Discovery of Interstellar Propylene (CH_2CHCH_3): Missing Links in Interstellar Gas-Phase Chemistry

TL;DR

The paper reports the first detection of interstellar propylene in a dark cloud, revealing gaps in current gas-phase chemical models and suggesting the need for revised understanding of interstellar hydrocarbon chemistry.

Contribution

This is the first identification of propylene in space, highlighting the presence of saturated hydrocarbons and challenging existing chemical models of dark clouds.

Findings

Propylene detected with a column density of 4×10^13 cm^-2.

Propylene abundance is comparable to other hydrocarbons like c-C_3H_2.

Current models lack formation pathways for propylene in gas phase.

Abstract

We report the discovery of propylene (also called propene, CH_2CHCH_3) with the IRAM 30-m radio telescope toward the dark cloud TMC-1. Propylene is the most saturated hydrocarbon ever detected in space through radio astronomical techniques. In spite of its weak dipole moment, 6 doublets (A and E species) plus another line from the A species have been observed with main beam temperatures above 20 mK. The derived total column density of propylene is 4 10^13 cm^-2, which corresponds to an abundance relative to H_2 of 4 10^-9, i.e., comparable to that of other well known and abundant hydrocarbons in this cloud, such as c-C_3H_2. Although this isomer of C_3H_6 could play an important role in interstellar chemistry, it has been ignored by previous chemical models of dark clouds as there seems to be no obvious formation pathway in gas phase. The discovery of this species in a dark cloud indicates that a thorough analysis of the completeness of gas phase chemistry has to be done.