Excitation and abundance study of CO⁺ in the interstellar medium

Institute for Astronomy, ETH Zurich, 8093 Zurich, Switzerland e-mail: pascal.staeuber@phys.ethz.ch

Received: 23 April 2009
Accepted: 22 July 2009

Abstract

Context. 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. Other uncertainties in models are the chemical network, the gas temperature or the geometry of the emitting source. Similar is true for other reactive ions that will be observed soon with the Herschel Space Observatory.

Aims. Chemical constraints are explored for observable CO⁺ abundances. The influence of an anomalous excitation mechanism on CO⁺ line intensities is investigated. Model results are compared to observations.

Methods. 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. Collisional excitation by atomic and molecular hydrogen as well as by electrons is studied for conditions appropriate to dense photon-dominated regions (PDRs) and star-forming environments.

Results. 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 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 ≤ 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.