Excitation and abundance study of CO+ in the interstellar medium
Abstract
Observations of CO+ suggest column densities on the order 1012 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+)=1012 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 >106 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 Nup <= 3. Significantly different line fluxes are calculated with an anomalous excitation mechanism, however, for transitions with higher upper levels and densities >106 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 106 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.