Speaker
Prof.
Nuria Calvet
(University of Michigan)
Description
Stars are born surrounded by protoplanetary disks, formed as rotating molecular cloud cores collapse conserving angular momentum. The makeup of these disks is originally similar to the ISM, namely, around 99% gas and 1% solid grains, “dust”. These proportions change with time with most of the gas ending in the stars and planets or being photoevaporated, and solids remaining in rocky planets and smaller bodies or in cores of giant planets. Understanding the details of this evolution is at the forefront of current research. At the same time, the advent of high spatial resolution instruments has provided astonishing images of substructures in protoplanetary disks, giving further clues to disk evolution. A key parameter affecting evolution -- determining the development of instabilities, the location of condensation fronts of volatiles and refractory materials, the chemical structure, the drift of material towards the star or leaving the disk -- is the disk temperature. Disks are heated mostly by absorption of radiation fields from the central engine: the photosphere and the X-rays and UV radiation due to magnetic activity on the stellar surface, plus the high-energy fields resulting from the accretion shock formed as the stellar magnetic field brings disk material onto the star. The ability of disk matter to absorb these radiation fields -- its opacity -- is therefore a crucial factor determining disk structure and evolution. I will review the current understanding of disk opacities and how they are used in disk modeling. These include both gas -- molecular and atomic -- and dust opacities, from the X-rays to the far-IR. Directly related to this is the equation of state appropriate to disks because the dust and gas phases are linked. For instance, molecular hydrogen, the dominant component of the disk, forms on grain surfaces so formation rates are dependent on grain mass abundance and size distribution. Condensation fronts of different species may induce the formation of pressure traps conducive to planet formation; the location of those fronts depends on the density-temperature structure of disks as well as on the exposure to desorbing radiation fields. I will describe these and other effects, as well as refer to the databases of molecular and dust opacities most frequently adopted in current efforts.
Primary author
Prof.
Nuria Calvet
(University of Michigan)
