First Detection of HC ₅ N in a Class II Disk around TW Hya

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 HC 5 N toward the TW Hya protoplanetary disk, representing the largest cyanopolyyne identified to date in a Class II system. We derive a HC 5 N 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 HC 5 N and HC 3 N formation mechanisms and analyze the HC 3 N/HC 5 N ratio. We use a chemical model to estimate the expected abundance and emitting layer of HC 5 N in a TW Hya–like disk. Although HC 5 N emission is spatially unresolved and 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.