We have detected CO ^ { + } toward the photon-dominated region ( PDR ) associated with the reflection nebula NGC 7023 .
This is the first detection of CO ^ { + } in the vicinity of a Be star .
A CO ^ { + } column density of \sim 3 10 ^ { 11 } cm ^ { -2 } has been derived toward the PDR peak .
We have , however , not detected CO ^ { + } in a well shielded clump of the adjacent molecular cloud , where the CO ^ { + } /HCO ^ { + } abundance ratio is at least 100 times lower than in the PDR .
Our results show , for the first time , that CO ^ { + } column densities as large as \sim 3 10 ^ { 11 } cm ^ { -2 } can be produced in regions with incident UV fields of just a few 10 ^ { 3 } ( in units of Habing field ) and densities of \leq 10 ^ { 5 } cm ^ { -3 } .
Furthermore , since the ionization potential of CO is larger than 13.6 eV , our data rule out the direct photoionization of CO as a significant CO ^ { + } formation mechanism .