The Curie line in protoplanetary disks and the formation of Mercury-like planets

Abstract

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.

Composition and Sticking of Hot Chondritic Dust in a Protoplanetary Hydrogen Atmosphere

“…This coincides with the known reduction reaction of olivines, breaking down to iron-poorer olivines, metallic Fe(Ni), and pyroxene (Boland & Duba 1981). After heating at 1400 K in H 2 gas, the sample consists of nearly equal amounts of olivine, pyroxene, and Fe(Ni) phases, and the latter might experience enhanced magnetic aggregation (Bogdan et al 2023).…”

“…One first result of this study, somewhat related to adhesion but with a focus on magnetic properties, has already been reported in Bogdan et al (2023). We showed that tempering Allende in the hydrogen atmosphere results in the formation of ferromagnetic iron-nickel constituents, which might be important in the context of magnetic aggregation and the formation of Mercury-like planets (Kruss & Wurm 2018.…”

“…The atmosphere that meteorite dust particles are subjected to highly influences compositional changes caused by high temperatures-ranging from significant to almost unnoticed upon tempering. Especially the formation of metallic iron phases might influence the aggregation within a protoplanetary disk's magnetic fields apart from simple adhesion (Kruss & Wurm 2018Bogdan et al 2023).…”

Forming super-Mercuries: Role of stellar abundances

“…As disc viscosity is ramped up in the MRI-active region, the growth of dust grains in this region could possibly be inhibited, posing potental problems for the growth of pebbles (and planets). On the other hand, it has been showed that large bodies could still grow near the silicate sublimation line at ∼ 0.1 AU (Flock et al 2019) or within the 'Curie line' at T ∼ 1000 K (Bogdan et al 2023). Detailed modelling of disc temperature and density profile in the inner region of protoplanetary discs is beyond the scope of this paper, but would be needed in future studies to understand the mechanism of dust growth in regions very close to the star.…”

“…In contrast to the work of Schneider & Bitsch (2021), we make the assumption that all the iron in the disc are locked only in pure Fe grains (metallic iron) with condensation temperature of 1357 K and not partitioned into other oxidised species such as Fe 2 O 3 or Fe 3 O 4 . This assumption is based on the results of equilibrium chemistry modelling (Jorge et al 2022) and laboratory experiments (Bogdan et al 2023) which show that iron in the disc is present in metallic form where temperatures are higher than ∼ 1000 K.…”