Astronomical Detection of the Interstellar Anion C10H− toward TMC-1 from the GOTHAM Large Program on the Green Bank Telescope

Abstract

Using data from the Green Bank Telescope (GBT) Observations of TMC-1: Hunting for Aromatic Molecules (GOTHAM) survey, we report the first astronomical detection of the C _10 H ^− anion. The astronomical observations also provided the necessary data to refine the spectroscopic parameters of C _10 H ^− . From the velocity stacked data and the matched filter response, C _10 H ^− is detected at >9 σ confidence level at a column density of ${4.04}_{-2.23}^{+10.67}\times {10}^{11}$ cm ^−2 . A dedicated search for the C _10 H radical was also conducted toward TMC-1. In this case, the stacked molecular emission of C _10 H was detected at a ∼3.2 σ confidence interval at a column density of ${2.02}_{-0.82}^{+2.68}\times {10}^{11}$ cm ^−2 . However, as the determined confidence level is currently <5 σ , we consider the identification of C _10 H as tentative. The full GOTHAM data set was also used to better characterize the physical parameters including column density, excitation temperature, line width, and source size for the C _4 H, C _6 H, and C _8 H 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 C _10 H ^− /C _10 H column density ratio is ∼ ${2.0}_{-1.6}^{+5.9}$ —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.

Keywords

• Astrochemistry
• Dark interstellar clouds