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| Paper: |
Chemical Models of Star-Forming Cores |
| Volume: |
476, New Trends in Radio Astronomy in the ALMA Era |
| Page: |
197 |
| Authors: |
Aikawa, Y. |
| Abstract: |
We review chemical models of low-mass star forming cores including our own work.
Chemistry in molecular clouds are not in equilibrium. Molecular abundances in star forming cores
change not only with physical conditions in cores but also with time.
In prestellar cores, temperature stays almost constant ∼ 10 K, while the gas density increases as the core collapses.
Three chemical phenomena are observed in this cold phase: molecular depletion, chemical fractionation,
and deuterium enrichment. They are reproduced by chemical models combined with isothermal gravitational collapse.
The collapse timescale of prestellar cores depends on the initial ratios of thermal, turbulent and magnetic
pressure to gravitational energy.
Since the chemical timescales, such as adsorption timescale of gas particle onto grains, are comparable to
the collapse timescale, molecular abundances in cores should vary depending on the collapse timescale.
Observations found that molecular abundances in some cores deviate from those in other cores, in spite of
their similar central densities; it could originate in the pressure to gravity ratio in the cores.
As the core contraction proceeds, compressional heating eventually overwhelms radiative cooling, and the
core starts to warm up. Temperature of the infalling gas rises, as it
approaches the central region. Grain-surface reactions of adsorbed molecules occur in this warm-up phase, as well as
in prestellar phase. Hydrogenation is efficient at T ≤ 20 K, whereas radicals can migrate on grain surface and
react with each other
to form complex organic molecules (COMs) at T ≥ 30 K. Grain-surface species are sublimated to the gas phase
and re-start gas-phase reactions; e.g. unsaturated carbon chains are formed from sublimated methane.
Our model calculation predicts that COMs increases as the warm region extends outwards and
the abundances of unsaturated carbon chains depend on the gas density in the CH4 sublimation zone.
Recent detection of COMs in prestellar cores may indicate that
a fraction of COMs formed in the vicinity of a protostar could be distributed to ambient clouds by outflows.
COMs and carbon chains in protostellar phase inherit the high D/H ratio of their mother molecules, which
originate mostly in cold prestellar phase. |
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