Sensitivity of gas-grain chemical models to surface reaction barriers:
  Effect from a key carbon-insertion reaction, C + H$_2$ $\rightarrow$ CH$_2$
  The feasibility of contemporary gas-grain astrochemical models depends on the
availability of accurate kinetics data, in particular, for surface processes.
We study the sensitivity of gas-grain chemical models to the energy barrier Ea
of the important surface reaction between some of the most abundant species: C
and H2 (surface C + surface H2 = surface CH2). We used the gas-grain code
ALCHEMIC to model the time-dependent chemical evolution over a 2D grid of
densities (nH: 10^{3} - 10^{12}cm^{-3}) and temperatures (T: 10 - 300 K),
assuming UV-dark (Av = 20 mag) and partly UV-irradiated (Av = 3 mag) conditions
that are typical of the dense interstellar medium. We considered two values for
the energy barrier of the surface reaction, Ea = 2500 K (as originally
implemented in the networks) and Ea = 0 K (as measured in the laboratory and
computed by quantum chemistry simulations). We find that if the C + H2 = CH2
surface reaction is barrierless, a more rapid conversion of the surface carbon
atoms into methane ice occurs. Overproduction of the CHn hydrocarbon ices
affects the surface formation of more complex hydrocarbons, cyanides and
nitriles, and CS-bearing species at low temperatures < 10-15 K. The surface
hydrogenation of CO and hence the synthesis of complex (organic) molecules
become affected as well. As a result, important species whose abundances may
change by more than a factor of two at 1 Myr include atomic carbon, small
mono-carbonic (C1) and di-carbonic (C2) hydrocarbons, CO2, CN, HCN, HNC, HNCO,
CS, H2CO, H2CS, CH2CO, and CH3OH (in either gas and/or ice). The abundances of
key species, CO, H2O, and N2 as well as O, HCO+, N2H+, NH3, NO, and most of the
S-bearing molecules, remain almost unaffected. Further accurate laboratory
measurements and quantum chemical calculations of the surface reaction barriers
will be crucial to improve the accuracy of astrochemical models.
