Excitation and abundance study of CO+ in the interstellar medium

TL;DR

This study investigates CO+ excitation and abundance in the interstellar medium, highlighting the importance of high temperatures and potential anomalous excitation mechanisms to explain observed column densities.

Contribution

It introduces a detailed chemical and radiative transfer model that accounts for formation excitation and explores conditions producing high CO+ abundances and line fluxes.

Findings

High CO+ abundances require G0 > 100 and T > 600 K in FUV environments.

Anomalous excitation significantly affects high-level transition fluxes at high densities.

Herschel observations can detect these excitation effects in protoplanetary disks.

Abstract

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.