An international team of astronomers has made the first confirmed detection in space of an extremely rare and heavy form of methanol around the protostar IRAS 4A2, in the Perseus molecular cloud about 1,000 light years from Earth.
Heavy methanol abundance exceeds model predictions
Measurements showed that the detected molecule is about 100 times more abundant than current chemical models predict. The researchers used the Atacama Large Millimetre/submillimetre Array, ALMA, to capture the spectral fingerprint of fully deuterated methanol, known as CD3OD, around the object, which is still in the process of forming.
The significance of the finding goes beyond adding a new molecule to the list of substances detected in space. The large gap between the measurements and predictions suggests that a key step may be missing from the current understanding of how organic molecules form. Methanol is the simplest alcohol, but it plays an important role in interstellar chemistry.
Reactions involving ice-covered dust grains help form larger organic compounds, making the study of methanol’s different forms a way to understand the pathways through which chemical complexity increases in environments preceding the formation of stars and planets. Deuterium is a heavy form of hydrogen, and deuterium-rich molecules are scientifically important because they form efficiently in the extremely cold environments that precede the birth of stars.
These molecules can act as preserved chemical traces, recording what matter was exposed to before the young star began heating its surroundings. Arno Belloche, an astronomer at the Max Planck Institute for Radio Astronomy, led the study as part of the COMAPASS project, a spectral survey using ALMA to study 11 young stellar systems resembling the early stages of the Sun’s development.
First detection of a fourfold-deuterated molecule between stars
Around IRAS 4A2, the team found a clear signature of a form of methanol in which all four hydrogen atoms had been replaced by the heavy hydrogen isotope deuterium. It is also the first interstellar molecule detected after undergoing four deuterium substitutions.
Silvia Spezzano, head of a research group at the Max Planck Institute for Extraterrestrial Physics, said that deuteration makes it possible to trace the chemical evolution of molecules during the formation of stars and planets. She added that this helps explain how molecular complexity moves from interstellar clouds into planetary systems.
A chemical gap reveals an unknown process
The main surprise emerged when the observed amount of fully deuterated methanol was compared with the model results: its abundance was about 100 times the expected level, while the available chemical models were unable to reproduce this ratio. Belloche said the finding probably means that a major process responsible for forming multiply deuterated methanol is missing from current models.
The COMAPASS project’s results also revealed signs of what is known as a methanol maser — intense natural radio emissions — in more than half of the protostars included in the study.
This behaviour had previously been associated mainly with regions where massive stars are born, but its appearance around low-mass protostars suggests that extreme physical conditions may also occur during the birth of Sun-like stars. The IRAS 4A2 data contain other unidentified spectral lines that scientists suspect belong to additional forms of multiply deuterated methanol.
If their nature is confirmed, they could help reconstruct the chain of reactions that carries organic matter from cold icy clouds to the discs from which planets form, and clarify how much of the chemistry inherited by planetary systems comes from the stages preceding the birth of their stars.
