Organic molecules in space

Abstract

The chemistry of interstellar molecular clouds is reviewed. Emphasis has been given to recent observational and theoretical advances. At present approximately 70 identified molecular species have been detected in interstellar molecular clouds and circumstellar shells. Identification of the newly detected species rests mainly on the exacting test between interstellar and laboratory spectra. A large number of unidentified interstellar lines indicates the wide chemical variety in synthesizing molecules in a low temperature, low density environment. Although most of the interstellar molecules are organic, (presently 53) some of the most abundant interstellar molecules are inorganic, such as H₂, H₂O, NH₃. The recent detection of the H₂D⁺ molecular ion is astro-chemically important, since the existence of the primary ion H ⁺₃ has been postulated by the ion-molecule gas phase reaction scheme. Amongst the newly detected molecules are additional carbon chain molecules and the first two ring molecules SiCC and C₃H₂.

Recent theoretical gas phase model calculations leading to the observed molecular complexity are discussed together with a critical evaluation of gas phse versus grain syntheses of interstellar molecules. Finally the question of how large interstellar molecules can be is addressed, seen in the light of chemical evolution.