Periodic Reporting for period 4 - DOC (The Dawn of Organic Chemistry)
Berichtszeitraum: 2022-04-01 bis 2022-09-30
From a “simple” chemical point of view, all terrestrial living organisms, from microbes to humans, are made up of the same basic components: amino acids, fatty acids, sugars, nucleobases, etc. In total, we are referring to about 50 “small” molecules containing less than 100 atoms of carbon, with hydrogen, oxygen, nitrogen and other elements in smaller quantities: as we know, terrestrial life is based on organic chemistry.
Of course, this is not by chance, because the electronic structure of carbon makes it the element that constitutes the backbone of the terrestrial life “small” molecules. However, this would not be enough if carbon was not also available for making these molecules. Luckily for us, carbon atoms form molecules relatively complex also in space and, particularly important, in regions where future Solar-like planetary systems are born. Astronomers call these molecules iCOMs, for interstellar Complex Organic Molecules. During the formation of the Solar System, iCOMs could have been transmitted to the small bodies, such as comets and asteroids (whose small fragments are called meteorites when reach Earth). Indeed, we see organic molecules in cometary and meteoritic material, even amino acids.
This led the Nobel laureate C. De Duve to affirm: “the chemical seeds of life are universal” and “life is an obligatory manifestation of matter, written into the fabric of the Universe”.
The objective of the DOC project is to understand the dawn of organic chemistry, namely the start of organic chemistry in systems similar to the progenitor of the Solar System, with the ultimate goal to understand how organic chemistry builds up and evolves in these systems and, consequently, to understand how universal the chemical seeds of life are.
To do that, we need a reliable theory for the organic chemistry in nascent Solar type systems and obtaining it is the immediate goal of DOC.
Of course, this is not by chance, because the electronic structure of carbon makes it the element that constitutes the backbone of the terrestrial life “small” molecules. However, this would not be enough if carbon was not also available for making these molecules. Luckily for us, carbon atoms form molecules relatively complex also in space and, particularly important, in regions where future Solar-like planetary systems are born. Astronomers call these molecules iCOMs, for interstellar Complex Organic Molecules. During the formation of the Solar System, iCOMs could have been transmitted to the small bodies, such as comets and asteroids (whose small fragments are called meteorites when reach Earth). Indeed, we see organic molecules in cometary and meteoritic material, even amino acids.
This led the Nobel laureate C. De Duve to affirm: “the chemical seeds of life are universal” and “life is an obligatory manifestation of matter, written into the fabric of the Universe”.
The objective of the DOC project is to understand the dawn of organic chemistry, namely the start of organic chemistry in systems similar to the progenitor of the Solar System, with the ultimate goal to understand how organic chemistry builds up and evolves in these systems and, consequently, to understand how universal the chemical seeds of life are.
To do that, we need a reliable theory for the organic chemistry in nascent Solar type systems and obtaining it is the immediate goal of DOC.
All the work done by the DOC team members aimed at building up a reliable theory for the organic chemistry in nascent Solar type systems.
First, we investigated why some young protostars are enriched of iCOM while others are rather enriched of carbon chains. The first ones are called hot corinos while the second class of protostars are known as WCCC (Warm Carbon Chain Chemistry) sources. We found that the probability to form hot corinos is larger than that to form WCCC sources in a relatively quiet region, NGC1333, where only Solar type planetary systems are currently formed. However, the situation is inverted in the Orion Molecular Clouds, where also high-mass stars are formed. These results are intriguing because the comparison that we obtained between the measured iCOMs relative abundances in comets and hot corinos would suggest that the Solar System passed through a hot corino phase, whereas various evidences suggest that the Solar System was born in an environment similar to that of the Orion Molecular Clouds.
In fact, the situation is even more complicated, as we showed that the observations in the millimetre, where traditionally hot corinos are searched for and studied, are actually plagued by the dust in front of the embedded hot corinos. This dust absorbs the photons emitted by the hot corinos and can totally mask their presence or, at best, falsify the derivation of the iCOM abundances. Our studies paved the road for new searches and studies of hot corinos in the radio wavelengths, where we may harvest more of these sources.
Hot corinos are enriched of gaseous iCOMs. Thus, one crucial parameter to understand the real chemical composition of hot corinos is the iCOM binding energy (BE), which governs whether the molecule is in the gas-phase or frozen onto the dust grains. Following up our discovery of the heavy impact of the dust in the hot corino’s studies, we used the combination of multiwavelength observations with new calculated BEs and discovered that hot corinos have a chemical onion-like structure. This was possible thanks to the tight collaboration of the chemist and astronomer members of the DOC team. While the latter obtained amazingly sharp images of two hot corinos, SVS13A and HH212-mm, the former provided state-of-the-art quantum mechanical (QM) computations of the BEs on large clusters of water ice. More in general, our enlarged team provided the BEs of several species computed, for the first time, on large clusters of frozen water. With a few exceptions, these BEs were unknown and simply guessed in the astrochemical models.
We also developed a new methodology, based on the radio observations of methanol and ammonia to better understand how the chemical composition of a protostar depends on its pas history and, therefore, environment. Since methanol and ammonia are formed on the grain surfaces at different times, as showed by our astrochemical modelling which includes our latest QM chemistry calculations of the two species formation processes, their measured abundance ratio constrains the past history of the hot corino. In this way, we discovered the violent birth of the protostar called NGC1333-IRAS4A, which was born after (or because of) the crash of a bubble of gas, probably created by the explosion of a distant supernova, with the NGC1333 molecular cloud. Again, this opens up a new way to study the full history of a protostar since its incubation period.
