A disinfectant in the galactic center region

An international group of researchers led by Arnaud Belloche (MPIfR, Bonn, Germany) reports the first identification of isopropanol in interstellar space, a substance used on Earth as a disinfectant. Isopropanol is the largest alcohol detected to date, demonstrating the increasing complexity of members of one of the most abundant classes of molecules to be found in space. The identification was made possible by observations of the star-forming region Sagittarius B2 (Sgr B2) close to the center of our galaxy, where many molecules have already been detected. It is the target of an extensive study of its chemical composition with the ALMA telescope in Chile.

The quest for molecules in space has been going on for over 50 years. To date, astronomers have identified 276 molecules in the interstellar medium. The Cologne Database for Molecular Spectroscopy (CDMS) provides spectroscopic data to detect these molecules, contributed by many research groups, and in many cases has played an important role in their detection.

The aim of this work is to understand how: organic molecules shapes in the interstellar medium, particularly in regions where new stars are born, and how complex these molecules can be. The underlying motivation is to make connections to the chemical makeup of bodies in the Solar System, such as comets, as provided by the Rosetta mission to Comet Churyumov-Gerasimenko a few years ago.

A notable star-forming region in our galaxy where many molecules have been detected in the past is Sagittarius B2 (Sgr B2), which is close to the famous source Sgr A*, the supermassive black hole at the center of our galaxy.

“Our group started investigating the chemical composition of Sgr B2 more than 15 years ago with the IRAM 30-m telescope,” said Arnaud Belloche of the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn/Germany, the lead author of the detection. paper. “These observations were successful and notably led to the first interstellar detection of several organic molecules, among many other results.”

With the advent of the Atacama Large Millimeter/submillimeter Array (ALMA) ten years ago, it became possible to go beyond what could be achieved toward Sgr B2 with a single-dish telescope and a long-term study of the chemical composition of Sgr B2 was started taking advantage of ALMA’s high angular resolution and sensitivity.

So far, the ALMA observations have led to the identification of three new organic molecules (isopropyl cyanide, N-methylformamide, urea) since 2014. The latest result within this ALMA project is now the detection of propanol (C₃huh₇OH).

Propanol is an alcohol and is now the largest in this class of molecules detected in interstellar space† This molecule exists in two forms (“isomers”), depending on which carbon atom the hydroxyl (OH) functional group is attached to: 1) normal-propanol, with OH attached to a terminal carbon atom of the chain, and 2) iso-propanol , with OH bonded to the central carbon atom in the chain. Isopropanol is also known as the most important ingredient in hand sanitizers on the planet. Both isomers of propanol in Sgr B2 were identified in the ALMA dataset. It is the first time isopropanol has been detected in the interstellar medium and the first time normal propanol has been detected in a star-forming region. The first interstellar detection of normal propanol was obtained shortly before the ALMA detection by a Spanish research team using single-dish radio telescopes in a molecular cloud not far from Sgr B2. However, the detection of isopropanol towards Sgr B2 was only possible with ALMA.

“The detection of both isomers of propanol is uniquely powerful in determining the formation mechanism of each. Because they are so similar, they behave physically in very similar ways, meaning the two molecules are in the same places at the same time,” says Rob Garrod of the University of Virginia (Charlottesville/USA). “The only open question is the exact amounts that are present – this makes their interstellar 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 determine the mechanisms by which they act.” to shape.”

The ALMA telescope network was essential for the detection of both isomers of propanol towards Sgr B2, thanks to its high sensitivity, high angular resolution and wide frequency coverage. One 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”, which is known from laboratory measurements.

“The larger the molecule, the more spectral lines at different frequencies it produces. In a source like Sgr B2, so many molecules contribute to the observed radiation that their spectra overlap and it is difficult to untangle their fingerprints and identify them individually,” , says Holger Müller of the University of Cologne, where laboratory work was mainly carried out on normal propanol.

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 those detected at larger scales with the IRAM 30-m radio telescope. The narrowness of these lines reduces spectral confusion, and this was key for the identification of both isomers of propanol in Sgr B2. The sensitivity of ALMA also played a key role: it would not have been possible to identify propanol in the collected data if the sensitivity had been only twice as high.

This research is a long-term effort to investigate the chemical makeup of sites in Sgr B2 where new stars are forming, and thereby understand the chemical processes at work during star formation. The aim is to determine the chemical composition of the star-forming sites and possibly identify new interstellar molecules. “Propanol has long been on our list of molecules to look for, but it is only thanks to the recent work done in our lab to characterize its rotational spectrum that we were able to robustly identify its two isomers,” Oliver says. Zingsheim, also from the University of Cologne.

By detecting closely related molecules that differ slightly in their structure (such as normal- and isopropanol or, as was done in the past: normal- and iso-propyl cyanide) and measuring their abundance ratio, the researchers can identify specific parts of the chemical reaction network that leads to their production in the interstellar medium†

“There are still many unidentified spectral lines in the ALMA spectrum of Sgr B2, meaning much work remains to be done to decipher its chemical composition. In the near future, extending ALMA instrumentation to lower frequencies is likely to help us reduce spectral confusion even further and potentially allow for the identification of additional organic molecules in this spectacular resource,” concludes Karl Menten, director of the MPIfR and Head of Millimeter and Submillimeter Astronomy Research Department.

The imaging spectral line study performed by ReMoCA with high angular resolution ALMA and the results of a recent spectroscopic study of propanol were used to search for the iso- and normal isomers of the propanol molecule in the nearby hot molecular core Sgr B2(N2) from the galactic center. The interferometric spectra were analyzed under the assumption of local thermodynamic equilibrium. The reaction network of the astrochemical model MAGICKAL was extended to investigate propanol formation pathways and to place the observational results in a broader astrochemical context.

The corresponding studies have been published in: Astronomy and Astrophysics†