Speaker
Description
The harsh radiation environment of disk surfaces is thought to be inhospitable to organic molecules. Indeed, the disk surface is sweltering atomic gas of several thousand Kelvin, well in excess of the dust temperature. However, as radiation is shielded by increasing column densities of dust the potential for molecules to form increases as both the photodissociation rates and temperatures decline. A critical transition occurs when the decreasing rate of molecular hydrogen destruction feeds back to the decrease in temperature and the increase in molecular line cooling. A molecular shielding layer forms that dramatically attenuates the stellar radiation. We will describe the observations that support this description of the hot molecular gas in the terrestrial-planet forming regions of disks and detail of the physical processes driving the heating. Below this hot molecular layer of species such as H$_2$, CO, H$_2$O, and OH, the organic molecules that would otherwise dissociate may form. Dust temperatures below the shielding layer are in a temperature regime that sensitvely determines the reaction rates for molecule formation. The dissipation of turbulent energy into heating the gas has the potential to dramatically increase the abundance of warm molecules in this layer. Indeed, measurements that constrain the abundance of warm prebiotic molecules may be diagnostic dynamical models of disk accretion and evolution.
