Drifting inwards in protoplanetary discs II: The effect of water on sticking properties at increasing temperatures

Abstract

In previous laboratory experiments, we measured the temperature dependence of sticking forces between micrometer grains of chondritic composition. The data showed a decrease in surface energy by a factor ∼ 5 with increasing temperature. Here, we focus on the effect of surface water on grains. Under ambient conditions in the laboratory, multiple water layers are present. At the low pressure of protoplanetary discs and for moderate temperatures, grains likely only hold a monolayer. As dust drifts inwards, even this monolayer eventually evaporates completely in higher temperature regions. To account for this, we measured the tensile strength for the same chondritic material as was prepared and measured under normal laboratory conditions in our previous work, but now introducing two new preparation methods: drying dust cylinders in air (dry samples), and heating dust pressed into cylinders in vacuum (super-dry samples). For all temperatures up to 1000 K, the data of the dry samples are consistent with a simple increase in the sticking force by a factor of ∼ 10 over wet samples. Up to 900 K super-dry samples behave like dry samples. However, the sticking forces then exponentially increase up to another factor ∼ 100 at about 1200 K. The increase in sticking from wet to dry extends a trend that is known for amorphous silicates to multimineral mixtures. The findings for super-dry dust imply that aggregate growth is boosted in a small spatial high-temperature region around 1200 K, which might be a sweet spot for planetesimal formation.

Collisional evolution of dust and water ice in protoplanetary discs during and after an accretion outburst

“…However, there is still a large parameter space to explore by laboratory experiments concerning the impact of multiple ice species on the collisional properties of dust grains, and whether the many-seeds response can be extended to the sublimation of other abundant ices. We also note that dust in high temperature environments (𝑇 > 1200 K) is thought to become more sticky (Pillich et al 2021). Accretion outbursts could provide the necessary temperature to lead to boosted growth in a more extended fractions of the inner disc.…”

First steps of planet formation around very low mass stars and brown dwarfs

“…We consider that the fragmentation velocity (v frag ) is 10 m s −1 . The actual value of these velocities is currently debatable because laboratory experiments are limited to temperatures that are higher than the typical disk miplane temperatures [67,68].…”