Discovery of the Seven-ring Polycyclic Aromatic Hydrocarbon Cyanocoronene (C₂₄H₁₁CN) in GOTHAM Observations of TMC-1

This paper asks whether a specific, large polycyclic aromatic hydrocarbon (PAH) derivative—cyanocoronene, formula ring PAH cyanocoronene ring PAH cyanocoronene ring PAH cyanocoronene (C${}_{24}$H${}_{11}$CN)—can be identified in the interstellar medium (ISM) via its rotational spectrum. The question matters because PAHs are widely implicated in key astrophysical processes (carbon sequestration, gas heating, catalytic chemistry, and the aromatic infrared band/AIB phenomenon), yet individual PAH carriers have historically been difficult to confirm. Radio spectroscopy provides a direct route: if a molecule has a permanent dipole moment, its rotational transitions can be measured in the laboratory and then searched for in cold molecular clouds.

The authors place their work in the context of the GOTHAM program, which has already discovered several PAH-related nitriles in the cold dark cloud TMC-1 (e.g., cyanonaphthalenes, cyanoacenaphthylenes, and cyanopyrenes). A striking prior result motivating this study is that, within the cyano-substituted PAH family, the derived column densities in TMC-1 are roughly similar (within about a factor of 2) across a range of sizes, rather than decreasing strongly with increasing molecular size as is often expected for carbon-chain chemistry. Cyanocoronene is the next step: coronene (C${}_{24}$H${}_{12}$) is a “prototypical” compact PAH, but coronene itself has no permanent dipole moment and therefore lacks a pure rotational spectrum. Attaching a CN group creates a dipole and enables rotational detection.

Methodologically, the paper combines (i) chemical synthesis and laboratory rotational spectroscopy to obtain an accurate line catalog, and (ii) targeted radioastronomical searches in GOTHAM observations of TMC-1 using the Green Bank Telescope (GBT). For the laboratory component, cyanocoronene is synthesized from coronene via formylcoronene and formylcoronene-oxime, ultimately yielding cyanocoronene as a yellow solid with an approximate yield of 59% (reported in the synthesis section). For spectroscopy, the authors use a laser-ablation assisted cavity-enhanced Fourier transform microwave (FTMW) spectrometer (Balle-Flygare type). The sample is compressed into a rod, ablated with a 532 nm Nd:YAG laser, and entrained in neon for a supersonic expansion that produces rotational temperatures around 2 K. They measure and assign 71 rotational transitions between 6.8 and 10.6 GHz, corresponding to 38 unique line frequencies. Spectroscopic constants are obtained by least-squares fitting in a Watson A-reduction (IIIl representation) using SPCAT/SPFIT. The fitted rotational constants (A, B, C) agree closely with theory, with a mean absolute percentage error (MAPE) of 0.39%. They also report that the asymmetry parameter is  = 20.12, indicating cyanocoronene is far from the prolate/oblate symmetric-top limits. They neglect ${}^{14}$N nuclear electric quadrupole hyperfine splitting because no splittings are resolved in the measured high-J transitions.

For the astronomical component, the authors analyze GOTHAM data covering 29 GHz between 4 and 36 GHz, with typical RMS noise between 3.8 and 15.0 mK. They emphasize that ultrasensitive X-band observations (9.38–10.96 GHz) enable individual transition detection; in this range they reach an RMS of 1.5 mK. They focus on the “cyanopolyyne peak” of TMC-1 (given by J2000 coordinates) and use a Markov Chain Monte Carlo (MCMC) framework to infer physical parameters from the observed spectra. The MCMC analysis incorporates priors informed by previous PAH detections for velocity components ($v_{\mathrm{LSR}}$), source size, and linewidth $\Delta V$, while allowing the total column density and excitation temperature $T_{\mathrm{ex}}$ to vary with uniform priors. Observational uncertainties are computed as the quadrature sum of local RMS noise and an additional 20% systematic uncertainty. The model includes four Doppler components, and the total column density is obtained by summing the component column densities.

The key astronomical results are a detection of cyanocoronene with a best-fit total column density and excitation temperature of $N({\mathrm{C}}_{24}{\mathrm{H}}_{11}\mathrm{CN})=2.69_{-0.23}^{+0.26}\times 10^{12}\mathrm{c}{\mathrm{m}}^{-2}$ and $T_{\mathrm{ex}}=6.05_{-0.37}^{+0.38}\mathrm{K}$. To quantify detection significance, they perform a velocity-stack and matched filtering analysis using the 100 brightest SNR lines of cyanocoronene. This yields a robust detection significance of 17.3$\sigma$ in the matched filter response. The paper also reports that the strongest contributing transitions fall in the C-, X-, and Ku-bands, with individual detections particularly near 10.515 GHz.

A central interpretive step is comparing cyanocoronene’s abundance to other cyano-substituted PAHs in TMC-1. The authors state that the derived column density of cyanocoronene is comparable to that of the 4-ring PAH cyanopyrene (and to cyano-substituted naphthalene and acenaphthylene), which “defies the trend” of decreasing abundance with increasing molecular size and complexity observed for carbon chains. They further estimate the column density of the parent unsubstituted coronene using a CN/H ratio proxy (assuming CN addition to aromatic double bonds is barrierless). This yields $N({\mathrm{C}}_{24}{\mathrm{H}}_{12})\approx 2\times 10^{13}\mathrm{c}{\mathrm{m}}^{-2}$. They discuss how this challenges existing cold-ISM chemical expectations, because low-temperature bottom-up gas-phase formation routes for compact medium-sized PAHs like pyrene are not known. Instead, they argue that the detected CN-derivatives correspond to members of the most thermodynamically favorable high-temperature PAH polymerization route (e.g., HACA-like ring growth), suggesting a possible inheritance from earlier high-temperature environments.

The paper acknowledges limitations implicitly through its modeling choices and through what is not yet measured. On the observational side, the detection relies on accurate laboratory spectroscopy and on the assumption that the MCMC model (four Doppler components with constrained $v_{\mathrm{LSR}}$, linewidth, and source size priors) correctly represents the emission. The authors also add a systematic 20% uncertainty to account for calibration and modeling imperfections, but they do not provide an alternative model comparison (e.g., different component counts). On the chemical-interpretation side, the coronene abundance is inferred rather than directly measured (because coronene lacks a rotational spectrum), and the CN/H proxy depends on assumptions about CN addition kinetics and the relevant CN/H formation ratio. They also note that searches for other cyaninated PAH intermediates (e.g., 9-cyanophenanthrene) were not successful, which could reflect incomplete spectroscopy coverage, isomer abundance differences, or sensitivity limits.

Practically, the results matter for astrochemistry and for the broader effort to identify specific PAH carriers. The detection establishes cyanocoronene as the largest individual PAH discovered in space via radio astronomy and provides evidence that large compact PAHs can be present in substantial quantities in cold molecular clouds. This supports the “PAH hypothesis” for aromatic emission phenomena and suggests that PAH chemistry in dense ISM environments may be governed not only by formation pathways but also by resilience to destruction (e.g., ion-neutral reaction survivability and grain-surface sputtering effects). The work also provides a concrete laboratory spectroscopic catalog and publicly accessible analysis products, enabling follow-up searches for related cyanoderivatives and other large PAHs in TMC-1 and similar sources.

Overall, the paper’s core contribution is the combined laboratory-to-astronomy identification of cyanocoronene in TMC-1, with quantified physical parameters and a statistically strong detection, thereby extending the known PAH nitrile inventory to the seven-ring coronene family and challenging prevailing abundance-size expectations for complex carbonaceous molecules in cold interstellar gas.