] Blum, J., “Dust agglomeration”, <em>Advances in Physics</em>, vol. 55, pp. 881–947, 2006. doi:10.1080/00018730601095039.</p><p>[2] Güttler, C., Blum, J., Zsom, A., Ormel, C. W., and Dullemond, C. P., “The outcome of protoplanetary dust growth: pebbles, boulders, or planetesimals?. I. Mapping the zoo of laboratory collision experiments”, <em>Astronomy and Astrophysics</em>, vol. 513, 2010. doi:10.1051/0004-6361/200912852.</p><p>[3] Schubert, B., “ICAPS Sounding Rocket - Particle Growth”, 2020. doi:10.5194/epsc2020-567.</p><p> </p>

<p>The ICAPS experiment (Interactions in Cosmic and Atmospheric Particle Systems) was part of the Texus-56 sounding rocket flight in November of 2019. ICAPS studies the agglomeration of 1.5 µm-sized, monomeric silica grains under microgravity conditions, as would be present in the early stages of dust growth in protoplanetary disks, which our study aims at describing.</p><p>For this, a cloud of dust was injected into a vacuum chamber with ~7000 monomer grains per mm³. A thermal trap was then utilized to stabilize the dust cloud against any external disturbances during the flight. Two overview cameras and a long-distance microscope with a high-speed camera were used for the in-situ observations of the particles (see Figs. 1 and 2).</p><p>This talk focuses on the data analysis and results of ICAPS, in particular with respect to Brownian motion and aggregate growth. From the total experiment time of six minutes of almost perfect weightlessness, we extracted the masses and translational friction times of 414 dust aggregates from their translational Brownian motion. For a subset of 69 of these particles, we were also able to derive their moments of inertia and rotational friction times from their Brownian rotation. With these data, we derived a fractal dimension close to 1.8 for the ensemble of dust aggregates. We compared this unambiguous physical method for the determination of the fractal dimension with an optical approach, in which the mass is derived through the particle extinction and the moment of inertia is derived from the microscopic images.</p><p>The combination of both methods then facilitates the growth analysis, for which the overview cameras were also used. We observed an initial rapid, charge-induced growth of aggregates, which was followed by a slower growth rate, which was dominated by ballistic cluster-cluster agglomeration. As a surprise, around 100 seconds into the flight, clear indication for runaway growth (or the onset of gelation) was observed.</p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.857ce136c38261213082561/sdaolpUECMynit/2202CSPE&app=m&a=0&c=7d2aa19a4c04fdb856b2dfee4d2634bb&ct=x&pn=gnp.elif&d=1" alt="" width="471" height="327"></p><p>Fig. 1: Examples of dust aggregates from the long-distance<br>microscope images.</p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.655e12c6c38266623082561/sdaolpUECMynit/2202CSPE&app=m&a=0&c=945438ed81e63b83298712bbcd057afe&ct=x&pn=gnp.elif&d=1" alt="" width="467" height="351"></p><p>Fig. 2: Image from one of the overview cameras after 100 s of experiment time (1024 x 768 pixels, about 12 x 9 mm²).</p>