2019ApJ...875...73B

Small hydrocarbons are an important organic reservoir in protostellar and protoplanetary environments. Constraints on desorption temperatures and binding energies of such hydrocarbons are needed for accurate predictions of where these molecules exist in the ice versus gas phase during the different stages of star and planet formation. Through a series of temperature programmed desorption experiments, we constrain the binding energies of 2- and 3-carbon hydrocarbons (C₂H₂-acetylene, C₂H₄-ethylene, C₂H₆-ethane, C₃H₄-propyne, C₃H₆-propene, and C₃H₈-propane) to 2200-4200 K in the case of pure amorphous ices, to 2400-4400 K on compact amorphous H₂O, and to 2800-4700 K on porous amorphous H₂O. The 3-carbon hydrocarbon binding energies are always larger than the 2-carbon hydrocarbon binding energies. Within the 2- and 3-carbon hydrocarbon families, the alkynes (i.e., least-saturated) hydrocarbons exhibit the largest binding energies, while the alkane and alkene binding energies are comparable. Binding energies are ∼5%-20% higher on water ice substrates compared to pure ices, which is a small increase compared to what has been measured for other volatile molecules such as CO and N₂. Thus in the case of hydrocarbons, H₂O has a less pronounced effect on sublimation front locations (i.e., snowlines) in protoplanetary disks.