Circumstellar Disks

The building blocks of planets are forged in the dusty material around stars, which we investigate by studying the radiation that is emitted by circumstellar material. This includes evaluating the physics and chemistry that is necessary to interpret these diagnostics. In the terrestrial-planet-forming region of the disks around young stars (at roughly the distance that the Earth is from the Sun) stellar blackbody radiation peaks in the near-infrared region (at several hundred nm) and heats the dust to temperatures of a few hundred Kelvin, which leads to thermal dust emission at far-infrared and sub-millimeter wavelengths.

The circumstellar gas is a hundred times for massive than the dust (at least initially) and is heated (at least in part) by stellar X-ray and FUV radiation. The gas temperature soars as this short-wavelength radiation heats the gas to over a thousand Kelvin. Atoms, ions, and molecules in the hot gas have line emission spectra that can be recorded by observatories on the ground (such as at the IRTF, Keck, and the VLT ) as well as observatories in space (for example, Spitzer and JWST). Our ongoing research is to understand and interpret the interesting properties of the emission spectra from disks, in order to understand and how they inform the early stages of planet formation.

Thermal-Chemical Models

We model the layered structure of the disk gas (illustrated below) and use this information to predict and interpret observations of atomic and molecular emission. Note the very high temperatures in the hot atomic surface layer, and the interesting transition to molecular conditions as the chemical abundance of carbon monoxide, water, and other molecules increase, while the temperature in the same region plummets.

Some Outstanding Questions

How does the mass and radiation field of a star determine the molecular composition of a circumstellar disk? Is there any gas remaining after the (millimeter-sized) dust has coagulated into (kilometer-sized) planetesimals? Is there an evolutionary trend of disk properties that is predictive of exoplanet characteristics? These are the types of questions that we think about and explore with numerical models and observations. The launch of JWST with mark the beginning of an era of new observations and constraints on circumstellar material, and

Additional Reading

Our recent work considering the role of Lyman-alpha radiation and photochemical heating builds on pioneering studies of how X-rays heat disk gas. These studies complement work that considered the photo-electric heating of disk gas, which is important high in the disk surface layers, where the size of the dust grains is small and similar to dust in the interstellar medium. As the disk evolves and the dust grains grow and settle to the midplane, the role of the photoelectric effect decreases. Below the irradiated layer, where accretion is thought to occur, heating via turbulence may have an observable signatures on the abundance and temperature of molecules.

Prof. Ádámkovics

213 Kinard Laboratory