Detection of Reactive Ions Towards UC Hii Regions

Abstract

We report the first detection of the reactive ions CO⁺ and HOC⁺ towards ultracompact (UC) Hii regions, in particular Mon R2 and G29.96-0.02. Two positions in Mon R2, the peak of the UC Hii region and a position in the high density molecular cloud which bounds it, were observed. CO⁺ and HOC⁺ were detected at the UC Hii region but not in the molecular cloud, as expected if the CO⁺ and HOC⁺ emissions arise in the PDR surrounding the UC Hii region. The measured CO⁺ and HOC⁺ column densities are of order of ≈ 10¹¹ cm^–2 in both sources, which yields a strikingly low [HCO⁺]/[HOC⁺] abundance ratio of 460 in Mon R2. Our values are similar to those found in some other well-known PDRs, like NGC 7023 or the Orion Bar.