Non-Detection of HC$_{11}$N toward TMC-1: Constraining the Chemistry of Large Carbon-Chain Molecules

TL;DR

This study reports the non-detection of HC$_{11}$N in TMC-1, challenging previous findings and suggesting new chemical pathways like cyclization reactions may influence large carbon-chain molecule abundances in space.

Contribution

It provides the first upper limit for HC$_{11}$N in TMC-1 and explores chemical models to explain its non-detection, proposing cyclization as a key process.

Findings

HC$_{11}$N not detected, upper limit below previous reports

Chemical models cannot fully explain non-detection

Cyclization reactions may deplete large carbon-chain molecules

Abstract

Bell et al. (1997) reported the first detection of the cyanopolyyne HC$_{11}$N toward the cold dark cloud TMC-1; no subsequent detections have been reported toward any source. Additional observations of cyanopolyynes and other carbon-chain molecules toward TMC-1 have shown a log-linear trend between molecule size and column density, and in an effort to further explore the underlying chemical processes driving this trend, we have analyzed GBT observations of HC$_9$N and HC$_{11}$N toward TMC-1. Although we find an HC$_9$N column density consistent with previous values, HC$_{11}$N is not detected and we derive an upper limit column density significantly below that reported in Bell et al. Using a state-of-the-art chemical model, we have investigated possible explanations of non-linearity in the column density trend. Despite updating the chemical model to better account for ion-dipole interactions, we are not able to explain the non-detection of HC$_{11}$N, and we interpret this as evidence of previously unknown carbon-chain chemistry. We propose that cyclization reactions may be responsible for the depleted HC$_{11}$N abundance, and that products of these cyclization reactions should be investigated as candidate interstellar molecules.