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Dense Interstellar Clouds

In our home galaxy, the Milky Way, as well as in external galaxies, the space between the stars is filled with an Interstellar Medium (ISM) consisting of gas and dust, like the beautiful Orion Nebula. While accounting for only a small fraction of the Galaxy's mass, the interstellar medium is nevertheless an important part of the Galactic ecosystem. The ISM is composed mainly of three types of clouds: dark clouds (Av > 5 mag), translucent clouds (1 mag < Av < 5 mag), and diffuse clouds (visual extinction, Av ~ 1 mag). Interstellar clouds are neither uniform nor dynamically quiescent on long timescales. They display "clumpy" structures, are continually evolving as new stars form, and are enriched by material ejected from dying stars that was formed during stellar nucleosynthesis.

Dense interstellar clouds are the birth sites of stars of all masses and their planetary systems. Interstellar molecules and dust become the building blocks for protostellar disks, from which planets, comets, asteroids, and other macroscopic bodies eventually form. Observations at infrared, radio, millimeter, and sub-millimeter frequencies show that a large variety of gas phase organic molecules are present in the dense interstellar medium. These include classes such as nitriles, aldehydes, alcohols, acids, ethers, ketones, amines, and amides, as well as many long-chain hydrocarbon compounds. Such species display large compositional variations between quiescent dark clouds and star-forming regions, as well as strong abundance gradients on small spatial scales within each type of cloud.

Radioastronomical molecular line surveys of well-known sources, such as the dense star-forming cores in the Orion and Sagittarius molecular clouds, show that molecules of considerable complexity can be found in these regions. Gas phase reactions can be important for forming some of the observed species but the presence of high abundances of saturated species, and the large deuterium fractionation ratios, indicates that catalysis on dust grains also contributes to the composition of these so-called hot molecular cores around protostars, theoretical studies indicate that a combination of gas and surface chemistries is necessary to explain many of their observed characteristics: evaporation of simple molecular mantles drives a complex gas phase chemistry. For example, surface-formed methanol, ethanol, and higher alcohols, can act as precursors of very large interstellar molecules through alkyl cation transfer reactions.

Diffuse clouds have moderate extinctions (< 1 mag) and densities of roughly 100 - 300 per cm3. They are characterized by an average temperature of ~ 100 K and a UV radiation field of approximately ~ 108 photons per cm2 s. Since the initial discovery of simple diatomic molecules in interstellar space, CH, CN neutrals and CH ions, many more molecules have been detected in the photon-dominated diffuse medium, although at lower abundances than found in dense clouds. Species detected include: HCO ions, CO, OH, C2, HCN, HNC, CN, CS and H2CO and recently C2H and c-C3H2 molecules in several extragalactic diffuse clouds. The C2H abundance varies little from diffuse to dense clouds whereas the c-C3H2 abundance is markedly higher in dense clouds. Reactions of the neutral molecules CH and CH2 molecules with C ions can lead to the formation and build-up of polyatomic hydrocarbons. However, the presence of large carbon-bearing species is strongly dependent on their formation and survival rate because the diffuse medium is controlled by photochemistry. The larger carbonaceous molecules that enter the diffuse interstellar gas are detected in circumstellar envelopes around late-type stars.

Fig. 1: Orion system. (c) C.R. O'Dell and S.K. Wong, Rice University and NASA

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