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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
, is highly volatile and it may be the last one
left ``standing''. The rate at which nitrogen is broken out of
N 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.