摘要：The CO+ reactive
ion is thought to be a tracer of the boundary between a Hii region and the hot
molecular gas. In this study, we present the spatial distribution of the CO+ rotational emission toward the
Mon R2 star-forming region. The CO+ emission presents a clumpy ring-like morphology, arising
from a narrow dense layer around the Hii region. We compared the
CO+ distribution
with other species present in photon-dominated regions (PDR), such as [Cii] 158
μm,
H2 S(3)
rotational line at 9.3 μm, polycyclic aromatic hydrocarbons (PAHs), and
HCO+. We find
that the CO+
emission is spatially coincident with the PAHs and [Cii] emission. This confirms
that the CO+
emission arises from a narrow dense layer of the Hi/H2 interface. We determined the
CO+ fractional
abundance relative to C+ toward three positions. The abundances range from 0.1 to
1.9 × 10-10 and
are in good agreement with the previous chemical model, which predicts that the production
of CO+ in PDRs only
occurs in dense regions with high UV fields. The CO+ linewidth is larger than those
found in molecular gas tracers, and their central velocity are blueshifted with respect to
the molecular gas velocity. We interpret this as a hint that the CO+ is probing photoevaporating
clump surfaces.
关键词：astrochemistry;photon-dominated region;Hii regions;radio lines: ISM
