CIPDISKS · Chemistry in Protoplanetary Disks

Chemistry in Protoplanetary Disks

The main goal of the current project is to get insight into the chemical composition of protoplanetary disks by studying the physical and chemical processes at work in such systems. Protoplanetary disks are nebulae of gas and dust surrounding young stars and provide the environment and material from which planets form. They are extremely complex systems to study from a theoretical point of view since almost all the processes that are important in the interstellar medium (photochemistry, grain-surface interactions, accretion flows, time dependence,...) play a role in determining its physical and chemical structure. The interplay among the different processes makes a self-consistent solution to be a very complicated task. Therefore, the different processes must be treated decoupled to make the problem feasible and also to properly understand their effects. In the practice, current models usually treat in detail some of the processes but use simple approximate approaches to deal with the remaining processes. Among the different processes at work in protoplanetary disks, the interaction of the ultraviolet radiation emitted by the star with the gaseous and dusty material of the disk determines to a large extent the chemical structure in the disk surface and the emergent spectrum. One of the main goals of the current project is to treat in a detailed way such processes, which are treated by approximate approaches by most of the state-of-the-art models. To address the above mentioned objective, Marcelino Agúndez has been deeply involved in the development of a scientific code able to compute the chemical composition of a generic protoplanetary disk. This task, being by itself a long-term project, has taken most of the work time of Marcelino during the two-years duration of the project. The code is able to compute the radiative transfer of ultraviolet radiation across the disk including all the discrete transitions due to the most abundant molecular species (H2, CO, H2O, ...). The detailed calculation of the ultraviolet radiation field at each point of the disk allows to compute the photodissociation and photoionisation rates of the different species through the relevant cross sections. The rigorous treatment of the photochemistry in protoplanetary disks is one of the most important added values of this code, as compared with most of the state-of-the-art models that treat photochemistry through approximate methods. A preliminary version of the code has already been used to interpret recent observations of the protoplanetary disk AB Auriga carried out with the IRAM 30-m telescope, a study which has been recently published in Astronomy & Astrophysics (524, A19; 2010). A first important outcome of the project, a systematic study of different types of protoplanetary disks and of the influence of several stellar and disk parameters on the chemical composition, is being carried out and will soon be published. Preliminary results indicate that the detailed treatment of the photochemistry in protoplanetary disks produce important differences as compared with approximate methods. Marcelino has also studied in detail fundamental processes which are important for the chemistry of protoplanetary disks, such as the chemistry of vibrationally excited molecular hydrogen. The study, published in The Astrophysical Journal (713, 662; 2010), finds that some species such as the cation CH+ is very efficiently formed by reactions involving vibrationally excited H2 in dense regions illuminated by a high ultraviolet field. Interestingly, the cation CH+ has been recently observed by the Herschel Space Observatory confirming our theoretical prediction. The scientific code developed during this project will become very useful to interpret observations of disks that are being carried out with the Herschel Space Observatory and those that will be carried out in the very near future with the radio interferometer ALMA.

Data: CORDIS, © European Union