DETECTION OF CO~+ TOWARD THE REFLECTION NEBULA NGC 7023

摘要

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 3 X 10~(11) cm~(-2) has been derived toward the PDR peak. However, we have 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 3 X 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 ≤ 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.