Speaker
Description
Until recently, it was thought that the production of larger interstellar molecules proceeds by definite chemistries depending upon the physical conditions of the source. For cold and pre-stellar cores at 10 K, it was thought that ion-molecule processes dominate the large molecule chemistry and lead to the production of very unsaturated carbon-chain type species. For hot cores, it was thought that the production of more saturated, large terrestrial-like “complex” organic species (COMs) observed in the gas occurs mainly by a radical-radical grain surface chemistry that begins during the warm-up stage from pre-stellar to hot core and transfers the reaction products into the gas via thermal desorption at later stages. But recent observations, experiments, and theory have shown that this picture is hopelessly simplistic. The detection of gaseous COMs in cold and pre-stellar cores at 10 K indicates a complex picture in which these molecules are formed either in the gas-phase mainly by neutral-neutral reactions including radiative association, or on and in ice mantles by a variety of surface processes, some quite unusual, and then undergo non-thermal desorption into the gas. The exotic processes can occur via non-thermal mechanisms or via three-body mechanisms in which a collision on the surface activates a molecule to react with another species. The formation of COMs in warming cores by surface radical-radical association reactions has come under some theoretical attack, which shows that surface reactions are more complex than thought, and that some surface association reactions don’t occur or occur very slowly. On the other hand, assumed reductions of barriers against diffusion for surface reactions have led to enhanced reaction rates for association reactions on surfaces. Moreover, new extensions of ion-molecule theory to higher temperatures have been reported, in which thermal or non-thermal desorption leads to gas-phase species, which can be precursors to new sets of reactions. Examples of precursor molecules are (1) methane, which can lead to a gaseous warm carbon-chain chemistry (WCCC), leading to the production of C7H among other unsaturated hydrocarbons, and (2) ammonia, which can enhance the synthetic capability of ion-molecule reactions by reacting with molecular ions to produce NH4+ and neutrals significantly larger than those produced by dissociative recombinations with electrons. This latter mechanism is based on the premise that large abundances of ammonia can be desorbed into the gas via accretion shocks during warm-up. Finally, a completely new theory of the formation of larger molecules by radiolysis of ice mantles of dust grains by cosmic rays suggests that COMs and other species can be formed in both cold and hot regions. The radiolysis theory is supported by laboratory experiments. In summary, the production and efficiency of complex molecules is currently recognized to be far more complex than considered five years ago, and more work is needed to determine which of the many processes considered are dominant, if any. At least, we know that liquids are not involved, or do we?
