  In laboratory experiments, we heated chondritic material up to 1400K in a
hydrogen atmosphere. Moessbauer spectroscopy and magnetometry reveal that, at
high temperatures, metallic iron forms from silicates. The transition
temperature is about 1200K after 1 h of tempering, likely decreasing to about
1000K 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.
