Ices and X-rays to explain chemical abundances in protoplanetary disks

To understand how planets form, it is mandatory to study the evolution of protoplanetary disks, e.g. disks of gas and dust grains orbiting around stars during the first 3-5 million years of their evolution, a that may evolve into planetary systems. In the last years, several observations have been done in order to detect the molecules, mainly organic, in these structures, allowing to trace the chemical evolution of disks and its connection with the planetary formation. This research line will allow us to understand how the chemistry of planets develops and it can be enriched by the interaction with the circumstellar material. The contribution from the disks, in fact, can enrich the pool of organic molecules in planets, triggering the prebiotic evolution.


The main instrument to study the chemistry of disks is the Atacama Large Millimetre Array (ALMA). ALMA observations, in fact, have allowed astronomers to identify and localize in disks molecules such as CO (carbon monoxide), CO2 (carbon dioxide), HCO, and H2CO (formaldehyde). To date, complex molecules have not been observed in disks, while molecules such as CH3OH (methanol) e CH3CN (acetonitrile) are not abundant as expected. INAF researchers are strongly involved in this research line, with important projects such as The Cradle of Life – Genesis-Ska. 


In order to understand the origin of such a variegate abundance of chemical species in disks, the team of researchers led by A. Ciaravella (INAF – Astronomical Observatory of Palermo) set up experiments that simulate the radiative environment where the dust grains with ice envelopes evolve, farther from the “snow lines” (e.g., regions of the disks where the temperature is low enough to allow given chemical species to froze). In the experiment these mantles are irradiated with X-rays since young solar-type stars are 3-4 order of magnitudes brighter in this energetic band than stars with the same mass but in the main sequence. Besides, X-rays can easily penetrate the circumstellar material, and thus they can play an important role in regulating the chemical evolution of disks.


In this experiment, the dust grains are covered by an external layer of ice as inferred by observations and theoretical studies: an outer layer with a 3:1 mixture of CO:CH3OH and an internal layer of H2O:CH4:NH3 ice (2:1:1). During the irradiation, the methanol is rapidly converted into new species, mainly CO, HCO, H2CO e CO. In these experiments no significant photo-evaporation of methanol has been observed, while a large fraction of its products desorb during the irradiation. The experiment has been repeated after adding in the inner ice layer isotopes 13C and 15N of methane (13CH4) and ammonia (15NH3), with the result that some molecules in the inner layer reach the surface of the ice layer and desorb. Complex molecules form in the inner layer, remaining in the ice.


These results can explain the non-detection of methanol in the gas in disks irradiated by stellar X-rays, together with the presence of CO, HCO e H2CO. The experiment, thus, demonstrates that the irradiation by X-rays of the dust grains covered by ice layers can reproduce the observed chemical abundances in disks. The study is described in the paper: “X-ray processing of a realistic ice mantle can explain the gas abundances in protoplanetary disks“, recently appeared on Proceedings of the National Academy of Sciences of United States of America, with the collaboration of the astronomers Antonio Jimenez-Escobar and Cesare Cecchi-Pestellini of INAF – Astronomical Observatory of Palermo, together with astronomers of the Centro de Astrobiologìa of Madrid and the National Central University of Taiwan.


The figure (click here to visualize the entire image), shows a scheme of the experiment, with the composition of the two ice layers covering the dust grains and the effects of the X-ray irradiation on the grains.


Mario Giuseppe Guarcello  ( follow mguarce)