A disinfectant in the realm of the galactic center

An international research group led by Arnaud Belloche (MPIfR Bonn) has for the first time detected the molecule isopropanol in a substance used on earth as a disinfectant. Iso-propanol is the largest alcohol molecule discovered in space to date and demonstrates the increasing complexity of members of this class of molecules so common in space. The discovery was made possible by observing the star-forming region Sagittarius B2 (Sgr B2) near the center of our Milky Way, in which numerous molecules have already been detected. Sgr B2 is the target of an extensive chemical composition study with the ALMA telescope in Chile.

The search for molecules in space has been going on for more than 50 years. To date, astronomers have been able to identify 276 molecules in the interstellar medium. The Cologne Molecular Spectroscopy Database (CDMS) presents spectroscopic data for the detection of these molecules, contributed by many research groups, and in many cases has contributed to the identification of these molecules.

The aim of the present work is to understand how organic molecules form in the interstellar medium, especially in regions where new stars are born, and how complex these molecules can be. One motivation behind this is to make connections to the chemical composition of bodies in the solar system such as comets, such as that provided by the Rosetta mission to comet Churyumov-Gerasimenko a few years ago.

A particularly suitable star-forming region in our galaxy, in which many molecules have already been detected in the past, is Sagittarius B2 (Sgr B2), very close to Sgr A*, the supermassive black hole at the center of the Milky Way.

“Our group started investigating the chemical composition of Sgr B2 with the 30 m IRAM radio telescope more than 15 years ago,” says Arnaud Belloche from the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, the first author of the Publication on the discovery of iso-propanol. “These observations were successful and in particular led to the first interstellar detection of a number of organic molecules, among many other results.”

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With the Atacama Large Millimeter/submillimeter Array (ALMA), which began operations ten years ago, it became possible to go beyond what could be achieved with single radio telescopes towards Sgr B2 and became a long-term chemical composition study started by Sgr B2 with high angular resolution, which also takes advantage of ALMA’s high sensitivity.

The ALMA observations since 2014 have led to the identification of three new organic molecules (isopropyl cyanide, N-methylformamide, urea). The latest result from this ALMA project is the detection of propanol (C3H7OH).

Propanol is the largest alcohol molecule discovered in interstellar space. This molecule exists in two forms (“isomers”), depending on which carbon atom the hydroxyl (OH) functional group is attached to: 1) regular propanol, in which the OH is attached to a terminal carbon atom of the chain, and 2) iso- Propanol where the OH is attached to the central carbon atom of the chain. Both isomers of propanol in Sgr B2 could be identified in the ALMA data set. It is the first time iso-propanol has been detected in the interstellar medium and normal propanol in a star-forming region. The first interstellar detection of normal propanol was made shortly before the ALMA detection by a Spanish research group with single telescopes in a molecular cloud not far from Sgr B2. However, the detection of isopropanol in the direction of Sgr B2 was only possible with ALMA.

“The detection of both propanol isomers is of unique significance when it comes to determining the formation mechanism of the two isomers. Because they are so similar, they also behave very similar physically, which means that the two molecules should be present in the same places and at the same times,” says Rob Garrod of the University of Virginia (Charlottesville/USA). “The the only open question is the exact amounts that are present – this makes their interstellar abundance ratio much more precise than would be the case for other pairs of molecules.It also means that the chemical network can be tuned much more precisely to reveal the mechanisms for to determine their origin.”

The ALMA telescope network was crucial in detecting both isomers of propanol towards Sgr B2 thanks to its high sensitivity, high angular resolution and wide frequency coverage. A difficulty in identifying organic molecules in the spectra of star-forming regions is spectral confusion. Each molecule emits radiation at specific frequencies, its spectral “fingerprint” known from laboratory measurements.

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“The larger the molecule, the more spectral lines at different frequencies it will emit. In a source like Sgr B2, there are so many molecules contributing to the observed radiation that their spectra overlap and it’s difficult to unravel their fingerprints and to identify them individually,” says Holger Müller from the University of Cologne, where laboratory measurements were carried out on normal propanol in particular.

Thanks to ALMA’s high angular resolution, it was possible to isolate parts of Sgr B2 that emit very narrow spectral lines – five times narrower than the lines detected on larger scales with the 30 m IRAM radio telescope! The narrowness of these lines reduces the confusion in the spectrum, and this was also key to identifying the two isomers of propanol in Sgr B2. The sensitivity of ALMA also played an important role: if the sensitivity was twice as bad, it would not have been possible to identify propanol in the data collected.

This research is a long-term project to study the chemical composition of different star-forming regions of Sgr B2 in order to understand the chemical processes involved in star formation. The goal is to determine the chemical composition of the star-forming regions and possibly identify new interstellar molecules. “Propanol has long been on our list of molecules to be tracked down, but only recent comparative work in the laboratory to characterize the spectrum of rotational transitions has enabled us to clearly identify the two isomers of propanol,” says Oliver Zingsheim, also from the University of Cologne.

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Detecting closely related molecules that differ slightly in structure (such as normal and isopropanol, or, as has been the case in the past, normal and isopropyl cyanide) and measuring their abundance ratio allows researchers to identify specific parts of the chemical to investigate the reaction network that leads to the production of molecules in the interstellar medium.

“There are still many unidentified lines in the ALMA spectrum of Sgr B2 and there is still a lot of work to unravel the chemical composition of this important source. In the near future, extending the ALMA instrumentation to lower frequencies will likely help us to further reduce the confusion in the spectrum and potentially identify more organic molecules in Sgr B2,” concludes Karl Menten, Director at the MPIfR and head of the Millimeter research department – and submillimeter astronomy.