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arXiv · 2606.02815

First Detection of HC5N in a Class II Disk around TW Hya

Steven C. Wampler·Ryan A. Loomis·Amina Diop·L. Ilsedore Cleeves·Yuri Aikawa·Romane Le Gal·Viviana Guzman·Charles J. Law·Karin Öberg·Jamila Pegues·Richard Teague·Catherine Walsh
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Abstract

Over the last decade of ALMA's operation the molecular inventory of protoplanetary disks has expanded rapidly, revealing a diverse set of nitrogen-bearing organics and carbon-chain molecules that trace both prebiotic chemistry and fundamental disk physics. Despite this progress, detections of larger species such as cyanopolyynes have remained limited, leaving larger carbon-chain chemistry in Class II disks largely unconstrained. Here, we report the first detection of HC5N toward the TW Hya protoplanetary disk, representing the largest cyanopolyyne identified to date in a Class II system. We derive a HC5N column density for two rotational transitions J = 41-40 and J = 37-36, N_T ~ 10^12 cm^-2 for assumed T_rot = 20-50 K and optically thin emission in LTE. We compare HC5N and HC3N formation mechanisms and analyze the HC3N/HC5N ratio. We use a chemical model to estimate the expected abundance and emitting layer of HC5N in a TW Hya-like disk. Although HC5N emission is spatially unresolved, measured column densities suggest an origin in the warm molecular layer where CN-based pathways are active. This detection extends the known carbon-chain chemistry in Class II disks and demonstrates that long cyanopolyynes can form and persist in planet-forming environments.

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Steven C. Wampler, Ryan A. Loomis, Amina Diop, L. Ilsedore Cleeves, Yuri Aikawa, Romane Le Gal, Viviana Guzman, Charles J. Law, Karin Öberg, Jamila Pegues, Richard Teague, Catherine Walsh. 2026-06-01. First Detection of HC5N in a Class II Disk around TW Hya. https://doi.org/10.3847/2041-8213%2Fae66e5

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