Modeling the role of electron attachment rates on column density ratios
  for CnH-/CnH (n=4,6,8) in dense molecular clouds
  (abridged) The fairly recent detection of a variety of anions in the
Interstellar Molecular Clouds have underlined the importance of realistically
modeling the processes governing their abundance. To this aim, our earlier
calculations for the radiative electron attachment (REA) rates for C4H-, C6H-,
and C8H- are employed to generate the corresponding column density ratios of
anion/neutral (A/N) relative abundances. The latter are then compared with
those obtained from observational measurements. The calculations involved the
time-dependent solutions of a large network of chemical processes over an
extended time interval and included a series of runs in which the values of REA
rates were repeatedly scaled. Macroscopic parameters for the clouds' modeling
were also varied to cover a broad range of physical environments. It was found
that, within the range and quality of the processes included in the present
network,and selected from state-of-the-art astrophysical databases, the REA
values required to match the observed A/N ratios needed to be reduced by orders
of magnitude for C4H- case, while the same rates for C6H- and C8H- only needed
to be scaled by much smaller factors. The results suggest that the generally
proposed formation of interstellar anions by REA mechanism is overestimated by
current models for the C4H- case, for which is likely to be an inefficient path
to formation. This path is thus providing a rather marginal contribution to the
observed abundances of C4H-, the latter being more likely to originate from
other chemical processes in the network, as we discuss in some detail in the
present work.Possible physical reasons for the much smaller differences against
observations found instead for the values of the (A/N) ratios in two other,
longer members of the series are put forward and analyzed within the
evolutionary modeling discussed in the present work.
