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PhD Thesis: the puzzle of complex organic molecule formation in
prestellar cores
One of the most fascinating questions in modern astrophysics is understanding
the origins of life ingredients on Earth. The advent of sensitive telescopes in the
radio and millimetre domains now makes this quest possible, and a new field
astrochemistry is developing. In the past decades, over 180 molecules have been
detected in the interstellar medium, among them many complex organic
molecules (i.e. molecules with more than 6 atoms, made of C, H, N and O) and
including organic molecules which are stable and common on Earth. These
organic molecules are seen in the warm gaz (~ 100 K) around nascent protostars,
a stadium through which our Sun went in the course of its formation. It is as of
yet unclear what the future of these species is during the star and planet
formation process, but they are likely destroyed, reformed, or incorporated into
protoplanetary disks, and then into planetesimals and small bodies like comets
and meteorites. Since these molecules are precursors of prebiotic molecules, their
study is essential to understand the emergence of molecular complexity as we
know it on Earth or as it possibly exists in (exo)planetary systems.
The scenarios invoked to account for the presence of the observed organic
species are that the molecules either result from hot gas-phase chemistry in the
direct vincinity of protostars, or that they are formed on interstellar dust grains
serving as catalysts. In this case, the reactants (small molecular species) that stick
to the grains at low temperatures make up the ice in which radical species form
that become mobile when the protostar heats up its environment. The surface
diffusion of radical species allows them to react together to form complex organic
molecules (COMs). According to current understanding, both considered reaction
paths need warm or hot gas to form COMs.
These hypotheses are now being questioned by the recent discovery of COMs in a
prestellar cores by our group (Bacmann et al. 2012, Astronomy & Astrophysics,
541, L12). Prestellar cores are the direct precursors of protostars and allow us to
study the gas before the warm-up phase due to the formation of the protostar.
The very low temperatures in these objects exclude the aforementioned scenarios
and indicate that non-thermal mechanisms may play an important role the the
formation of these molecules. Since the gas temperatures in these sources rarely
exceeds 12-15 K, radical mobility necessary to COM formation on grains cannot
come form thermal processes. Besides, the gas-phase reactions that have been
considered to form COMs lead to abundances several orders of magnitude lower
than observed abundances. The question of COM formation must therefore be
completely revisited.
The issues raised by this discovery are: is the species’ presence general at the
prestellar stage? What is the origin of the energy necessary to the formation of
these molecules? Should other formation schemes be considered, for example
alternative gas-phase reactions at very low temperature? Are the sources’
compositions similar? etc.
In order to answer these questions, the proposed work during the thesis is to
gather a comprehensive set of data on COMs in prestellar cores from large
radiotelescopes (IRAM 30m, GBT 100m). These observations will be analysed
(molecular contents, abundance, spatial distribution) and used to bring strong
constraints to chemical models of the cold interstellar medium, which are up to
now unable to account for the presence of COMs in these regions. Collaborations
with V. Taquet (NASA Goddard) and E. Herbst (University of Virginia) are foreseen.
Research Institution: Institut de Planétologie de d’Astrophysique de Grenoble
(IPAG) / Université Joseph Fourier
PhD Advisors: Aurore Bacmann & Alexandre Faure (ipag.osug.fr/bacmanna/
phd.html)
Duration: 3 years starting October 2014
Funding : Université Joseph Fourier (Grenoble). The PhD grant is associated with a
travel budget.