Evolution of the chemical composition of protoplanetary disks

(ORDO NEWS) — Planets form from gas and dust in the disks surrounding young stars. The chemicals in the disc material that evaporates easily are called volatiles and include water, carbon monoxide, nitrogen, and other simple organic molecules.

The amount of volatiles that accumulates during planet formation is a key factor affecting the composition of a planet’s atmosphere and its suitability for life. This amount, in turn, depends on the arrangement of the gas “reservoirs” in the disk at the time when the planets were formed.

Since the composition of the protoplanetary disk evolves over time, it is important to know the general principles of the evolution of the chemical composition of the disk matter in order to determine the composition of the planets. Astronomers know that the content of water and carbon monoxide in the material of young stellar systems is reduced, in comparison with the ordinary interstellar medium, sometimes by a factor of 100 or more.

According to the widespread version, this depletion in water and carbon monoxide is due to the fact that these substances freeze on the surface of dust particles and then accumulate in the plane of the disk, where they continue to remain in a solid-state. This hypothesis explains the structure of some of the observed systems, but not all of them.

In the new work, astronomers from the Harvard-Smithsonian Astrophysical Center, USA, led by Jennifer B. Bergner, used the ALMA observatory to observe the behavior of volatiles in five young protoplanetary disks. The modeling carried out according to the results of observations showed that the depletion of carbon monoxide in the studied systems proceeds very quickly – in the first 0.5-1 million years of the existence of the disk.

The researchers also found that the spectra of the youngest objects still in a “cocoon” of primary material exhibit characteristic features, probably related to the fact that the chemical compounds of the disk material in such systems are protected from the destructive effects of ultraviolet radiation emitted by the star.

The study is published in the Astrophysical Journal.

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