Limits on chemical complexity in diffuse clouds: search for CH3OH and HC5N absorption

TL;DR

This study investigates the chemical complexity of diffuse clouds by searching for specific molecules, CH3OH and HC5N, using radio telescopes, and finds their abundances are lower than expected, indicating limits to chemical complexity in such environments.

Contribution

The paper provides the first sensitive searches for CH3OH and HC5N in diffuse clouds, establishing upper limits on their abundances and highlighting constraints on chemical complexity.

Findings

Neither CH3OH nor HC5N detected, with column densities below known levels.

HCN/HC5N ratio is significantly higher in diffuse clouds than in TMC-1.

Chemical complexity in diffuse clouds is more limited than previously thought.

Abstract

Context: An unexpectedly complex polyatomic chemistry exists in diffuse clouds, allowing detection of species such as C2H, C3H2, H2CO and NH3 which have relative abundances that are strikingly similar to those inferred toward the dark cloud TMC-1 Aims: We probe the limits of complexity of diffuse cloud polyatomic chemistry. Methods: We used the IRAM Plateau de Bure Interferometer to search for galactic absorption from low-lying J=2-1 rotational transitions of A- and E-CH3OH near 96.740 GHz and used the VLA to search for the J=8-7 transition of HC5N at 21.3 GHz. Results: Neither CH3OH nor HC5N were detected at column densities well below those of all polyatomics known in diffuse clouds and somewhat below the levels expected from comparison with TMC-1. The HCN/HC5N ratio is at least 3-10 times higher in diffuse gas than toward TMC-1. Conclusions: It is possible to go to the well once (or more) too often