Resilience of small PAHs in interstellar clouds: Efficient stabilization of cyanonaphthalene by fast radiative cooling

TL;DR

This study demonstrates that recurrent fluorescence via vibronic coupling efficiently stabilizes the cationic form of cyanonaphthalene in interstellar conditions, explaining its unexpectedly high abundance in space.

Contribution

It provides the first measurements of dissociation and radiative cooling rates for 1-CNN+ and reveals a stabilization mechanism that challenges previous assumptions about PAH destruction in space.

Findings

Recurrent fluorescence significantly stabilizes 1-CNN+ in interstellar environments.

The measured cooling rates explain the high abundance of CNN in TMC-1.

Vibronic coupling enhances electronic transition probabilities, aiding stabilization.

Abstract

After decades of speculation and searching, astronomers have recently identified specific Polycyclic Aromatic Hydrocarbons (PAHs) in space. Remarkably, the observed abundance of cyanonaphthalene (CNN, C10H7CN) in the Taurus Molecular Cloud (TMC-1) is six orders of magnitude higher than expected from astrophysical modeling. Here, we report absolute unimolecular dissociation and radiative cooling rate coefficients of the 1-CNN isomer in its cationic form. These results are based on measurements of the time-dependent neutral product emission rate and Kinetic Energy Release distributions produced from an ensemble of internally excited 1-CNN + studied in an environment similar to that in interstellar clouds. We find that Recurrent Fluorescence - radiative relaxation via thermally populated electronic excited states - efficiently stabilizes 1-CNN+ , owing to a large enhancement of the electronic transition probability by vibronic coupling. Our results help explain the anomalous abundance of CNN in TMC-1 and challenge the widely accepted picture of rapid destruction of small PAHs in space.