Protoplanetary Disks
Protoplanetary disks form when a star is forming, during the gravitational collapse of a molecular cloud core. Much of the mass that eventually will end up on the star forming at the center will flow through the disk. Near the end of the star formation process, when the amount of new material falling down onto the disk, the accretion rate goes down, the disk becomes quieter. During this phase, we believe that planets are formed.
During this phase, the shape of the disk is often flaring, so the
surface is bowl-shaped. Radiation from the star can reach the surface
in many places, so that the disks can be detected in scattered light.
The disk is largely made of gas, but it also contains dust particles.
The small dust particles stay suspended in the disk and can be present
even at the surface of the disk. Larger grains sink down to the
midplane, where they may start to grow and form planets. The disk
surface is illuminated by the star, so it is warm and penetrated by
energetic UV radiation. That radiation breaks up molecules, so that
the surface layers are full of atoms and ions, but contain only few
molecules.
I am studying the structure and evolution of protoplanetary disks. We construct models of protoplanetary disks and use radiative transfer calculations to determine the temperatures distribution and the light emitted from the disk, so that these can be compared with observations. There is a wide range of temperatures present, and so the light is being emitted at many different wavelength. The inner parts of the disk shine in visible light. The further out we go, the longer the wavelength of the light becomes, so we get first near-infrared radiation with wavelenghts of a few micrometer, already invisible of the eye. Still further out, the disk emits mid- and far-infrared radiation, with wavelength of 10-60 micrometer or so. The coldest parts can be detected with radio telescopes.
We are particularly interested in the inner disk
regions. These regions are so hot that the dust particles start to
evaporate, and the material becomes more transparent. The figure to
the right (from Kama, Min, Dominik 2009, A&A 506,1199) shows this
region, with the temperature shown in color. There is a region where
the get quickly cold (dark area), and inside of this region there is
an extended area where only a small amount of dust can survive. The
"hole" in the disk is only the beginning. At later stages, when
planets are forming, they can clear out a much bigger hole. Such
disks give themselves away by only emitting a small amount of
radiation at near-infrared wavelengths.