[Paper Review] The IRAM-30m line survey of the Horsehead PDR: I. CF+ as a tracer of C+ and a measure of the Fluorine abundance

Viviana V. Guzmán, J. Pety+5 more|arXiv (Cornell University)|Jun 2, 2012

Astrophysics and Star Formation Studies23 references22 citations

TL;DR

This paper proposes CF⁺ as a ground-based proxy for C⁺ in photodissociation regions (PDRs), leveraging its detectability and direct proportionality to fluorine abundance. Using high-sensitivity IRAM-30m observations of the Horsehead PDR, the authors derive a gas-phase fluorine abundance of F/H = (0.6–1.5) × 10⁻⁸, confirming CF⁺ as a robust tracer of C⁺-dominated layers and validating its use for high-resolution ground-based studies of interstellar C⁺ emission.

ABSTRACT

C+ is a key species in the interstellar medium but its 158 μm fine structure line cannot be observed from ground-based telescopes. Current models of fluorine chemistry predict that CF+ is the second most important fluorine reservoir, in regions where C+ is abundant. We detected the J = 1-0 and J = 2-1 rotational lines of CF+ with high signal-to-noise ratio towards the PDR and dense core positions in the Horsehead. Using a rotational diagram analysis, we derive a column density of N(CF+) = (1.5 - 2.0) imes 10^12 cm^-2. Because of the simple fluorine chemistry, the CF+ column density is proportional to the fluorine abundance. We thus infer the fluorine gas-phase abundance to be F/H = (0.6 - 1.5) imes 10^-8. Photochemical models indicate that CF+ is found in the layers where C+ is abundant. The emission arises in the UV illuminated skin of the nebula, tracing the outermost cloud layers. Indeed, CF+ and C+ are the only species observed to date in the Horsehead with a double peaked line profile caused by kinematics. We therefore propose that CF+, which is detectable from the ground, can be used as a proxy of the C+ layers.

Motivation & Objective

• To identify a ground-observable tracer for C⁺, which is undetectable from Earth due to atmospheric opacity.
• To measure the gas-phase fluorine abundance in the Horsehead PDR using CF⁺ as a direct abundance proxy.
• To test the hypothesis that CF⁺ traces the same UV-illuminated, C⁺-rich layers as C⁺ itself.
• To validate CF⁺ as a kinematically resolved tracer of the outermost, ionized layers of interstellar clouds.

Proposed method

• Conducted deep, high-spectral-resolution observations of the J=1-0 and J=2-1 rotational transitions of CF⁺ using the IRAM-30m telescope.
• Performed rotational diagram analysis on the detected lines to derive the column density of CF⁺.
• Used a pure gas-phase chemical network with grain surface processes to model fluorine chemistry in the Horsehead PDR.
• Compared model-predicted CF⁺, HF, and C⁺ abundance profiles with observed data to validate the chemical model.
• Assessed the spatial and kinematic structure of CF⁺ emission to link it to the far-UV-illuminated PDR layers.
• Used the observed CF⁺ column density to infer the fluorine elemental abundance via the assumption that CF⁺ is the second most important fluorine reservoir.

Experimental results

Research questions

• RQ1 Can CF⁺ serve as a reliable ground-based proxy for C⁺ in interstellar PDRs?
• RQ2 What is the gas-phase fluorine abundance in the Horsehead PDR, as derived from CF⁺ observations?
• RQ3 Where in the cloud structure (in terms of visual extinction and radiation field) is CF⁺ predominantly formed and detected?
• RQ4 How do the kinematic profiles of CF⁺ and C⁺ compare, and what does this imply about their spatial co-location?
• RQ5 To what extent does the observed CF⁺ emission trace the same UV-illuminated, ionized layers as C⁺?

Key findings

• The J=1-0 and J=2-1 lines of CF⁺ were detected with high signal-to-noise ratio toward both the PDR and dense core positions in the Horsehead nebula.
• The derived column density of CF⁺ is N(CF⁺) = (1.5–2.0) × 10¹² cm⁻², based on rotational diagram analysis.
• The gas-phase fluorine abundance is inferred to be F/H = (0.6–1.5) × 10⁻⁸, with CF⁺ accounting for 4–8% of total fluorine.
• The model predicts a strong spatial and chemical overlap between CF⁺ and C⁺ in the outermost, UV-illuminated layers (A_V < 4) of the cloud.
• The double-peaked line profile of CF⁺ matches that of C⁺, indicating both trace kinematically distinct gas in the PDR, likely due to outflow or shock-like motions.
• CF⁺ emission is primarily located at the illuminated edge of the nebula, confirming its role as a tracer of the C⁺-dominated, far-UV-irradiated surface layers.