Prebiotic Material in the Universe Organic Material in the Universe

Observations carried out in recent decades at millimetre wavelengths by radio telescopes have shown that the interstellar medium is rich in complex organic molecules (Table 9.2). This wavelength region is particularly suitable for such studies because of the size of the molecules: relatively large, their fundamental frequencies are lower than those of smaller molecules.

Most of the polyatomic molecules have been detected in the interstellar medium, in particular in dense molecular clouds. The temperature there is sufficiently low (10—20 K) that any gaseous molecule that encounters a solid grain is immediately captured to form a layer of ice. The organic molecules incorporated in this layer are subject to ultraviolet radiation from nearby stars, which accelerates their evolution into even more complex refractory molecules. If the grain happens to become part of the denser and hotter environment of a star in the process of formation, these molecules, returning to the gaseous state, will enrich the ambient protoplanetary medium.

The circumstellar envelopes of evolved stars, in particular of stars rich in carbon, is also a medium where complex organic molecules are found in abundance, in particular the polycyclic aromatic hydrocarbons (PAH). When the envelope of one of these stars is ejected, these molecules enrich the surrounding interstellar medium. Currently more than 120 interstellar molecules are known (Table 9.2), the largest being HCnN, consisting of 13 atoms, 11 of which are carbon. In addition to these molecules, which have been identified individually in their gaseous phase by their spectrum at millimetre wavelengths, since the 1980s, PAHs have also been found. These are cyclic organic molecules, identified from their infrared spectra (Fig. 9.6), in which several tens of atoms are bound to their benzene groups.

Table 9.2 Molecules detected in the interstellar medium and in circumstellar envelopes [After Gilmour and Sephton, 2003]

2 atoms

2 atoms

Table 9.2 Molecules detected in the interstellar medium and in circumstellar envelopes [After Gilmour and Sephton, 2003]

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