Astronomers spot a precursor to organic molecules in a protoplanetary disc


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Using the Atacama Large Millimeter/submillimeter Array (ALMA), a millimeter wave observatory located in northern Chile, a team of astronomers has detected the largest molecule ever discovered in a planet-forming disc: it is methoxymethane (also called dimethyl ether), a molecule with nine atoms (CH3OCH3) considered as a precursor of even larger organic molecules, which could potentially lead to the emergence of life.

Understanding the formation and evolution of prebiotic molecules within protoplanetary disks can help to better understand how life originated in our own solar system. This is why scientists are so interested in this distant chemistry; they are trying to understand how this material essential to life is incorporated into planets at the time of their formation — which, at the same time, could give a better idea of ​​the potential for life in other planetary systems.

The protoplanetary disk in question surrounds Oph-IRS 80, a young star located approximately 472 light-years from Earth, in the constellation of Ophiuchus. In 2013, astronomers had discovered a particular region in the southern part of the disk, in which millimeter-sized dust grains were trapped and clumped together to form much larger objects, such as comets, asteroids and even potentially planets. This highly localized “dust trap” makes it the most asymmetrical protoplanetary disk detected to date. Recent observations of this area have identified several organic molecules there.

Molecules hidden in the ice

The IRS disk 20 has been the subject of extensive research . Last year, a team reported the detection of formaldehyde (CH2O ) and methanol (CH3OH) in this disc. The emission lines bound to these molecules displayed the same crescent shape as the dust continuum, revealing for the first time the direct link between the dust trap and complex organic molecules. The same team also detected sulfur monoxide (SO) and sulfur dioxide (SO2): this was the very first detection of sulfur volatiles in a protoplanetary disk.

This time, new observations have revealed the presence of methoxymethane (CH3OCH3); the researchers also attempted to detect methyl formate (HCOOCH3), a building block of even larger organic molecules. Methoxymethane is an organic molecule commonly seen in star-forming clouds, but this is the first time scientists have identified such large molecules within a protoplanetary disk. “ It’s really exciting to finally detect these large molecules in the discs. We thought for a long time that it would not be possible to observe them ”, explains in a press release Alice Booth, researcher at the Leiden Observatory, in the Netherlands, and co-author of the article describing this discovery.

protoplanetary disk dust trap

Cette image acquise par ALMA montre le piège à poussière (ici en vert) situé dans le disque protoplanétaire qui entoure le système Oph-IRS 48. L’anneau orange indique la localisation de particules de poussière plus fines (de l’ordre du micron). © ALMA (ESO/NAOJ/NRAO)/Nienke van der Marel

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This image acquired by ALMA shows the dust trap (here in green) located in the disc protoplanetary that surrounds the Oph-IRS system 80. The orange ring indicates the location of finer (micron-sized) dust particles. © ALMA (ESO/NAOJ/NRAO)/Nienke van der Marel

Experts believe that many complex organic molecules, such as methoxymethane, appear in the clouds of formation of stars, even before their birth. In these particularly cold environments, simple atoms and molecules like carbon monoxide (CO) stick to the dust grains. Then, the whole ends up forming a layer of ice in which various chemical reactions take place producing more complex molecules.

However, researchers have recently discovered that Oph-IRS disk dust trap 80 also constitutes an enormous reservoir of icy grains in the median plane of the disc, an “ice trap” containing complex organic molecules in the gaseous phase. So when the heat from the star sublimates the ice into gas, the trapped molecules are released and become detectable.

A disc particularly rich in complex molecules

This new discovery suggests that other complex molecules generally detected in star-forming clouds can potentially also be found within protoplanetary disks, trapped in ice. “ We now know that these larger complex molecules are available to power planets forming in the disk. This was not known before, because in most systems, these molecules are hidden in ice ,” says Alice Booth. These molecules are precursors to molecules essential for life, such as sugars and amino acids.

The team believe that methoxymethane and formate methyl could be formed via reactions involving radicals (CH3O + HCO → HCOOCH3 and CH3O + CH3 → CH3OCH3) — which assume a common formation pathway from the precursor CH3O. These reactions could be supported by UV irradiation of ice cream, causing the dissociation of methanol (according to the reaction CH3OH + hν → CH3 + OH or CH 3O+H).

The team also reports the first detection of nitric oxide (NO) in the disc (five transition lines in total), as well as an additional emission line for sulfur dioxide. Future observations will undoubtedly make it possible to discover still other molecular species. “ We hope that with further observations, we can get closer to understanding the origin of prebiotic molecules in our own system. solar,” said Nienke van der Marel, a researcher at the Leiden Observatory, who also participated in the study. Thanks to the Extremely Large Telescope (ELT) of the ESO, which is scheduled to go live later this decade, the team will be able to study more precisely the chemistry of the more internal regions of the disk.

Source: N. Brunken et al., Astronomy & Astrophysics