complex molecules are synthesized in the interstellar medium, the question of 

 the possible connections between the existence of these species and the exis- 

 tence of biological molecules on Earth is of interest. The first of these questions 

 can be addressed by both theoretical and observational approaches. It is feasible 

 to discuss whether the theoretical models predict large abundances of yet-to-be- 

 detected complex molecules, whether observational techniques can locate such 

 complex molecules if they exist in appreciable abundance, and what the best 

 frequencies and observational tools might be. The second of these questions is 

 very difficult to answer. The processes of star and planet formation from collaps- 

 ing interstellar clouds are not yet understood in any detail. An understanding of 

 the chemical relationship between organic interstellar molecules and those on 

 newly formed planets such as the primeval Earth requires an understanding of 

 the physical conditions experienced by the molecules during the transitions from 

 interstellar cloud to protosolar nebula and comets, from protosolar nebula to 

 planetesimals, and from planetesimals and comets to planets. (Some of these 

 issues are addressed in other sections of this report.) We will discuss answers to 

 the first of the two questions only. In addition, we will briefly discuss small 

 molecules of exobiological interest— chief among these is water. 



Interstellar clouds are giant accumulations of gas and dust particles that exist 

 primarily in the planes of the Milky Way and other spiral galaxies. Typical tem- 

 peratures in these clouds range from 10 to 100 K except in localized star- 

 forming regions, which are considerably warmer. The matter is mainly gaseous, 

 with perhaps only 1% (by mass) in the form of small dust particles, or grains, 

 roughly 0.1 pm in radius. The density of the gas in a typical, dense, interstellar 

 cloud is in the range 10 4 to 10 6 cm -3 ; that in the so-called diffuse interstellar 

 clouds is typically in the range 10 _1 to 10 2 cm -3 . The major constituents of the 

 gas are molecular hydrogen (H 2 ) and helium. Molecules involving heavier atoms, 

 of which close to 70 are known, possess lower abundances by significant factors. 

 With respect to H 2 , the (fractional) abundance of CO in dense clouds is typically 

 10" 4 ; that of water, as large as 10~ 5 ; that of methanol, 10~ 7 ; and that of even 

 more complex species 10" 8 to 10~ 10 . The molecules range in complexity from 

 diatomics such as H 2 and CO up to a 13-atom, linear, unsaturated nitrile 

 (HCj jN) and include many of the better-known small organic molecules. Note 

 that even though the detected molecules involving heavy atoms are trace con- 

 stituents of a rarified gas in the clouds, the clouds are so vast that the amount of 

 organic matter in them surpasses that on Earth. 



The mechanism by which gaseous molecules in dense interstellar clouds are 

 excited into emission in the microwave region of the spectrum is predominantly 

 inelastic collisions. These collisions convert translational energy into rotational 

 energy, promoting one of the collision partners into an excited rotational state 

 that then emits as it relaxes to a lower rotational level. In the bulk of interstellar 

 clouds, there is insufficient thermal energy to populate excited vibrational 

 states via inelastic collisions. Consequently, vibrational emission, which would be 



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