Polycyclic Aromatic Hydrocarbons Persistence in Space
Polycyclic aromatic hydrocarbons (PAHs) are flat molecules made of carbon and hydrogen. They are abundant in space and may have played a key role in the origin of life on Earth. Recent research has solved a long-standing puzzle about how certain PAHs survive harsh space conditions, particularly in the Taurus Molecular Cloud 1 (TMC1), a region 430 light-years away.
PAHs and Their Cosmic Significance
PAHs form about 20% of carbon in interstellar space. Their flat ring-like structure makes them stable and able to endure space radiation. Scientists believe meteors carrying PAHs helped seed early Earth with organic molecules.
The Mystery of PAHs in Taurus Molecular Cloud 1
TMC1 is rich in gas, dust, and plasma. It contains many small, closed-shell PAHs, molecules with paired electrons, which should break down under intense starlight. Yet, these PAHs persist in larger numbers than predicted. This contradicts earlier theories that expected rapid disintegration from radiation exposure.
Research on Indenyl Cation (C9H7+) and Cooling Mechanism
A team from Australia, Sweden, and the UK studied the indenyl cation, a charged fragment of indene (C9H8). They discovered these ions cool down quickly, preventing their destruction. This cooling occurs through a process called recurrent fluorescence, where the molecule emits light by electron transitions, releasing energy step-by-step.
Experimental Setup and Observations
Scientists used DESIREE, a facility in Stockholm with ion-storage rings cooled below –260ºC. Ions injected into the ring travel without collisions for minutes. By measuring neutral fragments from ion breakups, researchers tracked the rate of ion dissociation. The indenyl cation showed a faster cooling rate than other PAH ions, confirming an efficient energy loss mechanism.
Modelling Energy Loss in PAHs
The team created a model balancing three energy loss pathways – dissociation (bond breaking), infrared emission (vibrational energy loss), and recurrent fluorescence (light emission). Simulations including recurrent fluorescence matched experimental data closely. This confirmed that small PAHs can cool efficiently and avoid decomposition in space.
Implications for Astrochemistry and Planet Formation
This discovery refines models of PAH growth from small fragments to larger molecules. It explains how these molecules accumulate in clouds like TMC1. Such PAHs could contribute prebiotic carbon to forming planets. Recent radioastronomy has also detected many small PAHs, denoting their importance in cosmic chemistry.
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