Interstellar clouds, consisting of gas and dust particles, are strongly molecular in nature.  Although some molecules have been detected in the solid state, most of the almost 200 molecular species known are detected only in the gas.  “Complex” interstellar molecules; i.e., those with six or more atoms, are produced by a variety of local mechanisms, both in the gas phase and on bare dust surfaces or on and in ice mantles of dust particles. Many of the smaller complex molecules,  with sizes of 6-13 atoms, can be conveniently divided into two types: carbon-chain and terrestrial-like species.  The carbon-chain species consist of highly unsaturated and often exotic organic molecules including ions, radicals, and metastable isomers, while the terrestrial-like species consist of common oxygen-containing organic molecules such as esters, ethers, alcohols, etc.  Carbon-chain species, found mainly in cold cores at temperatures near 10 K, are understood to form via a variety of gas-phase processes, while the situation is more complex for terrestrial-like species. The synthesis of some of these molecules can involve individual steps of gas-phase and grain-surface/mantle processes.  Molecules on grains can desorb into the gas thermally, at temperatures over 100 K, and non-thermally from very cold grains utilizing the energy of photons or exothermic reactions.   On the grains, thermal chemical processes compete with non-thermal processes, activated through bombardment by photons, electrons, and cosmic rays, which are mainly protons.  In the past, most emphasis has been given to the role of photons in producing radicals on and in ice mantles that can subsequently react via a diffusive mechanism to form products, which eventually desorb into the gas.  Recently, more attention has been paid to the role of charged particle bombardment.  In this talk, the various surface processes will be discussed with an emphasis on the role of electron and proton bombardment, which can initiate a complex sequence of events in the grains leading to large molecules, in analogy with the results of many laboratory experiments.<br/>

Production mechanisms for complex interstellar molecules

253rd National Meeting (2017)

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