<< Back

Chemistry in regions of star formation (Originality)

Ionisation along the star formation trail
Ion-molecule reactions have long been known to be very important for interstellar chemistry because the Coulomb interaction promotes reactions involving ions even at the low temperatures of space. Indeed, ionic species are also known to be abundant in molecular clouds. Because of this high reactivity, molecules conversely control the degree of ionization inside molecular clouds. The degree of ionization in a cloud, on the other hand, directly influences the ``leakage'' of the supporting magnetic fields (ambipolar diffusion) and hence the formation of new stars and planets. However, the recent discovery of high abundances of large molecules such as polycyclic aromatic hydrocarbons and carbon chains in space has shifted the focus of this field somewhat. Because of their high electron affinity, these species may soak up all the free electrons in the gas and reactions involving such anionic species may dominate the chemistry and the ionization balance. Unfortunately, very little is known experimentally about the rates or products of reactions involving electrons, ions, and large anionic, neutral, or cationic PAHs or carbon chains. Hence, laboratory astrochemistry and astronomical modeling have to work hand in hand in order to explore this important interaction between ions, electrons and large molecules and to address the larger astronomical question of star formation.
Nitrogen chemistry as tracers of protostellar condensations
During the cold dense ambipolar diffusion phase preceding the actual collapse phase of the star formation, molecules are expected to freeze out on dust grains forming an icy mantle. The rate of this molecular freeze out will depend on the volatility of the species involved. Nitrogen bearing species are of particular relevance then because the main reservoir of nitrogen, N$_2$, is highly volatile and it may be the last one left ``standing''. The rate at which nitrogen is broken out of N$_2$ and incorporated into less volatile species will then determine the rate at which the gas phase loses its molecular signature. Studies of the reactivity and excitation of nitrogen-bearing species as well as their spectroscopy acquire, therefore, additional impetus, because such species may provide the only way to determine the physical conditions and dynamics of the proto-stellar phase of star formation.
Molecular tracers of shocks
The later phases of star formation are characterized by strong shocks driven into the environment by powerful protostellar winds. Sulfur monoxide, sulfur dioxide as well as silicon monoxide have long been used as shock indicators. However, the chemistry of sulfur in interstellar space is not well understood and the main reservoir of this element in molecular clouds has not yet been identified. Moreover, collisional excitation rates of these species are poorly known, considerably hampering the analysis of astronomical data in terms of the local physical conditions (density and temperature of the gas) as well as the abundances of these shock tracers. A combined program on the chemistry of sulfur- and silicon-bearing species, their abundances, their excitation, and their spectroscopic signatures, under astrophysically relevant conditions will be important to probe the interaction of protostars with their ``natal'' environment.