Finally, we used state-of-the-art QM computations to fill up the huge gap in our knowledge of the reactions occurring on the gas-phase and on the grain surfaces in the interstellar conditions.
First, we investigated why some young protostars are enriched of iCOM while others are rather enriched of carbon chains. The first ones are called hot corinos while the second class of protostars are known as WCCC (Warm Carbon Chain Chemistry) sources. We found that the probability to form hot corinos is larger than that to form WCCC sources in a relatively quiet region, NGC1333, where only Solar type planetary systems are currently formed. However, the situation is inverted in the Orion Molecular Clouds, where also high-mass stars are formed. These results are intriguing because the comparison that we obtained between the measured iCOMs relative abundances in comets and hot corinos would suggest that the Solar System passed through a hot corino phase, whereas various evidences suggest that the Solar System was born in an environment similar to that of the Orion Molecular Clouds.
In fact, the situation is even more complicated, as we showed that the observations in the millimetre, where traditionally hot corinos are searched for and studied, are actually plagued by the dust in front of the embedded hot corinos. This dust absorbs the photons emitted by the hot corinos and can totally mask their presence or, at best, falsify the derivation of the iCOM abundances. Our studies paved the road for new searches and studies of hot corinos in the radio wavelengths, where we may harvest more of these sources.
Hot corinos are enriched of gaseous iCOMs. Thus, one crucial parameter to understand the real chemical composition of hot corinos is the iCOM binding energy (BE), which governs whether the molecule is in the gas-phase or frozen onto the dust grains. Following up our discovery of the heavy impact of the dust in the hot corino’s studies, we used the combination of multiwavelength observations with new calculated BEs and discovered that hot corinos have a chemical onion-like structure. This was possible thanks to the tight collaboration of the chemist and astronomer members of the DOC team. While the latter obtained amazingly sharp images of two hot corinos, SVS13A and HH212-mm, the former provided state-of-the-art quantum mechanical (QM) computations of the BEs on large clusters of water ice. More in general, our enlarged team provided the BEs of several species computed, for the first time, on large clusters of frozen water. With a few exceptions, these BEs were unknown and simply guessed in the astrochemical models.
We also developed a new methodology, based on the radio observations of methanol and ammonia to better understand how the chemical composition of a protostar depends on its pas history and, therefore, environment. Since methanol and ammonia are formed on the grain surfaces at different times, as showed by our astrochemical modelling which includes our latest QM chemistry calculations of the two species formation processes, their measured abundance ratio constrains the past history of the hot corino. In this way, we discovered the violent birth of the protostar called NGC1333-IRAS4A, which was born after (or because of) the crash of a bubble of gas, probably created by the explosion of a distant supernova, with the NGC1333 molecular cloud. Again, this opens up a new way to study the full history of a protostar since its incubation period.
Finally, we used state-of-the-art QM computations to fill up the huge gap in our knowledge of the reactions occurring on the gas-phase and on the grain surfaces in the interstellar conditions.
When DOC started, it was known that Solar type protostars can be enriched of iCOMs, but a lot of fog existed on when and how exactly these very important molecules formed and how they evolved. This was prevalently due to (1) a shortage of astronomical observations with enough spatial resolution, (2) a knowledge deficiency of the gas-phase reactions that can occur in the interstellar conditions, and (3) a massive lack of the knowledge of the chemical processes taking place on the grain surfaces.
During the DOC project:
(1) we have obtained new astronomical observations with the best possible spatial resolution at the best world millimetre and radio interferometers, obtaining a sharper view of the objects which allowed a completely new view of the iCOM presence and evolution in young Solar type protostars;
(2) we have reviewed old and proposed new routes of gas-phase formation of several crucial iCOM and their deuterated counterparts by carrying out deep chemistry literature searches and new QM calculations, which, coupled with the astronomical observations, provided strong constraints to the formation routes of glycolaldehyde, acetaldehyde and formamide;
(3) we have for the first time computed the reactions between two radicals on the icy dust grain surfaces via beyond-the-art QM calculations, providing for the first time quantitative data on these reactions and showing that not all reactions lead to iCOMs, as previously assumed by astrochemical models; we also provided the binding energies of many species, necessary to simulate their diffusion and sublimation and, ultimately, their abundance in the gas-phase where they are detected.
DOC, therefore, greatly contributed to answer the question “in which nascent planetary systems the small molecules that may have triggered life on Earth are present and, ultimately, how much the Solar System is peculiar when it comes to organic chemistry”.
During the DOC project:
(1) we have obtained new astronomical observations with the best possible spatial resolution at the best world millimetre and radio interferometers, obtaining a sharper view of the objects which allowed a completely new view of the iCOM presence and evolution in young Solar type protostars;
(2) we have reviewed old and proposed new routes of gas-phase formation of several crucial iCOM and their deuterated counterparts by carrying out deep chemistry literature searches and new QM calculations, which, coupled with the astronomical observations, provided strong constraints to the formation routes of glycolaldehyde, acetaldehyde and formamide;
(3) we have for the first time computed the reactions between two radicals on the icy dust grain surfaces via beyond-the-art QM calculations, providing for the first time quantitative data on these reactions and showing that not all reactions lead to iCOMs, as previously assumed by astrochemical models; we also provided the binding energies of many species, necessary to simulate their diffusion and sublimation and, ultimately, their abundance in the gas-phase where they are detected.
DOC, therefore, greatly contributed to answer the question “in which nascent planetary systems the small molecules that may have triggered life on Earth are present and, ultimately, how much the Solar System is peculiar when it comes to organic chemistry”.