From Clouds to Planets: Tracking Organic Molecules Through Star Formation
Complex organic molecules (COMs) are molecules that contain at least six atoms, including carbon. They matter because they are thought to be the building blocks of pre-biotic chemistry, molecules that may one day lead to life. Observations have found COMs in many environments: cold interstellar clouds, young protostars, protoplanetary disks, and even comets. Yet, it is still unclear where exactly these molecules are created. Are they “inherited” from earlier stages of cloud evolution, or do they form later, during the collapse of a star and the building of its planet-forming disk? In this study, Pierre Marchand and collaborators investigate this question using computer simulations that track both the physical and chemical evolution of star-forming regions.
Methods
To model star formation, the authors used the Analytical Protostellar Environment (APE) code, which provides temperature and density maps as a cloud collapses to form a young star and disk. They then coupled this with the Nautilus chemical code, which follows thousands of chemical reactions in gas and on dust grains. This combined setup allowed them to simulate how 26 different COMs form, evolve, and move between solid ices and gas during the early stages of star and planet formation, specifically from the prestellar cloud core to the end of the Class I stage (about 150,000 years after the star’s birth).
Results: How Methanol Behaves
Methanol (CH₃OH), one of the most important COMs, is used as an example. At low temperatures, methanol sticks to grains as ice. When a forming star heats its surroundings past 100 K, methanol evaporates into the gas. The simulations show that methanol’s total abundance remains fairly constant during collapse, because while it is continuously destroyed in some reactions, it is also re-formed efficiently. This makes methanol a good tracer of inheritance from earlier cloud stages.
Results: Inheritance vs. New Formation
Out of the 26 COMs studied, Marchand and colleagues found that only two, methanol (CH₃OH) and ethanol (C₂H₅OH), are mainly inherited from the prestellar stage. Eight others, including propyne (CH₃CCH), propylene (C₃H₆), and methyl cyanide (CH₃CN), are partly inherited but also formed during collapse. The remaining 16, mostly heavier molecules, are created later, either during collapse or directly in the disk. This suggests that while some simple COMs are carried forward from early times, most of the chemical richness of disks is generated as material falls inward and heats up.
Results: How Abundances Change Over Time
During the first 150,000 years of disk evolution, many COMs show stable abundances, changing by less than a factor of two. Others, such as hydroxyacetone (CH₃COCH₂OH) or glycolaldehyde (HOCH₂CHO), vary by an order of magnitude. Interestingly, some molecules are destroyed in the hot inner disk, while others increase in abundance with time. These differences suggest that a forming planet’s chemical makeup will depend strongly on both where and when it forms in the disk.
Results: Sensitivity to Physical Conditions
The team also tested how changes in the initial environment affect outcomes. A warmer cloud temperature (15 K instead of 10 K) led to more COMs forming before collapse, especially nitrogen-bearing species. Higher cosmic-ray ionization, by contrast, destroyed many molecules and caused localized depletions. Surprisingly, increasing the total cloud mass had little effect on disk COM abundances. These experiments highlight how sensitive chemical outcomes can be to even small changes in physical conditions.
Discussion and Implications
The study shows that inheritance is important for a few simple COMs, but most complex molecules are built during collapse and early disk evolution. This means planets forming in disks inherit a mix of early and newly formed chemistry. Some molecules, such as methanol and ethanol, appear robust, staying constant across environments, while others are fragile and strongly shaped by local conditions. As observational facilities like the James Webb Space Telescope improve, astronomers may soon be able to test these predictions directly by looking at ices and gases in young disks.
Conclusion
Marchand and colleagues conclude that the chemical makeup of planet-forming disks is neither fully inherited nor fully new, but a combination of both. The simplest COMs survive from the cloud stage, while heavier ones are largely products of the collapse. These results underscore the importance of timing, environment, and location in shaping the chemical inventory available for planet and, eventually, life formation.
Source: Marchand
