Sticking properties rule the early phases of pebble growth in protoplanetary discs in which grains regularly travel from cold, water-rich regions to the warm inner part. This drift affects composition, grain size, morphology, and water content as grains experience ever higher temperatures. In this study we tempered chondritic dust under vacuum up to 1400 K. Afterwards, we measured the splitting tensile strength of millimetre-sized dust aggregates. The deduced effective surface energy starts out as γe = 0.07 J m−2. This value is dominated by abundant iron-oxides as measured by Mössbauer spectroscopy. Up to 1250 K, γe continuously decreases by up to a factor five. Olivines dominate at higher temperature. Beyond 1300 K dust grains significantly grow in size. The γe no longer decreases but the large grain size restricts the capability of growing aggregates. Beyond 1400 K aggregation is no longer possible. Overall, under the conditions probed, the stability of dust pebbles would decrease towards the star. In view of a minimum aggregate size required to trigger drag instabilities it becomes increasingly harder to seed planetesimal formation closer to a star.

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.

In laboratory experiments, we heated chondritic material up to 1400 K in a hydrogen atmosphere. Mössbauer spectroscopy and magnetometry reveal that, at high temperatures, metallic iron forms from silicates. The transition temperature is about 1200 K after 1 h of tempering, likely decreasing to about 1000 K for longer tempering. This implies that in a region of high temperatures within protoplanetary disks, inward drifting solids will generally be a reservoir of metallic iron. Magnetic aggregation of iron-rich matter then occurs within the magnetic field of the disk. However, the Curie temperature of iron, 1041 K, is a rather sharp discriminator that separates the disk into a region of strong magnetic interactions of ferromagnetic particles and a region of weak paramagnetic properties. We call this position in the disk the Curie line. Magnetic aggregation will be turned on and off here. On the outer, ferromagnetic side of the Curie line, large clusters of iron-rich particles grow and might be prone to streaming instabilities. To the inside of the Curie line, these clusters dissolve, but that generates a large number density that might also be beneficial for planetesimal formation by gravitational instability. One way or the other, the Curie line may define a preferred region for the formation of iron-rich bodies.

<p>Sticking properties play an important role in the early phase of planet formation. In the protoplanetary disc, grains drift towards the star, being exposed to increasingly higher temperatures. Tensile strength measurements by means of the Brazilian test along with results from Mössbauer spectroscopy suggest that there is a spacial region that favours planetesimal formation between 900 K and 1300 K [1,2].<span class="Apple-converted-space"> </span></p> <p>For the Brazilian test pieces of two meteorites, namely Sayh al Uhaymir 001 and Allende, were milled to micrometer dust, pressed into cylinders, and tempered at increasing temperatures up to 1400 K before the tensile strength measurement. The Sayh al Uhaymir is an L4/5 type chondrite that has undergone a slight thermal metamorphosis. The Allende however is classified as CV3. It is unequilibrated and therefore the closest to a realistic mix of minerals in the protoplanetary disc. Comparing sticking properties in terms of surface energies in relation to the heating temperature of the two different meteorite samples, they show no significant difference for heating under vacuum. Both datasets show a considerable increase in sticking around 1200 K by orders of magnitude. The new Allende data fits the older Sayh al Uhaymir data in this respect. This confirms the former found location in the warm inner disc that supports planetesimal formation best.</p> <p>The most abundant element in protoplanetary discs is gaseous hydrogen. To see how the presence of hydrogen might influence the results, we followed the same measurement procedure but tempering in a continuous hydrogen atmosphere. The heating chamber is flushed with hydrogen during the entire heating process. This also does not produce any significant change in the results. The relative surface energies still increase monotonously and rise by orders of magnitude around 1200 K.<span class="Apple-converted-space"> </span></p> <p>We see an influence of composition and atmosphere as well as water content, grain size and morphology on sticking properties. Overall, also our new results not only suggest subtle changes but imply a boost in surface energy for high-temperature dust. This continues to support the idea of a hot spot around 1200 K that favours aggregation and might trigger a high number of planetesimals and subsequently<span class="Apple-converted-space">  </span>planets in the inner part of protoplanetary discs [in prep].<span class="Apple-converted-space"> </span></p> <p><img src="" alt="" width="598" height="478" /></p> <p>[1] Bogdan, T., Pillich, C., Landers, J., Wende, H., & Wurm, G. (2020). Drifting inwards in protoplanetary discs I: Sticking of chondritic dust at increasing temperatures. Astronomy & Astrophysics, 638, A151.</p> <p>[2] Pillich, C., Bogdan, T., Landers, J., Wurm, G. & Wende, H., (2021). Drifting inwards in protoplanetary discs II: The influence of water on sticking properties at increasing temperatures. Astronomy & Astrophysics, 652, A106.</p>