Astronomical Detection of the Interstellar Anion C10H- towards TMC-1 from the GOTHAM Large Program on the GBT

TL;DR

This study reports the first astronomical detection of the C10H- anion in TMC-1 using GBT data, refining its spectroscopic parameters and comparing observed abundances with chemical models and machine learning predictions.

Contribution

The paper presents the first detection of C10H- in space, refines its spectroscopic data, and compares its abundance with models and machine learning, highlighting gaps in current chemical understanding.

Findings

C10H- detected at >9σ confidence with a column density of 4.04e11 cm-2.

Tentative detection of C10H at ~3.2σ confidence with a column density of 2.02e11 cm-2.

C10H-/C10H ratio is approximately 2.0, the highest observed between anion and neutral species.

Abstract

Using data from the GOTHAM (GBT Observations of TMC-1: Hunting for Aromatic Molecules) survey, we report the first astronomical detection of the C10H- anion. The astronomical observations also provided the necessary data to refine the spectroscopic parameters of C10H-. From the velocity stacked data and the matched filter response, C10H- is detected at >9{\sigma} confidence level at a column density of 4.04e11 cm-2. A dedicated search for the C10H radical was also conducted towards TMC-1. In this case, the stacked molecular emission of C10H was detected at a ~3.2{\sigma} confidence interval at a column density of 2.02e11 cm-2. However, since the determined confidence level is currently <5{\sigma}, we consider the identification of C10H as tentative. The full GOTHAM dataset was also used to better characterize the physical parameters including column density, excitation temperature, linewidth, and source size for the C4H, C6H and C8H radicals and their respective anions, and the measured column densities were compared to the predictions from a gas/grain chemical formation model and from a machine learning analysis. Given the measured values, the C10H-/C10H column density ratio is ~2.0 - the highest value measured between an anion and neutral species to date. Such a high ratio is at odds with current theories for interstellar anion chemistry. For the radical species, both models can reproduce the measured abundances found from the survey; however, the machine learning analysis matches the detected anion abundances much better than the gas/grain chemical model, suggesting that the current understanding of the formation chemistry of molecular anions is still highly uncertain.