[Paper Review] The first CO+ image: Probing the HI/H2 layer around the ultracompact HII region Mon R2

S. P. Treviño-Morales, A. Fuente+13 more|arXiv (Cornell University)|Jul 21, 2016

Astrophysics and Star Formation Studies28 references5 citations

TL;DR

This study presents the first spatially resolved CO+ (J=2–1) image toward the Mon R2 ultracompact HII region, revealing a clumpy, ring-like emission morphology tracing the H I/H2 interface in a dense, UV-irradiated photodissociation region (PDR). The CO+ emission is spatially coincident with [C II] and PAHs, confirming its origin in a narrow, dense layer at the PDR boundary, with fractional abundances of 0.1–1.9×10⁻¹⁰, consistent with chemical models predicting CO+ formation under high UV flux and high density conditions.

ABSTRACT

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 compare the CO+ distribution with other species present in photon-dominated regions (PDR), such as [CII] 158 mm, H2 S(3) rotational line at 9.3 mm, 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 have determined the CO+ fractional abundance, relative to C+ toward three positions. The abundances range from 0.1 to 1.9x10^(-10) and are in good agreement with 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 blue-shifted with respect to the molecular gas velocity. We interpret this as a hint that the CO+ is probing photo-evaporating clump surfaces.

Motivation & Objective

• To map the spatial distribution of CO+ emission in the Mon R2 star-forming region, a key site for studying the H I/H2 interface in a dense, UV-irradiated PDR.
• To determine whether CO+ traces the boundary between the ultracompact HII region and molecular gas, as predicted by chemical models.
• To investigate the physical conditions (density, UV field) required for CO+ production by comparing abundances with theoretical predictions.
• To explore the kinematic properties of CO+ and compare them with molecular gas tracers to infer dynamical processes such as photo-evaporation.

Proposed method

• Observations of the CO+ (J=2–1) transition at 236.0625 GHz were conducted using the IRAM-30m telescope with a 11″ beam, achieving a signal-to-noise ratio sufficient for imaging.
• Data were reduced using the CLASS/GILDAS package, with subsequent smoothing to 16″ resolution to improve signal-to-noise and enable spatial comparison with other tracers.
• The CO+ emission map was spatially compared with multi-wavelength data: [Ne II] (H II region), H2 S(3) (PDR), PAHs (11.3 μm), [C II] (158 μm), and H13CO+ (molecular gas) from Spitzer, Herschel, and previous studies.
• CO+ fractional abundance was derived at three positions (IF and MP2) using the ratio of CO+ brightness temperature to C+ emission, assuming local thermodynamic equilibrium.
• Kinematic analysis compared CO+ line profiles with those of HCO+ and H13CO+, focusing on velocity shifts and linewidths to infer outflow or evaporation signatures.
• Chemical model predictions were used to interpret CO+ abundances, particularly the dependence on H density (nH) and UV field (G0), to validate observed abundances.

Experimental results

Research questions

• RQ1 Does CO+ emission trace the H I/H2 interface in the Mon R2 PDR, as predicted by chemical models?
• RQ2 How does the spatial distribution of CO+ compare with other PDR tracers such as [C II], PAHs, and H2 lines?
• RQ3 What are the CO+ fractional abundances in different regions of Mon R2, and do they match theoretical predictions based on UV field and density?
• RQ4 Are the CO+ line profiles blue-shifted relative to molecular gas tracers, and what does this imply about gas kinematics?
• RQ5 Is the CO+ emission consistent with photo-evaporation of dense clumps at the PDR surface?

Key findings

• The CO+ emission exhibits a clumpy, ring-like morphology centered on the ultracompact HII region, spatially coinciding with [C II] and PAH emission, confirming its origin in a narrow, dense layer at the H I/H2 interface.
• The CO+ fractional abundance ranges from 0.1 to 1.9×10⁻¹⁰ at three positions, in good agreement with chemical models that predict CO+ formation only in high-density (nH ≥ 2×10⁴ cm⁻³) and high-UV (G₀ ≥ 10³) regions.
• CO+ linewidths are broader than those of standard molecular gas tracers like HCO+ and H13CO+, suggesting enhanced turbulence or outflowing motions.
• The central velocities of CO+ are systematically blue-shifted relative to the molecular gas, indicating that CO+ traces gas being ejected toward the observer, consistent with photo-evaporation of dense clumps.
• CO+ was not detected toward MP2 (upper limit <4×10⁻¹¹), while it is detected toward IF (abundance ~10⁻¹¹), supporting the requirement of high UV flux and density for CO+ production.
• The spatial morphology and kinematic offset suggest that the PDR is not a smooth layer but composed of fragmented, dense clumps undergoing photo-evaporation, with CO+ tracing the illuminated, evaporating surfaces.