First Detection Of Heavy Methanol In Space

Methanol containing heavy hydrogen is considered the building block of complex organic molecules and allows us to trace how stars and planets form from extremely cold interstellar gas and dust

To the point

• In a study led by Arnaud Belloche from the Max Planck Institute for Radio Astronomy (MPIfR), researchers succeeded in detecting a rare form of methanol in space for the first time.
• As part of the ALMA COMPASS program, the molecule was detected around the protostar IRAS4A2 in the Perseus molecular cloud.
• The observed abundance of the molecule cannot yet be reproduced by chemical models.

Methanol (CH₃OH) – the simplest organic alcohol – plays a central role in the formation of complex organic compounds in interstellar space. In a study led by Arnaud Belloche of the Max Planck Institute for Radio Astronomy, researchers have detected fully deuterated methanol (CD₃OD) in space for the first time, in which all hydrogen atoms have been replaced by heavy hydrogen (deuterium). This allows key predictions of chemical models to be tested. The results are published in the current issue of Astronomy & Astrophysics.

A key molecule

Complex molecules such as isopropanol are already present in very young stellar systems. Experiments suggest that they form on the surface of icy dust grains long before the star ignites. Methanol plays a key role in this process: on the grains, it can slowly react with atoms such as carbon, oxygen and hydrogen to form increasingly larger compounds. As soon as the star begins to shine and warms its surroundings, the complex organic molecules are released as gas. From this point onwards, their signatures can be observed using telescopes such as the Atacama Large Millimetre/submillimetre Array (ALMA), an array of radio telescopes in the Atacama Desert in Chile.

To understand which chemical pathways dominate the formation of complex molecules, a complete inventory of the chemical compounds around young stars is essential. Deuterated molecules are of particular interest because they form only under extremely cold conditions, typical of the early stages of star formation. At the centre is methanol: from the common form with regular hydrogen, through the rarer singly- and multiply-deuterated forms, to the fully deuterated form. Until now, there has been a gap in observations in this area.

A double first

As part of the ALMA program COMPASS (Complex Organic Molecules in Protostars with ALMA Spectral Surveys), the researchers observed eleven young stellar systems. Among them is the protostar IRAS4A2, located some 1,000 light-years away in the Perseus molecular cloud. Here, the team has now detected the unambiguous signature of fully deuterated methanol (CD₃OD) for the first time. This is also the first ever detection of a quadruply deuterated interstellar molecule.

Silvia Spezzano, research group leader at the Max Planck Institute for Extraterrestrial Physics (MPE) and co-author of the current study, explains: “Deuteration enables us to trace the chemical evolution of molecules during the formation of stars and planets. This helps us understand their chemistry and, ultimately, how molecular complexity is transferred from interstellar clouds to planetary systems.”

A challenge for chemical models

The team was not only able to detect CD₃OD around IRAS4A2, but also to determine the abundances of the various methanol variants. It turns out that the fully deuterated form is about a hundred times more abundant than theoretically predicted. None of the available chemical models is currently able to explain this high concentration of CD₃OD. “A key process driving the formation of multiply deuterated methanol therefore seems to be missing from all of them”, explains Arnaud Belloche.

There is even more to be found in the IRAS4A2 data: there are unidentified spectral lines. Given the detection of CD₃OD, the astronomers expect that some of these lines might originate from the multiply deuterated forms of methanol, CH₂DOD and CHD₂OD. Work is already underway on the relevant laboratory data, which is still needed for definitive confirmation. “At the Center for Astrochemical Studies at MPE, we are conducting a systematic laboratory program to study the spectroscopy of deuterated complex organic molecules, including CH₂DOD and CHD₂OD”, reports Silvia Spezzano. Arnaud Belloche adds: “This highlights the important interplay between observation, experimentation and modeling.” Alongside the current study, six others are being published that were produced as part of the COMPASS project. Together, they shed light on the diversity of complex organic molecules in space.