  Observations of CO+ suggest column densities on the order 10^12 cm^-2 that
can not be reproduced by many chemical models. CO+ is more likely to be
destroyed than excited in collisions with hydrogen. An anomalous excitation
mechanism may thus have to be considered when interpreting CO^+ observations.
Chemical models are used to perform a parameter study of CO^+ abundances. Line
fluxes are calculated for N(CO+)=10^12 cm^-2 and different gas densities and
temperatures using a non-LTE escape probability method. The chemical formation
and destruction rates are considered explicitly in the detailed balance
equations of the radiative transfer. In addition, the rotational levels of CO+
are assumed to be excited upon chemical formation according to a formation
temperature. It is found, that chemical models are generally able to produce
high fractional CO+ abundances (x(CO+) =10^-10). In a far-ultraviolet (FUV)
dominated environment, however, high abundances of CO+ are only produced in
regions with a Habing field G0 > 100 and T(kin) > 600 K, posing a strong
constraint on the gas temperature. For gas densities >10^6 cm^-3 and
temperatures > 600 K, the combination of chemical and radiative transfer
analysis shows little effect on intensities of CO+ lines with upper levels N_up
<= 3. Significantly different line fluxes are calculated with an anomalous
excitation mechanism, however, for transitions with higher upper levels and
densities >10^6 cm ^ -3. The Herschel Space Observatory is able to reveal such
effects in the terahertz wavelength regime. Ideal objects to observe are
protoplanetary disks with densities 10^6 cm^-3. It is finally suggested that
the CO+ chemistry may be well understood and that the abundances observed so
far can be explained with a high enough gas temperature and a proper geometry.
