An international team led by Arnaud Belloche (MPIfR, Bonn, Germany) reports the discovery of iso-propanol in interstellar space, a substance used as a sanitizer on Earth. Iso-propanol is the largest alcohol discovered thus far, demonstrating the increasing complexity of members of one of the most abundant classes of molecules in space. The identification was made possible by observations of the star-forming region Sagittarius B2 (Sgr B2), which is close to the center of our galaxy and has already detected many molecules. It is the subject of a lengthy investigation into its chemical composition using Chile’s ALMA telescope.
For more than 50 years, scientists have been looking for molecules in space. Astronomers have discovered 276 molecules in the interstellar medium so far. The Cologne Database for Molecular Spectroscopy (CDMS) provides spectroscopic data to detect these molecules, which has been contributed by many research groups, and has proven to be useful in many cases.
The current study aims to learn how organic molecules form in the interstellar medium, particularly in regions where new stars form, and how complex these molecules can be. The underlying motivation is to establish links to the chemical composition of bodies in the Solar System, such as comets, as delivered a few years ago by the Rosetta mission to comet Churyumov-Gerasimenko.
Sagittarius B2 (Sgr B2) is an outstanding star-forming region in our galaxy where many molecules have been detected in the past. It is located close to the famous source Sgr A*, the supermassive black hole at the centre of our galaxy.
“Our group began investigating the chemical composition of Sgr B2 with the IRAM 30-m telescope more than 15 years ago,” says Arnaud Belloche of the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn/Germany, the paper’s lead author. “These observations were successful, resulting in, among other things, the first interstellar detection of several organic molecules.”
With the arrival of the Atacama Large Millimeter/submillimeter Array (ALMA) ten years ago, it became possible to go beyond what a single-dish telescope could achieve toward Sgr B2, and a long-term study of the chemical composition of Sgr B2 was initiated, taking advantage of the high angular resolution and sensitivity provided by ALMA.
Since 2014, ALMA observations have resulted in the discovery of three new organic molecules (isopropyl cyanide, N-methyl formamide, and urea). The detection of propanol is the most recent ALMA project result (C3H7OH).
Propanol is an alcohol that has been detected in interstellar space as the largest molecule in this class. This molecule exists in two forms (“isomers”) depending on which carbon atom the hydroxyl (OH) functional group is attached to: 1) normal-propanol, with the OH functional group bound to the chain’s terminal carbon atom, and 2) iso-propanol, with the OH functional group bound to the chain’s central carbon atom. Iso-propanol is also widely used as a key ingredient in hand sanitizers around the world. The ALMA data set identified both isomers of propanol in Sgr B2.
It is the first detection of iso-propanol in the interstellar medium, as well as the first detection of normal-propanol in a star-forming region. A Spanish research team using single-dish radio telescopes in a molecular cloud near Sgr B2 obtained the first interstellar detection of normal-propanol shortly before the ALMA detection. However, only ALMA was capable of detecting iso-propanol toward Sgr B2.
“The detection of both propanol isomers is extremely useful in determining the mechanism of formation of each. Because they are so similar, they behave physically in very similar ways, which implies that the two molecules should be present in the same places at the same times “says Rob Garrod of the University of Virginia (Charlottesville, United States). “The only unknown is the precise amount present—this makes their interstellar ratio far more precise than it would be for other pairs of molecules. It also means that the chemical network can be tuned much more precisely to determine how they form.”
Because of its high sensitivity, high angular resolution, and broad frequency coverage, the ALMA telescope network was critical for detecting both isomers of propanol toward Sgr B2. The spectral confusion is a problem in identifying organic molecules in the spectra of star-forming regions. Each molecule emits radiation at specific frequencies, which can be determined using laboratory measurements.
“The larger the molecule, the more spectral lines at various frequencies it generates. Because there are so many molecules contributing to the observed radiation in a source like Sgr B2, their spectra overlap, making it difficult to separate their fingerprints and identify them individually “says Holger Müller of Cologne University, where laboratory work was done, particularly on normal-propanol.
Because of ALMA’s high angular resolution, researchers were able to isolate parts of Sgr B2 that emit very narrow spectral lines, which are five times narrower than lines detected on larger scales with the IRAM 30-m radio telescope. The narrowness of these lines reduces spectral confusion, which was critical for identifying both propanol isomers in Sgr B2. The sensitivity of ALMA was also important: if the sensitivity had been just twice as low, it would not have been possible to identify propanol in the collected data.
This study is part of a long-term effort to investigate the chemical composition of sites in Sgr B2 where new stars are forming in order to better understand the chemical processes at work during star formation. The goal is to identify new interstellar molecules and determine the chemical composition of star-forming sites. “Propanol has long been on our list of molecules to look for, but it wasn’t until recent work in our laboratory to characterize its rotational spectrum that we were able to identify its two isomers in a robust way,” says Oliver Zingsheim, also of Cologne University.
Detecting and measuring the abundance ratio of closely related molecules that differ slightly in structure (such as normal- and iso-propanol or, as previously done, normal- and iso-propyl cyanide) allows researchers to probe specific parts of the chemical reaction network that leads to their production in the interstellar medium.
“There are many unidentified spectral lines in the ALMA spectrum of Sgr B2, indicating that much work remains to be done to determine its chemical composition. In the near future, the expansion of the ALMA instrumentation to lower frequencies will most likely help us reduce spectral confusion even further and may allow us to identify additional organic molecules in this spectacular source “Karl Menten, Director of the MPIfR and Head of the Millimeter and Submillimeter Astronomy Research Department, concludes.
The results of a recent spectroscopic study of propanol, as well as the imaging spectral line survey ReMoCA performed with ALMA at the high angular resolution, were used to search for the iso and normal isomers of the propanol molecule in the hot molecular core Sgr B2(N2) in the neighbourhood of the galactic centre.
Under the assumption of local thermodynamical equilibrium, the interferometric spectra were examined. The reaction network of the astrochemical model MAGICKAL was expanded to investigate propanol formation routes and to contextualise the observational results.
The related research was published in Astronomy & Astrophysics.