[Paper Review] Probing non polar interstellar molecules through their protonated form: Detection of protonated cyanogen (NCCNH+)

M. Agúndez, J. Cernicharo+20 more|arXiv (Cornell University)|Jun 23, 2015

Astrophysics and Star Formation Studies44 references88 citations

TL;DR

This study presents the first detection of protonated cyanogen (NCCNH+) in interstellar space using radio telescopes, identifying its J = 5−4 and J = 10−9 rotational transitions toward the cold dark clouds TMC-1 and L483. The detection confirms the presence of non-polar cyanogen (NCCN), which lacks a permanent dipole moment and is undetectable via standard radio astronomy, by probing its protonated form, with inferred NCCN abundances reaching (1–10) × 10⁻⁸ relative to H₂.

ABSTRACT

Cyanogen (NCCN) is the simplest member of the series of dicyanopolyynes. It has been hypothesized that this family of molecules can be important constituents of interstellar and circumstellar media, although the lack of a permanent electric dipole moment prevents its detection through radioastronomical techniques. Here we present the first solid evidence of the presence of cyanogen in interstellar clouds through the detection of its protonated form toward the cold dark clouds TMC-1 and L483. Protonated cyanogen (NCCNH+) has been identified through the J=5-4 and J=10-9 rotational transitions using the 40m radiotelescope of Yebes and the IRAM 30m telescope. We derive beam averaged column densities for NCCNH+ of (8.6+/-4.4)e10 cm-2 in TMC-1 and (3.9+/-1.8)e10 cm-2 in L483, which translate to fairly low fractional abundances relative to H2, in the range (1-10)e-12. The chemistry of protonated molecules in dark clouds is discussed, and it is found that, in general terms, the abundance ratio between the protonated and non protonated forms of a molecule increases with increasing proton affinity. Our chemical model predicts an abundance ratio NCCNH+/NCCN of 1e-4, which implies that the abundance of cyanogen in dark clouds could be as high as (1-10)e-8 relative to H2, i.e., comparable to that of other abundant nitriles such as HCN, HNC, and HC3N.

Motivation & Objective

• To detect non-polar interstellar molecules like cyanogen (NCCN), which lack a permanent dipole moment and are undetectable by conventional radio astronomy.
• To test the hypothesis that protonated forms of such molecules can serve as indirect tracers in cold interstellar environments.
• To constrain the abundance of cyanogen in dark clouds using observed NCCNH+ emission and chemical modeling.
• To evaluate the role of proton affinity and ion-molecule reaction networks in governing the chemistry of protonated species in cold, dense clouds.

Proposed method

• Observations of the J = 5−4 (44.4 GHz) and J = 10−9 (88.8 GHz) rotational transitions of NCCNH+ were conducted using the 40 m Yebes radiotelescope and the IRAM 30 m telescope.
• Spectral line surveys were carried out in frequency-switching mode with high spectral resolution (50 kHz and 6 kHz) to detect weak emission lines.
• Antenna temperature was calibrated using sky and hot load measurements, with opacity corrections based on atmospheric models and weather data.
• Beam-averaged column densities were derived from integrated line intensities, accounting for beam efficiency and main beam brightness temperature conversion.
• Chemical modeling was performed using the UMIST RATE12 network to simulate time-dependent abundances of NCCNH+ and NCCN under dark cloud conditions.
• The abundance ratio [NCCNH+]/[NCCN] was calculated and compared with observational data to infer the true NCCN abundance.

Experimental results

Research questions

• RQ1 Can the protonated form of non-polar interstellar molecules like cyanogen (NCCN) be detected in cold dark clouds?
• RQ2 What is the inferred abundance of cyanogen (NCCN) in interstellar environments, given the detection of its protonated form NCCNH+?
• RQ3 How does the proton affinity of a molecule influence the abundance ratio [MH+]/[M] in cold, dense interstellar clouds?
• RQ4 To what extent do current chemical models accurately predict the formation and destruction pathways of NCCNH+?
• RQ5 Are there missing formation routes in the chemical network that could explain discrepancies between observed and modeled NCCNH+ abundances?

Key findings

• The J = 5−4 and J = 10−9 rotational transitions of NCCNH+ were detected toward TMC-1 and L483, with beam-averaged column densities of (8.6 ± 4.4) × 10¹⁰ cm⁻² and (3.9 ± 1.8) × 10¹⁰ cm⁻², respectively.
• The observed NCCNH+ abundances correspond to fractional abundances relative to H₂ in the range (1–10) × 10⁻¹².
• Chemical modeling predicts an [NCCNH+]/[NCCN] abundance ratio of approximately 10⁻⁴, implying that NCCN could be as abundant as (1–10) × 10⁻⁸ relative to H₂.
• The abundance ratio [MH+]/[M] increases with increasing proton affinity of the neutral molecule M, indicating a general trend in ion-molecule chemistry in dark clouds.
• The model underestimates the observed NCCNH+ abundance, suggesting possible missing formation pathways or inaccuracies in the rate constants for reactions involving NCCN.
• The study identifies HC₂S⁺ as a promising candidate for future detection due to its high proton affinity and measurable dipole moment.