Detection of reactive ions in the ultracompact H II regions Monoceros R2 and G29.96-0.02

Abstract

We report the first detection of the reactive ions CO+ and HOC+ toward ultracompact (UC) H II regions, particularly in Mon R2 and G29.96-0.02. We have observed two positions in Mon R2, namely, the peak of the UC H II region and the surrounding high-density molecular cloud. CO + and HOC+ were detected at the UC H II region but not at the molecular cloud, as expected if both ions arise in the photodissociation region (PDR) surrounding the UC H II region. The measured column densities are on the order of 1011 cm-2 in both sources, which yields a strikingly low [HCO+]/[HOC+] abundance ratio of 460 in Mon R2. These values are similar to those found in some other well-known PDRs, like NGC 7023 and the Orion bar. We briefly discuss the chemical implications of these results.